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TLS certificate check bypass with mbedTLS/PolarSSL

Project curl Security Advisory, May 18th 2016 - Permalink


libcurl did not check the server certificate of TLS connections done to a host specified as an IP address, or when explicitly asked to use SSLv3.

This flaw only exists when libcurl is built to use mbedTLS or PolarSSL as the TLS backend.

The documentation for mbedTLS and PolarSSL (wrongly) says that the API function ssl_set_hostname() is used only for setting the name for the TLS extension SNI. The set string is however even more importantly used by the libraries to verify the server certificate, and if no "hostname" is set it will just skip the check and successfully continue with the handshake.

libcurl would wrongly avoid using the function when the specified host name was given as an IP address or when SSLv3 is used, as SNI isn't supposed to be used then. This then leads to that all uses of TLS oriented protocols (HTTPS, FTPS, IMAPS, POPS3, SMTPS, etc) will allow connections to servers with unverified server certificates as long as they're specified as IP addresses or using SSLv3.

By tricking a libcurl-using client to use a URL with a host specified as IP address only, an application could be made to connect to an impostor server or Man In The Middle host without noticing.

Note: PolarSSL is the old name and releases of the library that nowadays is known and released under the name mbedTLS.


This flaw also affects the curl command line tool.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2016-3739 to this issue.

CWE-297: Improper Validation of Certificate with Host Mismatch

Severity: High


This flaw is relevant for all versions of curl and libcurl that support PolarSSL or mbedTLS.

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


In version 7.49.0, libcurl properly sets the "hostname" even when it is just an IP address and even when using SSLv3 that doesn't have SNI.


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

A - Upgrade curl and libcurl to version 7.49.0

B - Apply the patch to your version and rebuild

C - Build your libcurl with another TLS backend to work around this flaw.


It was first reported to the curl project on April 21st 2016. We contacted distros@openwall on May 8th.

libcurl 7.49.0 was released on May 18 2016, coordinated with the publication of this advisory.


Thanks a lot!