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Home » Hacking News » CA-2002-07-Double Free Bug in zlib Compression Library

CA-2002-07-Double Free Bug in zlib Compression Library

by Nikola Strahija on March 13th, 2002 There is a bug in the zlib compression library that may manifest itself as a vulnerability in programs that are linked with zlib. This may allow an attacker to conduct a denial-of-service attack, gather information, or execute arbitrary code.

It is important to note that the CERT/CC has not received any reports
of exploitation of this bug. Based on the information available to us
at this time, it is difficult to determine whether this bug can be
successfully exploited. However, given the widespread deployment of
zlib, we have published this document as a proactive measure.

I. Description

There is a bug in the decompression algorithm used by the popular zlib
compression library. If an attacker is able to pass a
specially-crafted block of invalid compressed data to a program that
includes zlib, the program's attempt to decompress the crafted data
can cause the zlib routines to corrupt the internal data structures
maintained by malloc.

The bug results from a programming error that causes segments of
dynamically allocated memory to be released more than once (i.e.,
"double-freed"). Specifically, when inftrees.c:huft_build() encounters
the crafted data, it returns an unexpected Z_MEM_ERROR to
inftrees.c:inflate_trees_dynamic(). When a subsequent call is made to
infblock.c:inflate_blocks(), the inflate_blocks function tries to free
an internal data structure a second time.

Because this bug interferes with the proper allocation and
deallocation of dynamic memory, it may be possible for an attacker to
influence the operation of programs that include zlib. In most
circumstances, this influence will be limited to denial of service or
information leakage, but it is theoretically possible for an attacker
to insert arbitrary code into a running program. This code would be
executed with the permissions of the vulnerable program.

The CERT/CC is tracking this issue as VU#368819. This reference number
corresponds to CVE candidate CAN-2002-0059.

II. Impact

This bug may introduce vulnerabilities into any program that includes
the affected library. Depending upon how and where the zlib routines
are called from the given program, the resulting vulnerability may
have one or more of the following impacts: denial of service,
information leakage, or execution of arbitrary code.

III. Solution

Upgrade your version of zlib

The maintainers of zlib have released version 1.1.4 to address this
vulnerability. Upgrade any software that is linked to or derived from
an earlier version of zlib. The latest version of zlib is available at

These are the MD5 checksums for zlib version 1.1.4:

abc405d0bdd3ee22782d7aa20e440f08 zlib-1.1.4.tar.gz

Apply a patch from your vendor

The zlib compression library is freely available and used by many
vendors in a wide variety of applications. Any one of these
applications may contain vulnerabilities that are introduced by this

Appendix A contains information provided by vendors for this advisory.
As vendors report new information to the CERT/CC, we will update this
section and note the changes in our revision history. If a particular
vendor is not listed below, we have not received their comments.
Please contact your vendor directly.

Appendix A. - Vendor Information

This appendix contains information provided by vendors for this
advisory. As vendors report new information to the CERT/CC, we will
update this section and note the changes in our revision history. If a
particular vendor is not listed below, we have not received their

Apple Computer, Inc.

Mac OS X and Mac OS X Server do not contain this vulnerability.

Compaq Computer Corporation

x-ref: SSRT0818 zlib

At the time of writing this document, Compaq continues to evaluate
this potential problem and impacts to Compaq released software. Compaq
will implement solutions based on the conclusion of this evaluation as
necessary. Compaq will provide notice of any new patches as a result
any required solution through standard patch notification procedures
and be available from your normal Compaq Services support channel.

Conectiva Linux

Conectiva Linux supported versions (5.0, 5.1, 6.0, 7.0, ferramentas
graficas and ecomerce) are affected by the zlib vulnerability. Updates
will be sent to our security mailing lists and be available at our ftp
site and mirrors. The updates will include a new version of zlib
itself and also other packages which include their own version of zlib
or are linked statically to the system-wide copy of zlib.


EnGarde Secure Linux Community and Professional are both vulnerable to
the zlib bugs. Guardian Digital addressed this vulnerability in
ESA-20020311-008 which may be found at:

EnGarde Secure Professional users may upgrade their systems using the
Guardian Digital Secure Network.


FreeBSD is not vulnerable, as the FreeBSD malloc implementation
detects and complains about several programming errors including this
kind of double free.


Fujitsu's UXP/V operating system is not affected by the zlib
vulnerability because it does not support zlib.

Hewlett-Packard Company

HP is not vulnerable.

IBM Corporation

IBM's AIX operating system, version 5.1, ships with open
source-originated zlib that is used with the Redhat Package Manager
(rpm) to install applications that are included in the AIX-Linux
Affinity Toolkit. zlib (libz.a) is a shared library in AIX. AIX 5.1 is
susceptible to the described vulnerability. AIX 4.3.x does not ship
with zlib, but customers who install zlib and use it will be similarly
vulnerable. IBM will make the patched version of zlib available as
soon as it is made available to us.


