OOB write via unchecked multiplication

Project curl Security Advisory, November 2, 2016 - Permalink

VULNERABILITY

In libcurl's base64 encode function, the output buffer is allocated as follows without any checks on insize:

malloc( insize * 4 / 3 + 4 )

On systems with 32-bit addresses in userspace (e.g. x86, ARM, x32), the multiplication in the expression wraps around if insize is at least 1GB of data. If this happens, an undersized output buffer will be allocated, but the full result will be written, thus causing the memory behind the output buffer to be overwritten.

If a username is set directly via CURLOPT_USERNAME (or curl's -u, --user option), this vulnerability can be triggered. The name has to be at least 512MB big in a 32bit system.

Systems with 64 bit versions of the size_t type are not affected by this issue.

We are not aware of any exploit of this flaw.

INFO

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2016-8617 to this issue.

CWE-131: Incorrect Calculation of Buffer Size

AFFECTED VERSIONS

This flaw exists in the following curl versions.

• Affected versions: curl 7.3 to and including 7.50.3
• Not affected versions: curl < 7.3 and curl >= 7.51.0

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

THE SOLUTION

In version 7.51.0, the overflow is avoided.

A patch for CVE-2016-8617 is available.

RECOMMENDATIONS

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

A - Upgrade curl and libcurl to version 7.51.0

B - Apply the patch to your version and rebuild

C - Do not use the CURLOPT_USERNAME option.

TIME LINE

It was first reported to the curl project on September 23 by Cure53.

We contacted distros@openwall on October 19.

curl 7.51.0 was released on November 2 2016, coordinated with the publication of this advisory.

CREDITS

This vulnerability was found during a Secure Open Source audit performed by Cure53.