Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
To wrap up this thread: after discussing this issue with our Windows admins over the past few months, we have concluded that the correct course of action here is to set the TRUSTED_FOR_DELEGATION flag in the userAccountControl attribute for all Linux host machine accounts that we control. This will ensure that AD sets the OK-AS-DELEGATE flag in the TGS-REP, which ensures that ssh credential forwarding will work correctly regardless of whether the specific Kerberos client implementation honors the OK-AS-DELEGATE flag by default. Our considerations were: * Kerberos clients are free to honor or ignore the OK-AS-DELEGATE flag, and it is not necessarily obvious which clients do or do not honor it by default. * Thus, if we do not set the OK-AS-DELEGATE flag, then we would need to continually identify all Kerberos client implementations in play in our enterprise, determine which ones honor the OK-AS-DELEGATE flag by default, determine a way to override that default to ignore the OK-AS-DELEGATE flag, and find a way to automate that override. This would require nontrivial effort. * Setting the TRUSTED_FOR_DELEGATION flag in the userAccountControl attribute is intended to indicate that the host is administered by reputable/trusted parties and that it can be trusted for delegation. This is the case for us, so we are using the TRUSTED_FOR_DELEGATION flag exactly as it was intended. Unfortunately, our Windows admins have yet to be able to find a way to delegate the ability to set the TRUSTED_FOR_DELEGATION flag in the userAccountControl attribute on a per-OU basis. They can delegate just about any other ability to us (the Linux admins) for hosts in the OUs that contain Linux hosts, but no matter what they try, we get an *Insufficient privileges* error whenever we attempt to set the TRUSTED_FOR_DELEGATION flag on a Linux host. (They are beginning to suspect that Active Directory might have some special hardcoded “only domain admins can set the TRUSTED_FOR_DELEGATION flag” rule that cannot be overridden or delegated.) Ultimately, they might have to resort to (e.g.) automating a Powershell script to enumerate the host machine accounts in our Linux OUs, and automatically setting the TRUSTED_FOR_DELEGATION flag for any hosts that are missing it. If we can figure out a way to delegate the ability to set the TRUSTED_FOR_DELEGATION flag, I’ll post it to this thread, for anyone in the future who might find themselves in the same position. My thanks to everyone who provided feedback in this thread. On Tue, Apr 30, 2024 at 12:49 PM Ken Hornstein wrote: > > I think the core issue here is that RFC4120§2.8 was unclear in > > defining the OK-AS-DELEGATE flag. > > I have to say that IMHO, the explanation in RFC 4120 is clear to me > and the current implementations within the MacOS X Kerberos code fall > squarely within the scope of the RFCs explanation. Yes, it was clearly written; yes, clients are not violating the RFC. And yet: the implementation of this feature still resulted in 5+ years of silent breakage, the avoidance of which required delegating users’ actual *passwords* to the remote systems. Had the systems in question had in fact been untrustworthy, this would have resulted in a far greater security risk than simply delegating the credentials in the first place. The OK-AS-DELEGATE flag might have satisfied the specific itch that Microsoft wanted to scratch, but it was poorly conceived and poorly implemented. Especially for people in heterogeneous environments, this flag is a treacherously subtle landmine. > You are, of course, free to express your opinions about the > implementation of these protocols extensions, but ... well, other > than the satisfaction of primal screaming into the void, I'm not > sure how helpful that will be. Poorly-conceived and implemented protocol extensions should be called out, if for no other reason than to avoid repeating their mistakes. Kerberos mailing list Kerberos@mit.edu https://mailman.mit.edu/mailman/listinfo/kerberos
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
>I looked at the Apple fork of Heimdal and didn't find any obvious code
>change to honor ok-as-delegate by default. In fact, it doesn't even
>implement enforce_ok_as_delegate. But both versions do implement a
>ccache config setting called "realm-config" and enforce ok-as-delegate
>if the 1 bit is set in the first byte of the value. Nothing in Heimdal
>or Apple's fork of it sets realm-config, but the macOS native ccache
>implementation or login system might do so.
You missed this code in kuser/kinit.c:
if (ok_as_delegate_flag || windows_flag || use_referrals_flag) {
unsigned char d = 0;
krb5_data data;
if (ok_as_delegate_flag || windows_flag)
d |= 1;
if (use_referrals_flag || windows_flag)
d |= 2;
data.length = 1;
data.data =
krb5_cc_set_config(context, ccache, NULL, "realm-config", );
}
However, if I run "kinit --ok-as-delegate" in my MIT-based realm, using
the MacOS X ssh client, "ssh -K foo'" still delegates a credential fine,
so something else is clearly going on.
