ecurity Advisory: FreeBSD-SA-00:25.alpha-dev-random

2000-06-13T00:00:00
ID SECURITYVULNS:DOC:332
Type securityvulns
Reporter Securityvulns
Modified 2000-06-13T00:00:00

Description

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============================================================================= FreeBSD-SA-00:25 Security Advisory FreeBSD, Inc.

Topic: FreeBSD/Alpha platform lacks kernel pseudo-random number generator, some applications fail to detect this.

Category: core Module: kernel Announced: 2000-06-12 Affects: FreeBSD/Alpha prior to the correction date. Corrected: 2000-05-10 (4.0-STABLE) 2000-04-28 (5.0-CURRENT) FreeBSD only: Yes

I. Background

The FreeBSD kernel provides a cryptographic-strength pseudo-random number generator via the /dev/random and /dev/urandom interfaces, which samples hardware measurements to provide a high-quality source of "entropy" (randomness).

II. Problem Description

The FreeBSD port to the Alpha platform did not provide the /dev/random or /dev/urandom devices - this was an oversight during the development process which was not corrected before the Alpha port "became mainstream". FreeBSD/i386 is not affected.

As a consequence, there is no way for Alpha systems prior to the correction date to obtain cryptographic-strength random numbers, unless an application "rolls its own" entropy gathering mechanism. This in itself is not a vulnerability, although it is an omission and a departure from the expected behaviour of a FreeBSD system.

The actual vulnerability is that some applications fail to correctly check for a working /dev/random and do not exit with an error if it is not available, so this weakness goes undetected. OpenSSL 0.9.4, and utilities based on it, including OpenSSH (both of which are included in the base FreeBSD 4.0 system) are affected in this manner (this bug was corrected in OpenSSL 0.9.5)

Therefore, cryptographic security systems on vulnerable FreeBSD/Alpha systems (including OpenSSH in the base FreeBSD 4.0 system) may have weakened strength, and cryptographic keys generated on such systems should not be trusted.

III. Impact

Cryptographic secrets (such as OpenSSH public/private keys) generated on FreeBSD/Alpha systems may be much weaker than their "advertised" strength, and may lead to data compromise to a dedicated and knowledgeable attacker.

PGP/GnuPG keys, and keys generated by the SSH or SSH2 ports, are not believed to be weakened since that software will correctly detect the lack of a working /dev/random and use alternative sources of entropy. OpenSSH and OpenSSL are currently the only known vulnerable applications.

IV. Workaround

None available.

V. Solution

One of the following three options, followed by step 2).

1a) Upgrade your FreeBSD/Alpha system to FreeBSD 4.0-STABLE after the correction date.

1b) install the patched 4.0-RELEASE GENERIC kernel available from:

ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:25/kernel.gz

e.g. perform the following steps as root:

fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:25/kernel.gz

fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:25/kernel.gz.asc

[ Verify the detached PGP signature using your PGP utility - consult your utility's documentation for how to do this ]

gunzip kernel.gz

cp /kernel /kernel.old

chflags noschg /kernel

cp kernel /kernel

chflags schg /kernel

1c) Download the kernel source patch and rebuild your FreeBSD/Alpha kernel, as follows:

fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:25/kernel.sys.diff

Download the detached PGP signature:

fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:25/kernel.sys.diff.asc

and verify the signature using your PGP utility.

Apply the patch:

cd /usr/src

patch -p < /path/to/kernel.sys.diff

Rebuild your kernel as described in

http://www.freebsd.org/handbook/kernelconfig.html

and reboot with the new kernel.

NOTE: Because of the significant improvements to the FreeBSD/Alpha platform in FreeBSD 4.0, it is not planned at this time to backport the necessary changes to FreeBSD 3.4-STABLE.

2) Immediately regenerate all OpenSSH-generated SSH keys and OpenSSL-generated SSL certificates, and any other data relying on cryptographic random numbers which were generated on FreeBSD/Alpha systems, whose strength cannot be verified. [Note: for most systems, the only significant vulnerability is likely to be from OpenSSH and OpenSSL-generated keys and certificates (e.g. for SSL webservers)]

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