Security Advisory August 12 2009
libcurl embedded zero in cert name
Project curl Security Advisory, August 12th 2009
SSL and TLS Server certificates contain one or more fields with server name
or otherwise matching patterns. These strings are stored as content and
length within the certificate, and thus there is no particular terminating
curl's OpenSSL interfacing code did faulty assumptions about those names and
patterns being zero terminated, allowing itself to be fooled in case a
certificate would get a zero byte embedded into one of the name fields. To
illustrate, a name that would show this vulnerability could look like:
This cert is thus made for "haxx.se" but curl would erroneously verify it
with no complaints for "example.com".
According to a recently published presentation, this kind of zero embedding
has been proven to be possible with at least one CA.
The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2009-2417 to this issue.
2. AFFECTED VERSIONS
Affected versions: curl and libcurl 7.4 to and including 7.19.5
Not affected versions: curl and libcurl 7.19.6 and later
This vulnerability is only present in OpenSSL-specific parts of the code.
If you build curl or libcurl to use GnuTLS or NSS instead, you must instead
make sure you use a secure version of those libraries. They have both
recently also been found vulnerable to this same flaw.
Also note that (lib)curl is used by many applications, and not always
advertised as such.
We have not researched curl versions earlier than 7.4 but we estimate that
usage of such old versions is very rare.
3. THE SOLUTION
libcurl 7.19.6 makes sure that the length from the cert is used for
verification and not the zero terminating byte.
We suggest you take one of the following actions immediately, in order of
A - Upgrade to curl and libcurl 7.19.6
B - Apply the suitable patch and rebuild
C - Rebuild curl with a safe version of GnuTLS or NSS
Note that both GnuTLS and NSS also suffered from this same vulnerability
so a "safe version" thus implies a version with this flaw already fixed!
5. TIME LINE
We were notified by Scott Cantor on July 30th, 2009.
We discussed solutions and a first patch was written and tested on July
Vendor-sec was contacted on August 3, 2009.
We agreed on and coordinated the synchronous disclosure of this problem
together with the curl 7.19.6 release.
curl 7.19.6 was released on August 12th 2009, just before this flaw was
Reported to us by Scott Cantor. Thanks a lot!
Daniel Stenberg wrote the primary patch and this advisory
Peter Sylvester for test case work and patch feedback
Michal Marek and Kamil Dudka provided the backported patches