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FTP PWD response parser out of bounds read

Project curl Security Advisory, October 4th 2017 - Permalink

VULNERABILITY

libcurl may read outside of a heap allocated buffer when doing FTP.

When libcurl connects to an FTP server and successfully logs in (anonymous or not), it asks the server for the current directory with the PWD command. The server then responds with a 257 response containing the path, inside double quotes. The returned path name is then kept by libcurl for subsequent uses.

Due to a flaw in the string parser for this directory name, a directory name passed like this but without a closing double quote would lead to libcurl not adding a trailing NUL byte to the buffer holding the name. When libcurl would then later access the string, it could read beyond the allocated heap buffer and crash or wrongly access data beyond the buffer, thinking it was part of the path.

A malicious server could abuse this fact and effectively prevent libcurl-based clients to work with it - the PWD command is always issued on new FTP connections and the mistake has a high chance of causing a segfault.

The simple fact that this issue has remained undiscovered for this long could suggest that malformed PWD responses are rare in benign servers.

We are not aware of any exploit of this flaw.

INFO

This bug was introduced in commit 415d2e7cb7, March 2005.

In libcurl version 7.56.0, the parser always zero terminates the string but also rejects it if not terminated properly with a final double quote.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2017-1000254 to this issue.

AFFECTED VERSIONS

curl is used by many applications, but not always advertised as such.

THE SOLUTION

A patch for CVE-2017-1000254 is available.

RECOMMENDATIONS

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

A - Upgrade curl to version 7.56.0

B - Apply the patch to your version and rebuild

C - Switch off FTP in CURLOPT_PROTOCOLS

TIME LINE

It was reported to the curl project on September 24, 2017. We contacted distros@openwall on September 25.

curl 7.56.0 was released on October 4 2017, coordinated with the publication of this advisory.

CREDITS

Reported by Max Dymond. Patch by Daniel Stenberg.

Thanks a lot!