--Ken
Kerberos mailing list Kerberos@mit.edu
https://mailman.mit.edu/mailman/listinfo/kerberos
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
On 4/30/24 12:49, Ken Hornstein via Kerberos wrote: First off, I would advise you to NOT look at upstream Heimdal, because that's not helpful because it's not actually the code in question. Instead maybe look at the actual Heimdal source code used on MacOS X? To expand on this: the Apple forks of open-source projects are available at opensource.apple.com, and at https://github.com/apple-oss-distributions (not sure if the latter is official or community-maintained). I looked at the Apple fork of Heimdal and didn't find any obvious code change to honor ok-as-delegate by default. In fact, it doesn't even implement enforce_ok_as_delegate. But both versions do implement a ccache config setting called "realm-config" and enforce ok-as-delegate if the 1 bit is set in the first byte of the value. Nothing in Heimdal or Apple's fork of it sets realm-config, but the macOS native ccache implementation or login system might do so. James could perhaps this test theory by setting KRB5CCNAME to FILE:something, running kinit -f, and seeing if ssh will forward those tickets. Kerberos mailing list Kerberos@mit.edu https://mailman.mit.edu/mailman/listinfo/kerberos
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
>I think the core issue here is that RFC4120§2.8 was unclear in >defining the ok-as-delegate flag. I have to say that IMHO, the explanation in RFC 4120 is clear to me and the current implementations within the MacOS X Kerberos code fall squarely within the scope of the RFCs explanation. The "wishy-washy" statements you describe are standard for introducing new protocol extensions. You are, of course, free to express your opinions about the implementation of these protocols extensions, but ... well, other than the satisfaction of primal screaming into the void, I'm not sure how helpful that will be. >> Simo already explained the thinking there, but I think the thing >> you're not considering is that not all services require delegated >> credentials. Yes, in your environment (and ours) delegated >> credentials for host principals is essential, but you don't normally >> need that for other types of credentials. I haven't run into Simo's >> experience of badly coded applications delegating credentials when >> they shouldn't, but I could believe it happens. > >Our experience is similar: while I agree it could happen, we have not >encountered it, so it is mystifying to us why Microsoft was so >insistent upon implementing this feature. In fairness ... in the normal Microsoft world, they do full credential delegation very rarely, and they spent a lot of effort on constrained delegation protocol extensions because the security issues associated with delegating a TGT. Being able to centrally manage these things from the KDC is not unreasonable! And yes, I do wonder what world people live in when they suggest ssh agent forwarding as an alternative to TGT delegation; to me ssh agent forwarding seems like a much worse solution, but to each his or her own. >It’s not a competency issue: our AD/Windows admins are highly >competent (if I do say so myself), and we Linux admins enjoy good >rapport with them. Rather, they expressed two concerns: >[...] I sympathize with these concerns, I do; it sounds like you're in a tough spot. >Upstream Heimdal has the enforce_ok_as_delegate krb5.conf >configuration option, the same as the MIT Kerberos, and like MIT >Kerberos, it defaults to false (only enforced when an application >specifically requests enforcement) (1). First off, I would advise you to NOT look at upstream Heimdal, because that's not helpful because it's not actually the code in question. Instead maybe look at the actual Heimdal source code used on MacOS X? >But from testing, ssh on Macs won’t delegate the credential unless the >service ticket of the target host has the ok-as-delegate flag. So >either that’s not the default for Apple’s version of Heimdal, or >something is overriding the default. Here's the thing: I, personally, cannot reproduce this in our realm. While we do provide our own ssh client, the stock MacOS X client still delegates credentials just fine for us. Clearly it does not work for you, so maybe investigating WHY it doesn't work for you would be helpful. My guess is that there is some internal realm configuration flag being set that tags your realm as a "windows" realm and triggers the enforcement of the ok-as-delegate flag. >> I will confess that we supply to our users Kerberos kits which are >> all MIT sources, including OpenSSH, for this exact reason; there's >> too much weird variance like this. > >In the past, we did the same thing, but we reverted to using the >macOS-provided Kerberos implementation, because decisions Apple made >in later macOS releases just made it too painful to replace it (or >even provide an alternative implementation alongside the macOS >implementation). I do not agree with this statement. We still do this today, our code works fine on everything up to and including Sonoma, and it interoperates with the MacOS X Kerberos code fine. I am not sure what decisions you are referring to. --Ken Kerberos mailing list Kerberos@mit.edu https://mailman.mit.edu/mailman/listinfo/kerberos
