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double free in curl_maprintf

Project curl Security Advisory, November 2, 2016 - Permalink


The libcurl API function called curl_maprintf() can be tricked into doing a double free due to an unsafe size_t multiplication, on systems using 32 bit size_t variables. The function is also used internally in numerous situations.

The function doubles an allocated memory area with realloc() and allows the size to wrap and become zero and when doing so realloc() returns NULL and frees the memory - in contrary to normal realloc() fails where it only returns NULL - causing libcurl to free the memory again in the error path.

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

This behavior can be triggered using the publicly exposed function.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2016-8618 to this issue.

CWE-415: Double Free

Severity: Medium


This flaw exists in the following curl versions (and again, only on 32bit versions).

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


In version 7.51.0, the memory growing functions will fail instead of letting the size wrap.


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 - Make really sure you never send strings larger than 1GB into this function


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.