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Date: Tue, 4 Apr 2017 17:09:25 +0200 (CEST)
From: Daniel Stenberg <>
To: curl security announcements -- curl users <>,,
        libcurl hacking <>,
Subject: [SECURITY ADVISORY] curl: --write-out out of buffer read

--write-out out of buffer read

Project curl Security Advisory, April 3, 2017 -


There were two bugs in curl's parser for the command line option `--write-out`
(or `-w` for short) that would skip the end of string zero byte if the string
ended in a `%` (percent) or `\` (backslash), and it would read beyond that
buffer in the heap memory and it could then potentially output pieces of that
memory to the terminal or the target file etc.

The curl security team did not report this as a security vulnerability due to
the minimal risk: the memory this would output comes from the process the user
itself invokes and that runs with the same privileges as the user. We could
not come up with a likely scenario where this could leak other users' data or
memory contents.

An external party registered this as a [CVE with]( and
we feel a responsibility to clarify what this flaw is about. The CVE-2017-7407
issue is specifically only about the `%` part of this flaw.

This flaw only exists 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-2017-7407 to this issue.


curl has supported this option since version 6.5 (released March 13, 2000).

This flaw exists in the following curl versions.

- Affected versions: 6.5 to and including 7.53.1
- Not affected versions: < 6.5 and >= 7.54.0


In version 7.54.0, the end of the buffer is properly acknowledged and we have
added tests that verify this functionality.

The curl project has (as of this writing) not yet released a version with this
problem fixed. It is however already fixed in curl's git repository, commits
[1890d59905414ab]( and
[8e65877870c1]( The fix is
also available as a [stand-alone


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

  A - Upgrade curl and libcurl to version 7.54.0

  B - Apply the patch to your version and rebuild

  C - Do not use the `--write-out` feature with unchecked input


It was first reported to the curl project on March 10. The Mitre CVE
registration was [brought to our
omment-21618166) on April 4, 2017. The first commit to fix this was made
public on March 11.

curl 7.54.0 is to be released on April 19 2017


Reported to the curl project by Brian Carpenter



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