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URL unescape heap overflow via integer truncation

Project curl Security Advisory, November 2, 2016 - Permalink


The URL percent-encoding decode function in libcurl is called curl_easy_unescape. Internally, even if this function would be made to allocate a destination buffer larger than 2GB, it would return that new length in a signed 32 bit integer variable, thus the length would get either just truncated or both truncated and turned negative. That could then lead to libcurl writing outside of its heap based buffer.

This can be triggered by a user on a 64bit system if the user can send in a custom (very large) URL to a libcurl using program.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2016-8622 to this issue.

CWE-122: Heap-based Buffer Overflow

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 parser function is fixed.


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


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.


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