Cisco IOS Misformed BGP Packet Causes Reload

Summary

• A Cisco device running IOS Border Gateway Protocol (BGP) is vulnerable to a Denial of Service (DoS) attack from a malformed BGP packet. Only devices with either the command bgp log-neighbor-changes configured or the command snmp-server enable traps bgp are vulnerable. The BGP protocol is not enabled by default, and must be configured in order to accept traffic from an explicitly defined peer. Unless the malicious traffic appears to be sourced from a configured, trusted peer, it would be difficult to inject a malformed packet.

  Cisco has made free software available to address this problem.

  This issue is tracked by CERT/CC VU#689326.

  This advisory will be posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050126-bgp.

Affected Products

• This section provides details on affected products.

  

  Vulnerable Products

  This vulnerability is present in any unfixed version of Cisco IOS, from the beginning of support for the BGP protocol, including versions 9.x, 10.x, 11.x and 12.x. This issue affects all Cisco devices configured for BGP routing and configured with either the bgp log-neighbor-changes command or snmp-server enable traps bgp. The bgp log-neighbor-changes command is on by default in 12.0(22)S, 12.0(11)ST, 12.1(10)E, 12.1(10) and later software.

  Cisco IOS XR is also affected.

  A router that is running the BGP process will have the following line in the configuration:

  router bgp <AS number>
  

  In order for the device to be impacted, one or both of the following configuration commands must be enabled:

  bgp log-neighbor-changes  
  
  

  or

  snmp-server enable traps bgp
  

  To determine the software running on a Cisco product, log in to the device and issue the show version command to display the system banner. Cisco IOS software will identify itself as "Internetwork Operating System Software" or simply "IOS ®." On the next line of output, the image name will be displayed between parentheses, followed by "Version" and the IOS release name. Other Cisco devices will not have the show version command or will give different output.

  The following example identifies a Cisco product running IOS release 12.0(3) with an installed image name of C2500-IS-L:

  Cisco Internetwork Operating System Software IOS (TM)
      
  2500 Software (C2500-IS-L), Version 12.0(3), RELEASE SOFTWARE

  The release train label is "12.0."

  The next example shows a product running IOS release 12.0(2a)T1 with an image name of C2600-JS-MZ:

  Cisco Internetwork Operating System Software IOS (tm)
  C2600 Software (C2600-JS-MZ), Version 12.0(2a)T1, RELEASE SOFTWARE (fc1)

  Additional information about Cisco IOS release naming can be found at: http://www.cisco.com/warp/public/620/1.html.

  Products Confirmed Not Vulnerable

  Products confirmed not to be vulnerable include devices that do not run Cisco IOS, such as the Cisco Guard, products that cannot participate in BGP or products that cannot be configured for BGP. No other Cisco products are currently known to be affected by this vulnerability.

Details

• The Border Gateway Protocol (BGP) is a routing protocol defined by RFC 1771, and designed to manage IP routing in large networks. An affected Cisco device running a vulnerable version of Cisco IOS software with the BGP protocol enabled will reload if a malformed BGP packet is already queued on the interface when a BGP neighbor change is logged. The device is not vulnerable unless either bgp log-neighbor-changes is configured or snmp-server traps enable bgp is configured.

  Malformed packets can not only come from malicious sources. A valid peering device such as another BGP speaking router that produces the specific malformed packet in error can trigger this behavior.

  In all cases, however, the packets must be sourced from an IP address that is explicitly configured.

  BGP runs over the Transport Control Protocol (TCP), a reliable transport protocol which requires a valid three way handshake before any further messages will be accepted. The Cisco IOS implementation of BGP requires the explicit definition of a neighbor before a connection can be established, and traffic must appear to come from that neighbor. These implementation details make it very difficult to maliciously send a BGP packet to a Cisco IOS device from an unauthorized source.

  This bug may also be triggered by other means which are not considered remotely exploitable. The use of the commands show ip bgp neighbors or debug ip bgp can cause a router to reload if the device has previously received a malformed packet.

