[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Benjamin Peterson]

Changes by Benjamin Peterson :


--
resolution:  -> duplicate
status: open -> closed
superseder:  -> Copy cgi.escape() to html

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Éric Araujo]

Éric Araujo  added the comment:

Markup nit fixed in r83999 (py3k) and r84001 (stupid typo), r84002 (3.1), 
r84003 (2.7).

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Fred L. Drake, Jr.]

Fred L. Drake, Jr.  added the comment:

Such constructs are notoriously tedious to grep for; patches are welcome.

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Georg Brandl]

Georg Brandl  added the comment:

No, that's just a relic from the olden LaTeX days, and I've not paid attention 
enough to fix it :)

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Éric Araujo]

Éric Araujo  added the comment:

Are 2.6 docs built by an older Sphinx version? I wonder why the text uses “the 
:func:`quoteattr` function in the :mod:`xml.sax.saxutils` module” and not 
“:func:`~xml.sax.saxutils.quoteattr” to get a direct link (or even just 
“consider using :func:`xml.sax.saxutils.quoteattr`.”).

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nosy: +merwok

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Georg Brandl]

Georg Brandl  added the comment:

Applied doc patch to 2.6 in r83539.

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priority: release blocker -> critical
versions:  -Python 2.5, Python 2.6

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Barry A. Warsaw]

Barry A. Warsaw  added the comment:

Unless someone can upload a specific patch to review in the next couple of 
hours, I'm going to reduce the priority for 2.6.6rc1.

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Georg Brandl]

Changes by Georg Brandl :


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priority: normal -> release blocker

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Craig Younkins]

Craig Younkins  added the comment:

> cgi.escape is for HTML attribute escaping only.

It is not safe for HTML attribute escaping because it does not encode single 
quotes.

> "More suitable" for HTML would be the correct interpretation rather make the 
> "input safe".

"More suitable, but not quite secure"

Regardless of the intended use of this method, many many people are using it 
for insecure HTML entity escaping.

> you should explain or point out to resources where 
> 'single quotes' representation in a non-entity format 
> in a HTML page has lead to XSS.

print "" % cgi.escape("' onload='alert(1);' bad='")

> The very next paragraph seems to address the security considerations
> while using the cgi module itself, rather than limiting it to
> cgi.escape. It says that:
> "To be on the safe side, if you must pass a string gotten from a form
> to a shell command, you should make sure the string contains only
> alphanumeric characters, dashes, underscores, and periods."

The security concerns related to output on the web are very different from the 
concerns related sending user input to a shell command. The needed escaping is 
completely different. Also, the security advice above is woefully inadequate. 

> Any doc change suggestions you propose?

Convert the characters '&', '<' and '>' in string s to their HTML entity 
encoded values. If the optional flag quote is true, the double-quotation mark 
character ('"') is also encoded. Note that the output of this method is not 
safe to put in an HTML attribute because it does not escape single quotes. If 
the value to be quoted might include single- or double-quote characters, or 
both, consider using the quoteattr() function in the xml.sax.saxutils module 
instead.

> If cgi.escape needs to escape single quotes, what should it be as:
> lsquo/rsquo (for XHTML) and ' or ' for Others?

Sorry, I should have included that in the OP. It should escape to ' 
It is also advised to escape the forward slash character ('/') to /

See OWASP.org for an explanation of the complexities of the escaping:
http://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet#RULE_.231_-_HTML_Escape_Before_Inserting_Untrusted_Data_into_HTML_Element_Content

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Senthil Kumaran]

Senthil Kumaran  added the comment:

On Wed, Jun 23, 2010 at 03:46:35PM +, Craig Younkins wrote:
> cgi.escape never escapes single quote characters, which can easily
> lead to a Cross-Site Scripting (XSS) vulnerability. This seems to be
> known by many, but a quick search reveals many are using cgi.escape
> for HTML attribute escaping.

cgi.escape is for HTML attribute escaping only.  I guess, you should
explain or point out to resources where 'single quotes' representation
in a non-entity format in a HTML page has lead to XSS.

> The intended use of this method is unclear to me. 

Escape HTML characters (most commonly), >,<, & and ". And mostly when
constructing responses where these characters are literally required.

> While the documentation says "if the value to be quoted might
> include single- or double-quote characters... [use the]
> xml.sax.saxutils module instead," it also implies that this method
> will make input safe for HTML. Because this method escapes 4 of the

"More suitable" for HTML would be the correct interpretation rather
make the "input safe". You might check the reference documentation
leading to xml.sax.saxutils.

> I suggest rewording the documentation for the method making it more
> clear what it should and should not be used for. 

The very next paragraph seems to address the security considerations
while using the cgi module itself, rather than limiting it to
cgi.escape. It says that:

"To be on the safe side, if you must pass a string gotten from a form
to a shell command, you should make sure the string contains only
alphanumeric characters, dashes, underscores, and periods."

With respect your bug report:

1. Any doc change suggestions you propose?  (After pointing out the
resources requested in first para)

2. If cgi.escape needs to escape single quotes, what should it be as:
lsquo/rsquo (for XHTML) and ' or ' for Others?

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Craig Younkins]

Craig Younkins  added the comment:

Proof of concept:
print "" % cgi.escape("' onload='alert(1);' bad='")

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Craig Younkins]

Changes by Craig Younkins :


--
type:  -> security

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[issue9061] cgi.escape Can Lead To XSS Vulnerabilities

[Craig Younkins]

New submission from Craig Younkins :

The method in question: http://docs.python.org/library/cgi.html#cgi.escape
http://svn.python.org/view/python/tags/r265/Lib/cgi.py?view=markup   # at the 
bottom
http://code.python.org/hg/trunk/file/3be6ff1eebac/Lib/cgi.py#l1031

"Convert the characters '&', '<' and '>' in string s to HTML-safe sequences. 
Use this if you need to display text that might contain such characters in 
HTML. If the optional flag quote is true, the quotation mark character ('"') is 
also translated; this helps for inclusion in an HTML attribute value, as in . If the value to be quoted might include single- or double-quote 
characters, or both, consider using the quoteattr() function in the 
xml.sax.saxutils module instead."

cgi.escape never escapes single quote characters, which can easily lead to a 
Cross-Site Scripting (XSS) vulnerability. This seems to be known by many, but a 
quick search reveals many are using cgi.escape for HTML attribute escaping.

The intended use of this method is unclear to me. Up to and including Mako 
0.3.3, this method was the HTML escaping method. Used in this manner, 
single-quoted attributes with user-supplied data are easily susceptible to 
cross-site scripting vulnerabilities.

While the documentation says "if the value to be quoted might include single- 
or double-quote characters... [use the] xml.sax.saxutils module instead," it 
also implies that this method will make input safe for HTML. Because this 
method escapes 4 of the 5 key XML characters, it is reasonable to expect some 
will use it for HTML escaping.

I suggest rewording the documentation for the method making it more clear what 
it should and should not be used for. I would like to see the method changed to 
properly escape single-quotes, but if it is not changed, the documentation 
should explicitly say this method does not make input safe for inclusion in 
HTML.

This is definitely affecting the security of some Python web applications. I 
already mentioned Mako, but I've found this type of bug in other frameworks and 
engines because the creators either called cgi.escape directly or modeled their 
own after it.

Craig Younkins

--
assignee: d...@python
components: Documentation, Library (Lib)
messages: 108457
nosy: Craig.Younkins, d...@python
priority: normal
severity: normal
status: open
title: cgi.escape Can Lead To XSS Vulnerabilities
versions: Python 2.5, Python 2.6, Python 2.7, Python 3.1, Python 3.2, Python 3.3

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