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
On Tue, Apr 16, 2024 at 9:31 PM Ken Hornstein wrote: > Simo already explained the thinking there, but I think the thing > you're not considering is that not all services require delegated > credentials. Yes, in your environment (and ours) delegated > credentials for host principals is essential, but you don't normally > need that for other types of credentials. I haven't run into Simo's > experience of badly coded applications delegating credentials when > they shouldn't, but I could believe it happens. Our experience is similar: while I agree it could happen, we have not encountered it, so it is mystifying to us why Microsoft was so insistent upon implementing this feature. We deploy ssh_config settings on all of our managed hosts that disable credential forwarding (as well as GSSAPI) for hosts outside of our domain. We also configure web browsers to perform GSSAPI only for hosts within our domain. So beyond the ok-as-delegate flag, we take efforts to disable all things Kerberos/GSSAPI (not just credential delegation) when connecting to hosts we do not control. > And, well .. one thing I guess I do not understand is, exactly, what > is your problem with turning on that setting on your KDC? If this > is just a cri de cœur about lousy AD admins, fair enough; I can > understand your pain there (most AD admins really don't know how > Kerberos works, sadly). It’s not a competency issue: our AD/Windows admins are highly competent (if I do say so myself), and we Linux admins enjoy good rapport with them. Rather, they expressed two concerns: 1. While setting the TRUSTED_FOR_DELEGATION flag in the userAccountControl attribute has the effect of setting the ok-as-delegate flag on service tickets issued for that host, it is not clear from Microsoft’s documentation what other effects (if any) setting the TRUSTED_FOR_DELEGATION flag will have in AD. Based on past experiences, they want to make sure that setting the TRUSTED_FOR_DELEGATION flag to achieve one particular effect won’t have unwanted side-effects (especially ones that may be subtle and not obvious at first). 2. Our AD admins have delegated the ability to create (join) and delete computer objects in two OUs that are specifically designed for Linux hosts (one OU for servers; another OU for clients). This permits Linux admins to [de]commission hosts on our own, without requiring them to intervene to create/destroy the computer objects. There are some non-trivial ACL/permission configurations required to make this work correctly, and they will need to figure out how to additionally delegate the “Linux admins in this group may set TRUSTED_FOR_DELEGATION flag on computer objects created in this OU” in the same way. #2, in particular, is a dealbreaker: we have a fair amount of Linux host churn (“cattle, not pets”), and thus Linux admins cannot go running to the AD admins to set the TRUSTED_FOR_DELEGATION flag every time we join a new computer object into one of the OUs designated for Linux hosts. If we need to set it, we (the Linux admins) must be able to set it ourselves. > Speaking about the OK-AS-DELEGATE flag, the ONLY thing that does is > set the flag in the ticket which the client uses as a hint (it is > free to ignore that hint). I cannot speak for the AD > TRUSTED_FOR_DELEGATION flag as a whole; it's possible it does > something else than set the OK-AS-DELEGATE ticket flag. Yes, that’s exactly the first concern. > So if there is NOT a delegation flag on the ticket, the realm-config > entry is checked. If the first byte has the least-significant bit > set, the all of the delegation flags are cleared. I suspect this is > what you're hitting. Perhaps? Upstream Heimdal has the enforce_ok_as_delegate krb5.conf configuration option, the same as the MIT Kerberos, and like MIT Kerberos, it defaults to false (only enforced when an application specifically requests enforcement) (1). But from testing, ssh on Macs won’t delegate the credential unless the service ticket of the target host has the ok-as-delegate flag. So either that’s not the default for Apple’s version of Heimdal, or something is overriding the default. > I will confess that we supply to our users Kerberos kits which are > all MIT sources, including OpenSSH, for this exact reason; there's > too much weird variance like this. In the past, we did the same thing, but we reverted to using the macOS-provided Kerberos implementation, because decisions Apple made in later macOS releases just made it too painful to replace it (or even provide an alternative implementation alongside the macOS implementation). (1) https://github.com/heimdal/heimdal/blob/master/lib/krb5/krb5.conf.5 Kerberos mailing list Kerberos@mit.edu https://mailman.mit.edu/mailman/listinfo/kerberos
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
