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FreeBSD-SA-08.11.arc4random Security Advisory
The FreeBSD Project
Topic: arc4random(9) predictable sequence vulnerability
Credits: Robert Woolley, Mark Murray, Maxim Dounin, Ruslan Ermilov
Affects: All supported versions of FreeBSD.
Corrected: 2008-11-24 17:39:39 UTC (RELENG_7, 7.1-PRERELEASE)
2008-11-24 17:39:39 UTC (RELENG_7_0, 7.0-RELEASE-p6)
2008-11-24 17:39:39 UTC (RELENG_6, 6.4-STABLE)
2008-11-24 17:39:39 UTC (RELENG_6_4, 6.4-RELEASE)
2008-11-24 17:39:39 UTC (RELENG_6_3, 6.3-RELEASE-p6)
CVE Name: CVE-2008-5162
For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:http://security.FreeBSD.org/>.
arc4random(9) is a generic-purpose random number generator based on the
key stream generator of the RC4 cipher. It is expected to be
cryptographically strong, and used throughout the FreeBSD kernel for a
variety of purposes, some of which rely on its cryptographic strength.
arc4random(9) is periodically reseeded with entropy from the FreeBSD
kernel's Yarrow random number generator, which gathers entropy from a
variety of sources including hardware interrupts. During the boot
process, additional entropy is provided to the Yarrow random number
generator from userland, helping to ensure that adequate entropy is
present for cryptographic purposes.
II. Problem Description
When the arc4random(9) random number generator is initialized, there may
be inadequate entropy to meet the needs of kernel systems which rely on
arc4random(9); and it may take up to 5 minutes before arc4random(9) is
reseeded with secure entropy from the Yarrow random number generator.
All security-related kernel subsystems that rely on a quality random
number generator are subject to a wide range of possible attacks for the
300 seconds after boot or until 64k of random data is consumed. The list
* GEOM ELI providers with onetime keys. When a provider is configured in
a way so that it gets attached at the same time during boot (e.g. it
uses the rc subsystem to initialize) it might be possible for an
attacker to recover the encrypted data.
* GEOM shsec providers. The GEOM shsec subsytem is used to split a shared
secret between two providers so that it can be recovered when both of
them are present. This is done by writing the random sequence to one
of providers while appending the result of the random sequence on the
other host to the original data. If the provider was created within the
first 300 seconds after booting, it might be possible for an attacker
to extract the original data with access to only one of the two providers
between which the secret data is split.
* System processes started early after boot may receive predictable IDs.
* The 802.11 network stack uses arc4random(9) to generate initial vectors
(IV) for WEP encryption when operating in client mode and WEP
authentication challenges when operating in hostap mode, which may be
* The IPv4, IPv6 and TCP/UDP protocol implementations rely on a quality
random number generator to produce unpredictable IP packet identifiers,
initial TCP sequence numbers and outgoing port numbers. During the
first 300 seconds after booting, it may be easier for an attacker to
execute IP session hijacking, OS fingerprinting, idle scanning, or in
some cases DNS cache poisoning and blind TCP data injection attacks.
* The kernel RPC code uses arc4random(9) to retrieve transaction
identifiers, which might make RPC clients vulnerable to hijacking
No workaround is available for affected systems.
NOTE WELL: Any GEOM shsec providers which were created or written to
during the first 300 seconds after booting should be re-created after
applying this security update.
Perform one of the following:
1) Upgrade your vulnerable system to 6-STABLE, or 7-STABLE, or to the
RELENG_7_0, or RELENG_6_3 security branch dated after the correction
2) To patch your present system:
The following patches have been verified to apply to FreeBSD 6.3 and
a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.
# fetch http://security.FreeBSD.org/patches/SA-08:11/arc4random.patch
# fetch http://security.FreeBSD.org/patches/SA-08:11/arc4random.patch.asc
# fetch http://security.FreeBSD.org/patches/SA-08:11/arc4random6x.patch
# fetch http://security.FreeBSD.org/patches/SA-08:11/arc4random6x.patch.asc
b) Apply the patch.
# cd /usr/src
# patch < /path/to/patch
c) Recompile your kernel as described in
<URL:http://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
VI. Correction details
The following list contains the revision numbers of each file that was
corrected in FreeBSD.
The latest revision of this advisory is available at
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