Tuesday, 13 May 2008

Debian OpenSSL Predictable Random Number Generator & Perl script

The media is currently overflown with this news. Debians openssl package included a patch that introduced a vulnerability to Debian systems and its deductions (like Ubuntu). From the announcement:
Luciano Bello discovered that the random number generator in Debian's
openssl package is predictable. This is caused by an incorrect
Debian-specific change to the openssl package (CVE-2008-0166). As a
result, cryptographic key material may be guessable.

This is a Debian-specific vulnerability which does not affect other
operating systems which are not based on Debian. However, other systems
can be indirectly affected if weak keys are imported into them.

It is strongly recommended that all cryptographic key material which has
been generated by OpenSSL versions starting with 0.9.8c-1 on Debian
systems is recreated from scratch. Furthermore, all DSA keys ever used
on affected Debian systems for signing or authentication purposes should
be considered compromised; the Digital Signature Algorithm relies on a
secret random value used during signature generation.
So it is adviced to upgrade the openssl package as soon as possible. In the announcement above there is a perl script to check whether your system is vulnerable to this kind of attack. You can grab it here:
http://security.debian.org/project/extra/dowkd/dowkd.pl.gz
http://security.debian.org/project/extra/dowkd/dowkd.pl.gz.asc (corresponding PGP signature)
After extracting and setting the executable bit (chmod +x dowkd.pl) try the following commands:
./dowkd.pl user
The upper example scans for weak keys for the executing user. This command here will scan the given host for the weakness:
./dowkd.pl host HOSTNAME
(Replace HOSTNAME with a valid hostname in your local network). For me, the latter example printed:
# 192.168.1.2 SSH-2.0-OpenSSH_4.7p1 Debian-8
# 192.168.1.2 SSH-2.0-OpenSSH_4.7p1 Debian-8
192.168.1.2: weak key
192.168.1.2: weak key
:|

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