On Tue, Apr 16, 2024 at 1:46 PM Simo Sorce wrote: > The correct action is for you to ask the Domain Administrators to > mark the target hosts as ok for delegation, it is unclear why MIT > Kerberos should make it easy to override Realm policies. I think the core issue here is that RFC4120§2.8 was unclear in defining the ok-as-delegate flag. Quoting RFC4120§2.8, emphasis mine: * The ticket flags … *MAY* have the ok-as-delegate flag set * A client *CAN* use the presence of this flag to *HELP* it decide * It is acceptable to *IGNORE* the value of this flag All of these statements are wishy-washy statements that provide no concrete guidance for server/client implementers: * Should server implementations set this flag by default unless administrators specifically elect to disable it, or should server implementations clear this flag by default unless administrators specifically elect to enable it? * Should client implementations honor this flag by default unless administrators specifically elect to ignore it, or should client implementations ignore this flag by default unless administrators specifically elect to honor it? * Given heterogeneous environments where there are likely to be multiple independent client implementations, how should this feature be deployed at a client and server level in order to minimize interoperability issues? In light of the failure of RFC4120§2.8 to answer any of these difficult questions, I would argue that the MIT Kerberos behavior is reasonable (arguably, the most reasonable): servers do not set the flag by default, and clients do not honor the flag by default. This leaves it up to individual sites to determine whether the additional control this feature offers is desired, and if so, how to deploy it without breaking things. > Delegating a whole TGT is generally a bad idea, and often clients > are misconfigured to broadly forward it everywhere. That is why > Microsoft took back control in the hands of administrators. It is > not a bad thing, and if your setup has been vetted such that TGT > delegation is an acceptable risk, then it should be easy to get it > fixed the proper way, by getting your domain admins to set the right > flag on the permitted target hosts. The issue is not that this feature exists; the issue is that until recently, we simply *did not know* that this feature exists. And why would we? * Windows admins never had any reason to be aware of this feature, because they use remote desktop to login to remote Windows hosts from other Windows hosts. * Linux admins never had any issues with delegating credentials from a Linux host to another Linux host, because MIT Kerberos clients ignore the flag by default. * We assumed that the inability to delegate credentials from Windows host to Linux hosts via PuTTY was a PuTTY implementation issue, because that was the only instance where we saw delegation failing. And PuTTY, at least with default logging, showed no warning/error messages. Furthermore, because so few people were regularly logging into Linux hosts from Windows hosts (people who did this often simply requested a Linux workstation), it wasn’t worth the time/effort to investigate further. This state continued for the better part of a decade. It was only when we began to officially support Mac desktops, and ssh clients on those Macs also failed to delegate credentials (the same as PuTTY on Windows), that we began to suspect there was a deeper issue at play. > Note that if ticket delegation is needed solely to allow jumping > from hosts to host, you should be able to achieve the same result > via agent forwarding, it would be a safer option. We use automounted NFS sec=krbp (AUTH_GSS security) home directories everywhere, so users *must* possess a credential in order to access their home directories. So even if we permitted publickey authentication, it would not solve the issue of needing a credential on the target host. > But all that said I would like to note that it is certainly > inappropriate to call people names before fully understanding the > scope of a security measure, just because it is a little > inconvenient. Speaking as a Linux administrator in a heterogeneous environment where our Linux hosts rely on Microsoft Active Directory, I assure you that I have been calling Microsoft names for quite some time. ;-) (Don’t even get me started about AD’s continued lack of support for aes256-cts-hmac-sha384-192 or aes128-cts-hmac-sha256-128, when it’s approaching 10 years since RFC8009 defined them, and Microsoft knows that the legacy SHA-1 AES encryption types are increasingly giving sites that are under the gun for FIPS compliance fits.) I suspect that Microsoft was the main proponent of adding §2.8 to RFC4120, and they cared little about clarifying behavior in the face of heterogeneous KDC/client Kerberos implementations, because they knew exactly how *they* intended to implement it for their KDC
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
>> I'm a LITTLE confused as to what you're describing here. As I
>> understand you, the TRUSTED_FOR_DELEGATION flag doesn't appear on
>> the wire and only in the account properties.
