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cert name check ignore with GnuTLS

Project curl Security Advisory, December 17th 2013 Permalink


(This issue is almost identical to the one named CVE-2013-4545, but this problem affects a different SSL backend.)

libcurl is vulnerable to a case of missing out the checking of the certificate CN or SAN name field when the digital signature verification is turned off.

libcurl offers two separate and independent options for verifying a server's TLS certificate. CURLOPT_SSL_VERIFYPEER and CURLOPT_SSL_VERIFYHOST. The first one tells libcurl to verify the trust chain using a CA cert bundle, while the second tells libcurl to make sure that the name fields in the server certificate meets the criteria. Both options are enabled by default.

This flaw had the effect that when an application disabled CURLOPT_SSL_VERIFYPEER, libcurl mistakenly also skipped the CURLOPT_SSL_VERIFYHOST check. Applications can disable CURLOPT_SSL_VERIFYPEER and still achieve security by doing the check on its own using other means.

The curl command line tool is not affected by this problem as it either enables both options or disables both at the same time.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2013-6422 to this issue.

CWE-297: Improper Validation of Certificate with Host Mismatch

Severity: Medium


This flaw only exists in the TLS backend that uses GnuTLS. If libcurl is built with another TLS backend, it is not affected.

libcurl is used by many applications, but not always advertised as such!


libcurl 7.34.0 makes sure that both options independently cause the operation to fail unless the criteria is fulfilled.


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

A - Upgrade to curl and libcurl 7.34.0

B - Apply the patch and rebuild libcurl

C - Make sure CURLOPT_SSL_VERIFYPEER is not disabled

D - Build libcurl with another TLS backend than GnuTLS


It was reported to the curl project on November 29th 2013. We contacted distros@openwall on December 3rd.

libcurl 7.34.0 was released on December 17th 2013, coordinated with the publication of this advisory.


Thanks a lot!