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URL sanitization vulnerability

Project curl Security Advisory, January 24th 2012 Permalink


curl is vulnerable to a data injection attack for certain protocols through control characters embedded or percent-encoded in URLs.

When parsing URLs, libcurl's parser is liberal and only parses as little as possible and lets as much as possible through as long as it can figure out what to do.

In the specific process when libcurl extracts the file path part from a given URL, it did not always verify the data or escape control characters properly before it passed the file path on to the protocol-specific code that then would use it for its protocol business.

This passing through of control characters could be exploited by someone who would be able to pass in a handcrafted URL to libcurl. Lots of libcurl using applications let users enter URLs in one form or another and not all of these check the input carefully to prevent malicious ones.

A malicious user might pass in %0d%0a to get treated as CR LF by libcurl, and by using this fact a user can trick for example a POP3 client to delete a message instead of getting it or trick an SMTP server to send an unintended message.

This vulnerability can be used to fool libcurl with the following protocols: IMAP, POP3 and SMTP.

Both curl the command line tool and applications using the libcurl library are vulnerable.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2012-0036 to this issue.

CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection')

Severity: High


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


libcurl 7.24.0 scans for a range of "bad codes" in the path part of URLs so that they are rejected before any protocol code even can consider using them.


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

A - Upgrade to curl and libcurl 7.24.0

B - Apply the patch and rebuild libcurl

C - Rebuild curl with support for vulnerable protocols IMAP, POP3 and SMTP disabled.

D - Disable the vulnerable protocols IMAP, POP3 and SMTP at runtime to forbid libcurl from using them. You can do this with the CURLOPT_PROTOCOLS option.


Dan Fandrich realized the problem and reported it to us on December 22 2011.

We discussed solutions and a first patch was written on the same day.

curl 7.24.0 was released on January 24th 2012, coordinated with the publication of this this flaw.


Thanks a lot!