>
>Yes. Apologies; I should have been more precise: when Microsoft AD is
>acting as the KDC, whether AD sets the the OK-AS-DELEGATE flag in the
>TGS-REP is determined by whether the userAccountControl attribute in
>Active Directory of the host for which a service ticket is being
>requested has the TRUSTED_FOR_DELEGATION flag set. If
>TRUSTED_FOR_DELEGATION is set, OK-AS-DELEGATE is set in the TGS-REP;
>otherwise, OK-AS-DELEGATE is not set in the TGS-REP.
Okay, THAT makes more sense.
>Heimdal appears to implement the same option (enforce_ok_as_delegate),
>but although the upstream source claims the default is false (do not
>enforce), Macs seem to enforce it by default. So either Apple has
>changed that default in the code, or else they are overriding the
>default somewhere in the Kerberos configuration.
>In terms of Windows, Microsoft’s Kerberos implementation seems to
>enforce compliance with the OK-AS-DELEGATE flag. (PuTTY on Windows
>will not delegate a credential unless the target host has the
>TRUSTED_FOR_DELEGATION flag set in AD.) Perhaps there is a way to
>disable this behavior, but we have not yet found a way to do so.
Well, I figured out what is going on there ... see below.
>> the RFC provides what I would consider a cognizant explanation:
>>
>> https://datatracker.ietf.org/doc/html/rfc4120#section-2.8
>
>Microsoft provides a similar explanation, but it is still an
>unsatisfying one, because it does not speak to our issue: if denying
>the ability to delegate a credential (in order to protect it from
>exposure to a possibly-untrustworthy host) forces the user to instead
>acquire a credential directly from the possibly-untrustworthy host
>(thereby exposing the user’s actual password), then this is not a
>security improvement. And while I acknowledge that RFC4120 is 19 years
>old and a lot has changed since it was originally published, neither
>the RFC authors nor Microsoft seems to have considered/predicted this
>scenario.
Simo already explained the thinking there, but I think the thing you're
not considering is that not all services require delegated credentials.
Yes, in your environment (and ours) delegated credentials for host
principals is essential, but you don't normallty need that for other
types of credentials. I haven't run into Simo's experience of badly
coded applications delegating credentials when they shouldn't, but I could
believe it happens. It does make sense to be able to control this
centrally based on KDC configuration (that's one of the advantages
to _having_ a KDC). And, well .. one thing I guess I do not understand
is, exactly, what is your problem with turning on that setting on your
KDC? If this is just a cri de cœur about lousy AD admins, fair enough;
I can understand your pain there (most AD admins really don't know how
Kerberos works, sadly).
Speaking about the OK-AS-DELEGATE flag, the ONLY thing that does is set
the flag in the ticket which the client uses as a hint (it is free to
ignore that hint). I cannot speak for the AD TRUSTED_FOR_DELEGATION flag
as a whole; it's possible it does something else than set the OK-AS-DELEGATE
ticket flag.
However, we use MIT Kerberos for our KDC and we occasionally use the
MacOS X ssh client and it delegates credentials just FINE for us without
the use of OK-AS-DELEGATE, so I was curious as to what was going on there.
I did some digging into the MacOS X Heimdal code, and here's what I found.
- The OpenSSH code is mostly the same for our purposes in terms of the
Kerberos support; use of the -K flag turns on the right flags for
credential delegation (in addition to GSSAPI authentication support).
- In the gss_init_sec_context() code path I found this little snippet:
/*
* If the credential doesn't have ok-as-delegate, check if there
* is a realm setting and use that.
*/
if (!ctx->kcred->flags.b.ok_as_delegate && ctx->ccache) {
krb5_data data;
ret = krb5_cc_get_config(context, ctx->ccache, NULL,
"realm-config", );
if (ret == 0) {
/* XXX 1 is use ok-as-delegate */
if (data.length < 1 || unsigned char *)data.data)[0]) & 1) == 0)
req_flags &= ~(GSS_C_DELEG_FLAG|GSS_C_DELEG_POLICY_FLAG);
krb5_data_free();
}
}
So if there is NOT a delegation flag on the ticket, the realm-config
entry is checked. If the first byte has the least-significant bit
set, the all of the delegation flags are cleared. I suspect this is
what you're hitting.