OpenBSD is not vulnerable as OpenBSD's malloc implementation detects
double freeing of memory. The zlib shipped with OpenBSD has been fixed
in OpenBSD-current in January 2002.

Openwall GNU/*/Linux

All versions of Openwall GNU/*/Linux (Owl) prior to the 2002/02/15
Owl-current snapshot are affected by the zlib double-free
vulnerability. Owl-current after 2002/02/15 includes the proper fixes
in its userland packages. In order to not place the users of other
vendors' products at additional risk, we have agreed to delay
documenting this as a security change and including the fixes in Owl
0.1-stable until there's a coordinated public announcement. While we
don't normally support this kind of a policy (releasing a fix before
there's an announcement), this time handling the vulnerability in this
way was consistent with the state of things by the time the (already
publicly known) bug was first realized to be a security vulnerability.

The zlib bug could affect the following Owl packages: gnupg, openssh,
rpm, texinfo (not necessarily in a security sense). Of these, the
OpenSSH could potentially allow for an active remote attack resulting
in a root compromise. If only SSH protocol version 1 is allowed in the
OpenSSH server this is reduced to a local attack, but reverse remote
attack possibilities by a malicious server remain. Additionally, any
third-party software that makes use of the provided zlib library could
be affected.

Parts of the Linux 2.2 kernel included in Owl were also affected by
the vulnerability. Fortunately, those parts (Deflate compression
support for PPP and the experimental Deflate compression extension to
IrDA) are normally not used by the Owl userland. The bug has been
corrected starting with Linux 2.2.20-ow2 which has been made public
and a part of both Owl-current and Owl 0.1-stable on 2002/03/03. This
change, however, will only be documented in the publicly-available
change logs on the coordinated public announcement date.

Red Hat, Inc.

Red Hat Linux ships with a zlib library that is vulnerable to this
issue. Although most packages in Red Hat Linux use the shared zlib
library we have identified a number of packages that either statically
link to zlib or contain an internal version of the zlib code.

Updates to zlib and these packages as well as our advisory note are
available from the following URL. Users of the Red Hat Network can use
the up2date tool to automatically upgrade their systems.

Red Hat would like to thank CERT/CC for their help in coordinating
this issue with other vendors.


SGI acknowledges the zlib vulnerabilities reported by CERT and is
currently investigating. No further information is available at this

For the protection of all our customers, SGI does not disclose,
discuss or confirm vulnerabilities until a full investigation has
occurred and any necessary patch(es) or release streams are available
for all vulnerable and supported IRIX operating systems. Until SGI has
more definitive information to provide, customers are encouraged to
assume all security vulnerabilities as exploitable and take
appropriate steps according to local site security policies and
requirements. As further information becomes available, additional
advisories will be issued via the normal SGI security information
distribution methods including the wiretap mailing list on


XFree86 versions 4.0 through 4.2.0 include zlib version 1.0.8. XFree86
3.x includes zlib version 1.0.4. The zlib code included with XFree86
is only used on some platforms. This is determined by the setting of
HasZlib in the imake config files in the xc/config/cf source
directory. If HasZlib is set to YES in the platform's
file(s), then the system-provided zlib is used instead of the
XFree86-provided version. XFree86 uses the system-provided zlib by
default only on the following platforms:

FreeBSD 2.2 and later
NetBSD 1.2.2 and later
Debian Linux

The zlib code in XFree86 has been fixed in the CVS repository (trunk
and the xf-4_2-branch branch) as of 14 February 2002. A source patch
for XFree86 4.2.0 will be available from

The following XFree86 4.2.0 binary distributions provided by XFree86
include and use a vulnerable version of zlib:


When updated binaries are available, it'll be documented at

To check if an installation of XFree86 includes zlib, see if the
following file exists:


To check if an XFree86 X server is dynamically linked with zlib, look
for a line containing 'libz' in the output of 'ldd

Various vendors repackage and distribute XFree86, and may use settings
and configurations different from those described here.

All users of zlib versions 1.1.3 or earlier should obtain the latest
version, 1.1.4 or later, from, in order to avoid
this vulnerability as well as other possible vulnerabilities in
versions prior to 1.1.3 when decompressing invalid data.

Appendix B. - References


The CERT/CC thanks Owen Taylor and Mark Cox of Red Hat, Inc. for
reporting this vulnerability. We also thank Mark Adler of for
contributing to our research and Matthias Clasen for contributing to
the discovery of this vulnerability.

This document was written by Jeffrey P. Lanza.

This document is available from:

CERT/CC Contact Information

Email: [email protected]
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) /
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Copyright 2002 Carnegie Mellon University.

Revision History
Mar 12, 2002: Initial release

Version: PGP 6.5.8

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