Cisco IOS TCP Sequence Predictability

Reported March 1, 2001, by Cisco Systems.


All Cisco products using the Internetwork Operating System (IOS), including (but not limited to):

  • 800, 1000, 1005, 1400, 1600, 1700, 2500, 2600, 3600, MC3810, 4000, 4500, 4700, 6200, 6400 NRP, and 6400 NSP series Cisco routers

  • ubr900 and ubr920 universal broadband routers

  • Catalyst 2900 ATM, 2900XL, 2948g, 3500XL, 4232, 4840g, and 5000 RSFC series switches

  • 5200, 5300, and 5800 series access servers

  • Catalyst 6000 MSM, 6000 Hybrid Mode, 6000 Native Mode, 6000 Supervisor Module, and Catalyst ATM Blade

  • RSM, 7000, 7010, 7100, 7200, ubr7200, 7500, 10000 ESR, and 12000 GSR series Cisco routers

  • DistributedDirector

  • Catalyst 8510CSR, 8510MSR, 8540CSR, and 8540MSR series switches


A vulnerability exists in Cisco’s IOS that allows for initial TCP sequence prediction. This vulnerability affects only TCP connections that start from or end at the affected IOS device. Using TCP sequence predictability, an attacker can potentially inject data packets by using forged source IP addresses, causing potential man-in-the-middle attacks. Under some circumstances, if an attacker knows the SNMP community read-write strings, he can configure devices remotely without authorization.


Cisco has issued a notice regarding this vulnerability.

Discovered by Cisco Systems.

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