So, what sets that realm configuration? As far as I can tell, that
ONLY happens inside of kinit. Specifically, this code block:
if (ok_as_delegate_flag || windows_flag || use_referrals_flag) {
unsigned char d = 0;
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
The correct action is for you to ask the Domain Administrators to mark the target hosts as ok for delegation, it is unclear why MIT Kerberos should make it easy to override Realm policies. Delegating a whole TGT is generally a bad idea, and often clients are misconfigured to broadly forward it everywhere. That is why Microsoft took back control in the hands of administrators. It is not a bad thing, and if your setup has been vetted such that TGT delegation is an acceptable risk, then it should be easy to get it fixed the proper way, by getting your domain admins to set the right flag on the permitted target hosts. Note that if ticket delegation is needed solely to allow jumping from hosts to host, you should be able to achieve the same result via agent forwarding, it would be a safer option. But all that said I would like to note that it is certainly inappropriate to call people names before fully understanding the scope of a security measure, just because it is a little inconvenient. If you wanted a knob like the one you ask for it should probably be called: dishonor_trusted_for_delegation_mr_auditor_please_be_lenient As for users that type their passwords onto random hosts, that is a user education problem in general, but that can be addressed simply by forcing the use of 2FA authentication on the user's part, preferably via a hardware token and pkinit, but an OTP solution would work as well. Cheers, Simo. On Tue, 2024-04-16 at 03:03 -0400, James Ralston wrote: > On Mon, Apr 15, 2024 at 7:56 PM Ken Hornstein wrote: > > > I'm a LITTLE confused as to what you're describing here. As I > > understand you, the TRUSTED_FOR_DELEGATION flag doesn't appear on > > the wire and only in the account properties. > > Yes. Apologies; I should have been more precise: when Microsoft AD is > acting as the KDC, whether AD sets the the OK-AS-DELEGATE flag in the > TGS-REP is determined by whether the userAccountControl attribute in > Active Directory of the host for which a service ticket is being > requested has the TRUSTED_FOR_DELEGATION flag set. If > TRUSTED_FOR_DELEGATION is set, OK-AS-DELEGATE is set in the TGS-REP; > otherwise, OK-AS-DELEGATE is not set in the TGS-REP. > > > the [MIT Kerberos] library already provides an option to ignore that > > [the OK-AS-DELEGATE] flag and it seems that by default it DOES > > ignore that flag) > > I think the enforce_ok_as_delegate is the option you’re referring to? > > The man page claims it is disabled by default (unless an application > specifically requests it). That matches our experiences. > > Heimdal appears to implement the same option (enforce_ok_as_delegate), > but although the upstream source claims the default is false (do not > enforce), Macs seem to enforce it by default. So either Apple has > changed that default in the code, or else they are overriding the > default somewhere in the Kerberos configuration. > > In terms of Windows, Microsoft’s Kerberos implementation seems to > enforce compliance with the OK-AS-DELEGATE flag. (PuTTY on Windows > will not delegate a credential unless the target host has the > TRUSTED_FOR_DELEGATION flag set in AD.) Perhaps there is a way to > disable this behavior, but we have not yet found a way to do so. > > > the RFC provides what I would consider a cognizant explanation: > > > > https://datatracker.ietf.org/doc/html/rfc4120#section-2.8 > > Microsoft provides a similar explanation, but it is still an > unsatisfying one, because it does not speak to our issue: if denying > the ability to delegate a credential (in order to protect it from > exposure to a possibly-untrustworthy host) forces the user to instead > acquire a credential directly from the possibly-untrustworthy host > (thereby exposing the user’s actual password), then this is not a > security improvement. And while I acknowledge that RFC4120 is 19 years > old and a lot has changed since it was originally published, neither > the RFC authors nor Microsoft seems to have considered/predicted this > scenario. > > On Mon, Apr 15, 2024 at 8:40 PM Stephen Frost wrote: > > > I'd *strongly* encourage you to ignore what OSX comes with in terms > > of Kerberos "support" and push to move everything away from what OSX > > ships with and to instead use MIT Kerberos. In my experience, this > > is far from the only issue you're going to run into with the hacked > > up Kerberos from OSX and they don't seem to care to properly > > maintain it. > > It has been our experience that ripping out a vendor-supplied > library/package and replacing it with an in-house version almost > always has a higher long-term cost than simply living with whatever > warts the vendor-supplied library/package has. So we are reluctant to > take this approach unless there is truly no other choice. > > But this segues back to my original question: how are other sites that > use Microsoft AD as their KDCs handling this? > > Are other sites ensuring that the TRUSTED_FOR_DELEGATION flag
