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[SECURITY ADVISORY] Windows DLL hijacking

From: Daniel Stenberg <>
Date: Mon, 30 May 2016 08:30:10 +0200 (CEST)

Windows DLL hijacking

Project cURL Security Advisory, May 30th 2016 -


libcurl would load Windows system DLLs in a manner that may make it vulnerable
to a DLL hijacking (aka binary planting) attack in certain configurations.

libcurl has a unified code base that builds and runs on a multitude of
different versions of Windows. To make that possible, when libcurl is built
with SSPI or telnet is used, it dynamically loads some of the necessary system
DLLs at run-time by calling LoadLibrary(). No path is specified for these

To find a DLL when no path is specified LoadLibrary() follows [DLL search
to load it. If it is a "known DLL" no searching is done, the system copy is
used. If it is not a "known DLL": The application directory is searched first.
The current directory is searched next, if DLL safe search mode isn't enabled.
The system directory is searched next.

The 3 system DLLs libcurl loads dynamically are security.dll, secur32.dll and
ws2_32.dll (a "known DLL" when installed). These DLLs may not be present on
some versions of Windows, which is why they are loaded dynamically. Depending
on a number of factors outlined in the DLL search order document it may be
possible for an attacker to plant a DLL of the same name in the user's current
directory, application directory or other directory in the DLL search order,
thereby possibly causing it to be loaded first.

**Recent versions of Windows include all 3 of those dynamically loaded system
DLLs and also enable safe DLL search mode by default. Therefore in such a case
ws2_32.dll could not be planted, and security.dll or secur32.dll could only be
planted in the application directory.**

To address this issue we have changed libcurl so that any system DLL it
dynamically loads in Windows is done in the most secure way available.

Note if an attacker has the ability to write new files to your application
directory they can likely still plant DLLs to be loaded in any case, load-time
or run-time. This is by design in Windows DLL loading (refer the the DLL
search order doc). For example it may be possible to override DLL search paths
by planting an app.exe.local file or possibly a fake manifest. There is
nothing we can do to prevent against this. We advise you to guard write
permissions on your application directory.

**Also note it is may still be possible for planting attacks to be done
against load-time DLLs used by libcurl and the curl tool. This is because
Windows loads those DLLs and their dependencies without specifying a
path. There is nothing we can do to fix this, it is endemic in the design of
Windows. We advise you to guard write permissions on your application

We are not aware of any exploit of this flaw.


This flaw also affects the curl command line tool.
The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2016-4802 to this issue.
This flaw is relevant for all versions of curl and libcurl on Windows that
support SSPI or Telnet.
- Affected versions: libcurl 7.11.1 to and including 7.49.0
- Not affected versions: libcurl < 7.11.1 and libcurl >= 7.49.1
libcurl is used by many applications, but not always advertised as such!
In version 7.49.1, libcurl loads DLLs from the Windows system directory,
either via LoadLibraryEx() and LOAD_LIBRARY_SEARCH_SYSTEM32, where supported,
or by explicitly finding the system directory.
A [patch for CVE-2016-4802]( is available.
We suggest you take one of the following actions immediately, in order of
  A - Upgrade curl and libcurl to version 7.49.1
  B - Apply the patch to your version and rebuild
  C - Build your libcurl without SSPI and Telnet support enabled
Additionally in any case you should secure write permissions to your
application directory and if you are using the curl tool exercise judicious
use of `--remote-header-name` (also usable as `-J`). The `-J` option when
combined with `-O` lets the server choose the file name. A rogue server could
send you the name of a DLL or other file that could be used in a planting
It was first reported to the curl project on May 7th 2016. We contacted MITRE
on May 13.
libcurl 7.49.1 was released on May 30 2016, coordinated with the publication
of this advisory.
Reported by Guohui from Huawei WeiRan Labs. Patch by Steve Holme. Stefan
Kanthak and Jay Satiro brought valuable feedback.
Thanks a lot!
List admin:
Received on 2016-05-30