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Date: Wed, 2 Nov 2016 08:09:01 +0100 (CET)
From: Daniel Stenberg <>
To: curl security announcements -- curl users <>,,
        libcurl hacking <>,
Subject: [SECURITY ADVISORY] curl double-free in curl_maprintf

double-free in curl_maprintf

Project cURL Security Advisory, November 2, 2016 -


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 internallty in numerous

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

This behavior is triggable using the publicly exposed function.

We are not aware of any exploit of this flaw.


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


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

- Affected versions: curl 7.1 to and including 7.50.3
- Not affected versions: curl >= 7.51.0

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.

A [patch for CVE-2016-8618]( is


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

  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 funciton


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

We contacted distros@...nwall 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



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