Wireshark (formerly Ethereal) announced yesterday that there is a vulnerability which could cause it to crash, use up all available memory, or potentially execute arbitrary code.  It is highly recommended that we upgrade to version 0.99.2.  I would assume that those using the older Ethereal versions should move over to the new Wireshark branded versions as well.

Source:
http://www.wireshark.org/security/wnpa-sec-2006-01.html
http://secunia.com/advisories/21078/

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Scott Fendley
ISC Handler