Hello everyone, I sent this to BUGTRAQ yesterday. Here is the copy for the firewall list. Abstract: In their so called Knowledge Database, AltaVista Software states that Firewall97 for Digital Unix is not affected by the well known buffer overflow bug present in BIND versions prior to 4.9.7 since all DNS queries are proxied through the Firewall's DNS proxy (dnsd), that can either relay queries to name servers running on other hosts or to a name server running on the Firewall itself. See: http://support.altavista-software.com/kb/solutions/firewall/general/259-042398.asp If the name server is running on the firewall itself (which is an approved configuration described in the manual), then there is a very simple way to circumvent the dnsd and attack the named on the Firewall directly. So, everyone who relied on this assurance (given shortly after those BIND problems were discovered) and did therefore not replace the named binary on the firewall with a self compiled BIND-4.9.7 named had (and may still have) a major security hole on his/her firewall. This problem is worsened by the fact that installation of the firewall software will alter some system files and therefore the Digital Unix patch utility (dupatch) will, at least in some cases, refuse to install several operating system patches including those for BIND. The Detailed Story: To divide the DNS information available about the internal network into information that is to be given to the outside world and information that is meant for internal use only, the AltaVista Firewall97 uses a DNS proxy (dnsd) running on port 53 on the firewall that redirects queries appropriately: Queries from external hosts are redirected to a name server that holds information to be given to the outside world, while queries from internal hosts are redirected to a name server that holds information meant for internal use only. Each of those two name servers may be running on another host or on the firewall itself. If one chooses to run one of the two or both name servers on the firewall, then the named(s) will be configured to listen on port 8053 and/or port 8153 (Firewall97 + Service Pack 3). The secure_zone statement secure_zone IN TXT 127.0.0.1:H in the zone files is then used to ensure that only queries from localhost (coming from the dnsd) are answered. The problem is that the named(s) running on port 8053/8153 will take input from any host, logging these (unauthorized) queries like named[22343]: Unauthorized request nowhere.example.org from [129.69.xxx.yyy].1945 So, if we want to attack the named on the firewall directly, we simply have to aim at port 8053/8153 instead of aiming at port 53. As I had no exploit code for Digital Unix I took a tool to exploit the named buffer overflow on ix86 machines from www.rootshell.com, changed the target port to 8053/8153 and launched it against the firewall (running BIND-4.9.3). This caused a segmentation fault with core dump of the named. It should be possible to get a root shell out of that named with the appropriate exploit code for Digital Unix 4.0a and higher, where the stack is executable. Let's turn to the patching problems now: I installed Firewall97 on Digital Unix 4.0b + patch kit DUV40BAS00005-19971009 shortly after those BIND problems had been discovered last year. The Aggregate Selective patch kit duv40bas00008-19980821 released in August 1998 contained fixes for BIND. When I tried to apply this patch, the dupatch utility found that several system files had changed due to the installation of the firewall software and refused to install several patches to ensure that no altered system file got overwritten by a new one from the patch kit. Unfortunately, among the patches that were not installed was the patch for BIND. I don't know if there is a single patch kit (not an Aggregate Selective patch kit) for Digital Unix addressing only those BIND problems and if that one works. The fix for the problems described above is quite simple: Compile a BIND-4.9.7 named for Digital Unix and replace the named on the firewall with that one. However, what cannot be fixed is the fact that a lot of people were convinced that their firewall is secure, while in reality everyone, who had this knowledge about DNS on Firewall97 and the ability to write exploit code for Digital Unix, was probably able to immediately root-compromise their firewall. -- Jochen Bauer Institute for Theoretical Physics University of Stuttgart Germany PGP public key available from: http://www.theo2.physik.uni-stuttgart.de/jtb.html - [To unsubscribe, send mail to [EMAIL PROTECTED] with "unsubscribe firewalls" in the body of the message.]