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
On Mon, Apr 15, 2024 at 7:56 PM Ken Hornstein wrote: > I'm a LITTLE confused as to what you're describing here. As I > understand you, the TRUSTED_FOR_DELEGATION flag doesn't appear on > the wire and only in the account properties. Yes. Apologies; I should have been more precise: when Microsoft AD is acting as the KDC, whether AD sets the the OK-AS-DELEGATE flag in the TGS-REP is determined by whether the userAccountControl attribute in Active Directory of the host for which a service ticket is being requested has the TRUSTED_FOR_DELEGATION flag set. If TRUSTED_FOR_DELEGATION is set, OK-AS-DELEGATE is set in the TGS-REP; otherwise, OK-AS-DELEGATE is not set in the TGS-REP. > the [MIT Kerberos] library already provides an option to ignore that > [the OK-AS-DELEGATE] flag and it seems that by default it DOES > ignore that flag) I think the enforce_ok_as_delegate is the option you’re referring to? The man page claims it is disabled by default (unless an application specifically requests it). That matches our experiences. Heimdal appears to implement the same option (enforce_ok_as_delegate), but although the upstream source claims the default is false (do not enforce), Macs seem to enforce it by default. So either Apple has changed that default in the code, or else they are overriding the default somewhere in the Kerberos configuration. In terms of Windows, Microsoft’s Kerberos implementation seems to enforce compliance with the OK-AS-DELEGATE flag. (PuTTY on Windows will not delegate a credential unless the target host has the TRUSTED_FOR_DELEGATION flag set in AD.) Perhaps there is a way to disable this behavior, but we have not yet found a way to do so. > the RFC provides what I would consider a cognizant explanation: > > https://datatracker.ietf.org/doc/html/rfc4120#section-2.8 Microsoft provides a similar explanation, but it is still an unsatisfying one, because it does not speak to our issue: if denying the ability to delegate a credential (in order to protect it from exposure to a possibly-untrustworthy host) forces the user to instead acquire a credential directly from the possibly-untrustworthy host (thereby exposing the user’s actual password), then this is not a security improvement. And while I acknowledge that RFC4120 is 19 years old and a lot has changed since it was originally published, neither the RFC authors nor Microsoft seems to have considered/predicted this scenario. On Mon, Apr 15, 2024 at 8:40 PM Stephen Frost wrote: > I'd *strongly* encourage you to ignore what OSX comes with in terms > of Kerberos "support" and push to move everything away from what OSX > ships with and to instead use MIT Kerberos. In my experience, this > is far from the only issue you're going to run into with the hacked > up Kerberos from OSX and they don't seem to care to properly > maintain it. It has been our experience that ripping out a vendor-supplied library/package and replacing it with an in-house version almost always has a higher long-term cost than simply living with whatever warts the vendor-supplied library/package has. So we are reluctant to take this approach unless there is truly no other choice. But this segues back to my original question: how are other sites that use Microsoft AD as their KDCs handling this? Are other sites ensuring that the TRUSTED_FOR_DELEGATION flag is set for Linux hosts, so that all various Kerberos libraries (including ones that enforce OK-AS-DELEGATE by default) will correctly delegate credentials to Linux hosts? Are other sites configuring their Kerberos libraries (on a per-OS basis) to ignore the OK-AS-DELEGATE flag? Have few other sites that use Microsoft AD as their KDC even run into this, because they don’t have services (e.g. home directories mounted via NFSv4+RPCGSS) that require a Kerberos credential, and therefore don’t need to forward Kerberos credentials to the remote host when making an ssh connection to it? My read of the MIT Kerberos kdc.conf man page is that ok-as-delegate is not one of the flags in default_principal_flags that defaults to enabled. So heterogeneous sites that use MIT Kerberos as their KDCs (not AD) should also be seeing this issue. At least so far, we *think* the best course of action is to always set the TRUSTED_FOR_DELEGATION flag for Linux hosts in AD, so that credential delegation won’t depend on whether the Kerberos library that any specific host uses pays attention to the OK-AS-DELEGATE flag. And this does seem to be the intention of the TRUSTED_FOR_DELEGATION / OK-AS-DELEGATE flags: it’s advice to Kerberos clients that it’s OK to delegate credentials to the target host, which is exactly what we want to happen. The only thing we’re not completely sure about is whether setting the TRUSTED_FOR_DELEGATION flag in AD will have other security ramifications that aren’t clear from Microsoft’s documentation. Which is why I was hoping that others on this list have already been down this particular road and could
