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curl / Docs / Security Problems / inappropriate GSSAPI delegation
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inappropriate GSSAPI delegation

Project curl Security Advisory, June 23rd 2011 permalink

VULNERABILITY

When doing GSSAPI authentication, libcurl unconditionally performs credential delegation. This hands the server a copy of the client's security credentials, allowing the server to impersonate the client to any other using the same GSSAPI mechanism. This is obviously a very sensitive operation, which should only be done when the user explicitly so directs.

The GSS/Negotiate feature is only used by libcurl for HTTP authentication if told to, and only if libcurl was built with a library that provides the GSSAPI. Many builds of libcurl don't have GSS enabled.

There is no known exploit for this problem.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2011-2192 to this issue.

CWE-281: Improper Preservation of Permissions

AFFECTED VERSIONS

Note that libcurl is used by many applications, and not always advertised as such.

SOLUTION

libcurl 7.21.7 avoids setting the option that selects delegation.

A patch is available.

RECOMMENDATIONS

We suggest you take one of the following actions immediately, in order of preference:

A - Upgrade to curl and libcurl 7.21.7

B - Apply patch and rebuild libcurl

C - Disable credential forwarding. Assuming the GSSAPI mechanism is Kerberos, users can acquire an initial credential (TGT) which is not forwardable. On Unix platforms, both the MIT and Heimdal versions of kinit use the -F flag for this (note the capital "F"; -f does the opposite!). You can also set this as a global default in /etc/krb5.conf (or wherever that file lives in a particular installation):

 [libdefaults]
 forwardable = no

D - Stop using GSS/Negotiate

TIME LINE

Richard Silverman realized the problem exists and reported it to us on June 6th 2011.

We discussed solutions and a first patch was written on June 8th.

curl 7.21.7 was released on June 23rd 2011, coordinated with the publication of this this flaw.

CREDITS

Reported to us by Richard Silverman. Thanks a lot!

Daniel Stenberg wrote the primary patch and this advisory. Additional help and valuable feedback provided by Dan Fandrich and Julien Chaffraix.