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AUSCERT External Security Bulletin Redistribution
ESB-2004.0175 -- Squid Proxy Cache Security Update Advisory SQUID-2004:1
Squid-2.5.STABLE5 fixes and features for URL encoding tricks
02 March 2004
AusCERT Security Bulletin Summary
Publisher: The Squid HTTP Proxy developer team
Impact: Reduced Security
Access Required: Existing Account
CVE Names: CAN-2003-1025
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Squid Proxy Cache Security Update Advisory SQUID-2004:1
Advisory ID: SQUID-2004:1
Date: February 29, 2004
Summary: Squid-2.5.STABLE5 fixes and features for
URL encoding tricks.
Affected versions: Squid-2.x up to and including 2.5.STABLE4
This memo discusses two important changes to Squid that
deal with URL encoding issues. These changes are available
in Squid version 2.5.STABLE5.
The first is a workaround for a recently-discovered Microsoft
Internet Explorer bug. The MSIE bug causes certain specially
crafted URLs to be incorrectly displayed. In particular, the
user sees one hostname, while the request is sent to a different
origin server. This bug is triggered by creating a URL that has
a hostname in the userinfo credentials field followed by an
encoded, non-printable control character. (For additional
information, see http://www.kb.cert.org/vuls/id/652278)
To help address this problem, Squid now includes a new access
control type that can match patterns in the userinfo field.
The second fixes a bug in Squid that allows users to bypass
certain access controls. Squid versions 2.5.STABLE4 and earlier
contain a bug in the "%xx" URL decoding function. It may insert
a NUL character into decoded URLs, which may allow users to
bypass url_regex ACLs.
You can also find information on the changes by visiting our
patch archive for version Squid-2.5.STABLE5:
The MSIE bug does not pose any security problems to Squid itself.
However, it does allow your users to be fooled into visiting a
malicious site. To block such URLs with Squid, you can use the
new 'urllogin' ACL type:
acl UserInfoControlChar urllogin [[:cntrl:]]
http_access deny UserInfoControlChar
<additional http_access rules follow>
NOTE: regular expression libraries may vary from system to
system. Please double-check that the "[[:cntrl:]]" works on your
particular operating system.
The Squid decoding bug may allow clever users to bypass your
access controls that use 'url_regex' ACL types. If "%00" appears
in the URL, previous Squid versions insert a NUL character when
decoding. For example, consider this access control
acl BadSite url_regex www\.example\.com
http_access deny BadSite
and this URL requested by a user:
The vulnerable Squid will insert a NUL character after "foo" and
make a comparison between "http://foo" and "www\.example\.com".
The comparison does not result in a match, and the user's request
is not denied.
This bug has been fixed by leaving any occurrences of "%00" in
place while decoding.
The Squid-2.5.STABLE5 release contains fixes for these
problems. You can download the Squid-2.4.STABLE5 release from
or the mirrors (may take a while before all mirrors are updated).
For a list of mirror sites see
Individual patches to the mentioned issues can be found from our
patch archive for version Squid-2.5.STABLE4
The patches should also apply with only a minimal effort to
earlier Squid 2.5 versions if required.
If you are using a prepackaged version of Squid then please
refer to the package vendor for availability information on
Determining if your version is vulnerable:
To determine which version of Squid you are using, run the command
You are likely to be vulnerable to these issues if you are
running version 2.5.STABLE4 or earlier.
If you are using a binary or otherwise pre-packaged version
please verify with your vendor on which versions are affected as
some vendors ship earlier versions with the needed patches
applied. Note that unless you have upgraded to a version
released after 2003-01-14 you are most likely vulnerable to
There is no easy means to determine if your version is affected
other than by the Squid version number.
Other versions of Squid:
Versions prior to the 2.5 series are deprecated, please update
to Squid-2.5.STABLE5 if you are using a version older than 2.5.
These changes have also been made to the Squid-3 source tree.
To address the MSIE URL display bug, you may want to upgrade your
Explorer installations if and when a patch is available from
You may be able to work around the MSIE bug by developing a
Squid redirector. When the redirector program detects a
suspicious URL (e.g., with control characters in the userinfo
field), it can redirect the user to a local page that describes
The best way to avoid Squid's "%00" bug is to not use any
url_regex ACL types. You may want to use dst_domain and/or
urlpath_regex types instead.
Contact details for the Squid project:
For installation / upgrade support: Your first point of contact
should be your binary package vendor.
If your install is built from the original squid sources, then
the email@example.com mailing list is your primary
support point. (see <http://www.squid-cache.org/mailing-lists.html>
for subscription details).
For bug reporting, particularly security related bugs the
firstname.lastname@example.org mailing list is the appropriate forum.
It's a closed list (though anyone can post) and security related
bug reports are treated in confidence until the impact has been
established. For non security related bugs, the squid bugzilla
database should be used <http://www.squid-cache.org/bugs/>.
Mitch Adair reported %00 bug.
Duane Wessels, for patching the %00 bug and adding the urllogin
2004-01-14 21:10 GMT Initial release
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