  Additionally, using an SNMP management station to poll the SNMP Object Identifier (OID) bgpPeerEntry.bgpPeerLastError (1.3.6.1.2.1.15.3.1.14) may cause the device to reset. This may only happen after the device has received the malformed packet and the BGP session is reset.

  A Cisco IOS device receiving an invalid BGP packet will reset and may take several minutes to become fully functional. This vulnerability may be exploited repeatedly resulting in an extended DoS attack. This issue is documented in bug ID CSCee67450 ( registered customers only) .

  A Cisco IOS XR device will restart the BGP process. All the sessions will be dropped and BGP process will have to re-establish sessions with its peers. No other routing protocols will be affected.

  If graceful restart is enabled, forwarding should not be impacted.

  In IOS XR, the BGP process is not mandatory by default. Unless the user has explicitly configured the BGP process to be mandatory, the IOS XR device will not reload.

Workarounds

• The effectiveness of any workaround is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround is the most appropriate for use in the intended network before it is deployed.

  • Remove the configuration command bgp log-neighbor-changes. This feature is used to monitor BGP peer status and its removal may reduce network monitoring capabilities. More information on this command is available here: http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_a1g.html#wp1040601
    

  The use of networking best practices techniques can greatly reduce the probability of a network infrastructure attack. Best practices that may reduce risk in this case include:

  BGP MD5

  Under normal circumstances, due to inherent security factors in the TCP protocol, such as sequence number checks, it is difficult, but possible to forge an appropriate packet to exploit this problem. Configuring your Cisco IOS device for BGP MD5 authentication greatly increases the work necessary to forge a valid packet from a remote peer. This will not protect your peering session if a valid BGP peer generates an invalid packet.

  This can be configured as shown in the following example:

  router(config)# router bgp 
   router(config-router)# neighbor  password 
  
  

  It is necessary to configure the same shared MD5 secret on both peers and at the same time. Failure to do so will break the existing BGP session and the new session will not get established until the exact same secret is configured on both devices. For a detailed discussion on how to configure BGP, refer to the following document:

  http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a00800ca571.html

  Once the secret is configured, it is prudent to change it periodically. The exact period must fit within your company security policy but it should not be longer than a few months. When changing the secret, again it must be done at the same time on both devices. Failure to do so will break your existing BGP session. The exception is if your Cisco IOS software release contains the integrated CSCdx23494 ( registered customers only) fix on both sides of the connection. With this fix, the BGP session will not be terminated when the MD5 secret is changed only on one side. The BGP updates, however, will not be processed until either the same secret is configured on both devices or the secret is removed from both devices.

  Infrastructure Access Control Lists (iACLs)

  Although it is often difficult to block traffic transiting your network, it is possible to identify traffic that should never be allowed to target your infrastructure devices and block that traffic at the border of your network. Infrastructure ACLs are considered a network security best practice and should be considered as a long-term addition to good network security as well as providing some added protection for this specific vulnerability. The white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques for infrastructure protection ACLs:

  http://www.cisco.com/warp/public/707/iacl.html

  Workarounds for IOS XR

  Users of Cisco IOS XR can avoid exposure to this vulnerability by not enabling 'debug bgp' at a level of 'warning' or anything more verbose and not logging bgp neighbor state changes.

Fixed Software

• When considering software upgrades, please also consult http://www.cisco.com/en/US/products/products_security_advisories_listing.html and any subsequent advisories to determine exposure and a complete upgrade solution.

  In all cases, customers should exercise caution to be certain the devices to be upgraded contain sufficient memory and that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, contact the Cisco Technical Assistance Center ("TAC") for assistance.

  Cisco customers running IOS XR software should contact Cisco's TAC for repaired software.

