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[SECURITY ADVISORY] curl: printf floating point buffer overflow

From: Daniel Stenberg <>
Date: Wed, 21 Dec 2016 07:59:15 +0100 (CET)

printf floating point buffer overflow

Project curl Security Advisory, December 21, 2016 -


libcurl's implementation of the printf() functions triggers a buffer overflow
when doing a large floating point output. The bug occurs when the conversion
outputs more than 255 bytes.

The flaw happens because the floating point conversion is using system
functions without the correct boundary checks.

The functions have been documented as deprecated for a long time and users are
discouraged from using them in "new programs" as they are planned to get
removed at a future point. But as the functions are present and there's
nothing preventing users from using them, we expect there to be a certain
amount of existing users in the wild.

If there are any application that accepts a format string from the outside
without necessary input filtering, it could allow remote attacks.

This flaw does not exist in the command line tool.

We are not aware of any exploit of this flaw.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2016-9586 to this issue.
This flaw exists in the following libcurl versions.
- Affected versions: libcurl 7.1 to and including 7.51.0
- Not affected versions: libcurl >= 7.52.0
libcurl is used by many applications, but not always advertised as such!
In version 7.52.0, the conversion is limited to never generate a larger output
than what fits in the fixed size buffer.
A [patch for CVE-2016-9586]( is
We suggest you take one of the following actions immediately, in order of
  A - Upgrade curl and libcurl to version 7.52.0
  B - Apply the patch to your version and rebuild
  C - Do not use the `curl_mprintf()` functions
It was first reported to the curl project on November 8 by Daniel Stenberg.
We contacted distros_at_openwall on December 13.
curl 7.52.0 was released on December 21 2016, coordinated with the publication
of this advisory.
Reported and patched by Daniel Stenberg.
List admin:
Received on 2016-12-21