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
Greetings, * Ken Hornstein via Kerberos (kerberos@mit.edu) wrote: > >Has anyone else struggled with ssh clients being unable to delegate > >As far as we can tell, for reasons we still have been unable to > >fathom, Microsoft decided that simply permitting credential delegation > >based on whether the TGT has the forwardable flag set was > >insufficient. Instead, Microsoft implemented a new flag in the MS-SFU > >Kerberos Protocol Extensions, TRUSTED_FOR_DELEGATION. The flag is a > >property of the service principal of the *target* host: if the target > >host does not have the TRUSTED_FOR_DELEGATION flag set in the > >userAccountControl attribute of the host’s machine account in Active > >Directory, then if the Kerberos library that the ssh client uses > >honors the MS-SFU Kerberos Protocol Extensions and honors the > >TRUSTED_FOR_DELEGATION flag, it will refuse to delegate the user’s > >credentials to the target host, *even* if all other settings would > >permit credential delegation. > > I'm a LITTLE confused as to what you're describing here. As I > understand you, the TRUSTED_FOR_DELEGATION flag doesn't appear on the > wire and only in the account properties. What, exactly, is there for a > client implementation to honor or not honor? If you're talking about > the OK-AS-DELEGATE flag in the Kerberos ticket, MIT Kerberos does > implement that flag (but ... the library already provides an option > to ignore that flag and it seems that by default it DOES ignore that > flag). It seems like some versions of Heimdal also will ignore the > OK-AS-DELEGATE flag by default and you can configure Heimdal to respect > that flag but I am unclear what the OS X Heimdal does. Calling that a > Microsoft extension is incorrect, though, as that appears in RFC 4120. > As for the thinking behind this flaga, well, the RFC provides what I > would consider a cognizant explanation: > > https://datatracker.ietf.org/doc/html/rfc4120#section-2.8 > > If you're talking about something else, I would be curious as to what > you mean. I didn't think ssh could utilize any of the S4U stuff > but it's always possible that could have changed. Before delving too deeply here ... frankly, I'd *strongly* encourage you to ignore what OSX comes with in terms of Kerberos "support" and push to move everything away from what OSX ships with and to instead use MIT Kerberos. In my experience, this is far from the only issue you're going to run into with the hacked up Kerberos from OSX and they don't seem to care to properly maintain it. Thanks, Stephen signature.asc Description: PGP signature Kerberos mailing list Kerberos@mit.edu https://mailman.mit.edu/mailman/listinfo/kerberos
Re: honoring the TRUSTED_FOR_DELEGATION KDC MS-SFU Kerberos Protocol Extensions flag?
>Has anyone else struggled with ssh clients being unable to delegate >As far as we can tell, for reasons we still have been unable to >fathom, Microsoft decided that simply permitting credential delegation >based on whether the TGT has the forwardable flag set was >insufficient. Instead, Microsoft implemented a new flag in the MS-SFU >Kerberos Protocol Extensions, TRUSTED_FOR_DELEGATION. The flag is a >property of the service principal of the *target* host: if the target >host does not have the TRUSTED_FOR_DELEGATION flag set in the >userAccountControl attribute of the host’s machine account in Active >Directory, then if the Kerberos library that the ssh client uses >honors the MS-SFU Kerberos Protocol Extensions and honors the >TRUSTED_FOR_DELEGATION flag, it will refuse to delegate the user’s >credentials to the target host, *even* if all other settings would >permit credential delegation. I'm a LITTLE confused as to what you're describing here. As I understand you, the TRUSTED_FOR_DELEGATION flag doesn't appear on the wire and only in the account properties. What, exactly, is there for a client implementation to honor or not honor? If you're talking about the OK-AS-DELEGATE flag in the Kerberos ticket, MIT Kerberos does implement that flag (but ... the library already provides an option to ignore that flag and it seems that by default it DOES ignore that flag). It seems like some versions of Heimdal also will ignore the OK-AS-DELEGATE flag by default and you can configure Heimdal to respect that flag but I am unclear what the OS X Heimdal does. Calling that a Microsoft extension is incorrect, though, as that appears in RFC 4120. As for the thinking behind this flaga, well, the RFC provides what I would consider a cognizant explanation: https://datatracker.ietf.org/doc/html/rfc4120#section-2.8 If you're talking about something else, I would be curious as to what you mean. I didn't think ssh could utilize any of the S4U stuff but it's always possible that could have changed. --Ken Kerberos mailing list Kerberos@mit.edu https://mailman.mit.edu/mailman/listinfo/kerberos
