curl / Docs / curl CVEs / OOB write via unchecked multiplication


OOB write via unchecked multiplication

Project curl Security Advisory, November 2, 2016 - Permalink


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 is allocated, but the full result is 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.


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

CWE-131: Incorrect Calculation of Buffer Size

Severity: Medium


This flaw exists in the following curl versions.

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


In version 7.51.0, the overflow is avoided.


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.


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.


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