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Alert (TA04-111B)

Cisco IOS SNMP Message Handling Vulnerability

Systems Affected

  • Cisco routers and switches running vulnerable versions of IOS.

  • Vulnerable IOS versions known to be affected include:
    • 12.0(23)S4, 12.0(23)S5
    • 12.0(24)S4, 12.0(24)S5
    • 12.0(26)S1
    • 12.0(27)S
    • 12.0(27)SV, 12.0(27)SV1
    • 12.1(20)E, 12.1(20)E1, 12.1(20)E2
    • 12.1(20)EA1
    • 12.1(20)EW, 12.1(20)EW1
    • 12.1(20)EC, 12.1(20)EC1
    • 12.2(12g), 12.2(12h)
    • 12.2(20)S, 12.2(20)S1
    • 12.2(21), 12.2(21a)
    • 12.2(23)
    • 12.3(2)XC1, 12.3(2)XC2
    • 12.3(5), 12.3(5a), 12.3(5b)
    • 12.3(6)
    • 12.3(4)T, 12.3(4)T1, 12.3(4)T2, 12.3(4)T3
    • 12.3(5a)B
    • 12.3(4)XD, 12.3(4)XD1


There is a vulnerability in Cisco's Internetwork Operating System (IOS) SNMP service. When vulnerable Cisco routers or switches process specific SNMP requests, the system may reboot. If repeatedly exploited, this vulnerability could result in a sustained denial of service (DoS).

This vulnerability is distinct from the vulnerability described in US-CERT Technical Alert TA04-111A issued earlier today. Cisco has published an advisory about this distinct SNMP issue at the following location:


The Simple Network Management Protocol (SNMP) is a widely deployed protocol that is commonly used to monitor and manage network devices. There are several types of SNMP messages that are used to request information or configuration changes, respond to requests, enumerate SNMP objects, and send both solicited and unsolicited alerts. These messages use UDP to communicate network information between SNMP agents and managers.

There is a vulnerability in Cisco's IOS SNMP service in which attempts to process specific SNMP messages are handled incorrectly. This may potentially cause the device to reload.

Typically, ports 161/udp and 162/udp are used during SNMP operations to communicate. In addition to these well-known ports, Cisco IOS uses a randomly selected UDP port in the range from 49152/udp to 59152/udp (and potentially up to 65535) to listen for other types of SNMP messages. While SNMPv1 and SNMPv2c formatted messages can trigger this vulnerability, the greatest risk is exposed when any SNMPv3 solicited operation is sent to a vulnerable port.

Cisco notes in their advisory:

    SNMPv1 and SNMPv2c solicited operations to the vulnerable ports will perform an authentication check against the SNMP community string, which may be used to mitigate attacks. Through best practices of hard to guess community strings and community string ACLs, this vulnerability may be mitigated for both SNMPv1 and SNMPv2c. However, any SNMPv3 solicited operation to the vulnerable ports will reset the device. If configured for SNMP, all affected versions will process SNMP version 1, 2c and 3 operations.

Cisco is tracking this issue as CSCed68575. US-CERT is tracking this issue as VU#162451.


A remote, unauthenticated attacker could cause the vulnerable device to reload. Repeated exploitation of this vulnerability could lead to a sustained denial of service condition.


Upgrade to fixed versions of IOS

Cisco has published detailed information about upgrading affected Cisco IOS software to correct this vulnerability. System managers are encouraged to upgrade to one of the non-vulnerable releases. For additional information regarding availability of repaired releases, please refer to the "Software Versions and Fixes" section of the Cisco Security Advisory.


Cisco recommends a number of workarounds, including disabling SNMP processing on affected devices. For a complete list of workarounds, see the Cisco Security Advisory.

Appendix A. Vendor Information

This appendix contains information provided by vendors for this advisory. As vendors report new information to US-CERT, we will update this section and note the changes in our revision history. If a particular vendor is not listed below, we have not received their comments.

Cisco Systems

Please refer to Cisco Security Advisory: "Vulnerabilities in SNMP Message Processing". Cisco has published their advisory at the following location:

US-CERT thanks Cisco Systems for notifying us about this problem.

Feedback can be directed to the authors: Jeff Havrilla, Shawn Hernan, Damon Morda

The latest version of this document can be found at:

Copyright 2004 Carnegie Mellon University. Terms of use:

Revision History

  • April 20, 2004: Initial release

    Last updated

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