Signature for CVE ID: CVE-2008-1151 (CISCO PPTP memory leak - DoS)

Please see these links for more information on vulnerability:

http://nvd.nist.gov/nvd.cfm?cvename=CVE-2008-1151
http://www.cisco.com/en/US/products/products_security_advisory09186a0080969862.shtml

According to this vulnerability report,  PPTP process in CISCO routers
leak memory upon every PPTP termination. Eventually memory is used up
and no other PPTP connections are entertained.

How does one go about writing signatures for detecting exploits
targeting this vulnerability?
Only possibility I can think of, based on capabilities of signature
language, is to check for the rate at which these PPTP connections are
made. If it checks for higher rate, there could be false negatives
where attacker makes the connections at very slow rate.  If it checks
for lower rate,  then there is possibility of false positives. What is
the right way of writing signatures?


thanks
Ravi

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