  Major Release	Availability of Repaired Releases
  Affected 12.0-Based Release	Rebuild	Maintenance Release
  12.0	12.0(28b)
  12.0DA	Vulnerable; contact TAC
  12.0DB	Migrate to 12.3(4)T11 or later
  12.0DC	Migrate to 12.3(4)T11 or later
  12.0S	12.0(25)S5
  	12.0(26)S2d
  	12.0(26)S5
  	12.0(27)S2d
  	12.0(27)S4
  	12.0(28)S1
  		12.0(29)S
  12.0SC	Vulnerable; contact TAC
  12.0SP	Migrate to 12.0S or later
  12.0ST	Migrate to 12.0(26)S5 or later
  12.OSV	12.0(27)SV4
  12.0SX	Vulnerable; contact TAC
  12.0SY	Migrate to 12.0(26)S5 or later
  12.0SZ	Migrate to 12.0(26)S5 or later
  12.0W5		12.0(28)W5 (31)
  12.0WC	Vulnerable; contact TAC
  12.0WT	Vulnerable; contact TAC
  12.0WX	Vulnerable; contact TAC
  12.0XA	Migrate to 12.1 latest
  12.0XB	Migrate to 12.0(1)T or later
  12.0XC	Migrate to 12.1 latest
  12.0XD	Migrate to 12.1 latest
  12.0XE	Migrate to 12.1E latest
  12.0XF	Vulnerable; contact TAC
  12.0XG	Migrate to 12.1 latest
  12.0XH	Migrate to 12.1 or later
  12.0XI	Migrate to 12.1 or later
  12.0XJ	Migrate to 12.1 latest
  12.0XK	Migrate to 12.2 latest
  12.0XL	Migrate to 12.2 latest
  12.0XM	Migrate to 12.1 or later
  12.0XN	Migrate to 12.1 latest
  12.0XP	Vulnerable; contact TAC
  12.0XQ	Migrate to 12.1 or later
  12.0XR	Migrate to 12.2 latest
  12.0XS	Migrate to 12.1E latest
  12.0XT	Vulnerable; contact TAC
  12.0XU	Vulnerable; contact TAC
  12.0XV	Migrate to 12.1 or later
  Affected 12.1-Based Release	Rebuild	Maintenance Release
  12.1		12.1(26)
  12.1AA	Migrate to 12.2 latest
  12.1AX	12.1(14)AX3
  12.1AY	Migrate to 12.1(22)EA2 or later
  12.1AZ	Migrate to 12.1(22)EA2 or later
  12.1DA	Vulnerable; contact TAC
  12.1DB	Migrate to 12.3(4)T11 or later
  12.1DC	Migrate to 12.3(4)T11 or later
  12.1E	12.1(22)E3
  	12.1(23)E2
  		12.1(26)E
  12.1EA	12.1(22)EA2
  12.1EC	Vulnerable; contact TAC
  12.1EO	Vulnerable; contact TAC
  12.1EV	Migrate to 12.2S latest
  12.1EW	Migrate to 12.2(18)EW2 or later
  12.1EX	Migrate to 12.1E latest
  12.1EY	Migrate to 12.1E latest
  12.1T	Migrate to 12.2 or later
  12.1XA	Migrate to 12.2 or later
  12.1XB	Migrate to 12.2 or later
  12.1XC	Migrate to 12.2 or later
  12.1XD	Migrate to 12.2 or later
  12.1XE	Migrate to 12.1E latest
  12.1XF	Migrate to 12.3 or later
  12.1XG	Migrate to 12.3 or later
  12.1XH	Migrate to 12.2 or later
  12.1XI	Migrate to 12.2 latest
  12.1XJ	Migrate to 12.3 or later
  12.1XL	MMigrate to 12.3 or later
  12.1XM	Migrate to 12.3 or later
  12.1XP	Migrate to 12.3 or later
  12.1XQ	Migrate to 12.3 or later
  12.1XR	Migrate to 12.3 or later
  12.1XT	Migrate to 12.3 or later
  12.1XU	Migrate to 12.3 or later
  12.1XV	Vulnerable; contact TAC
  12.1YA	Migrate to 12.3 or later
  12.1YB	Migrate to 12.3 or later
  12.1YC	Migrate to 12.3 or later
  12.1YD	Migrate to 12.3 or later
  12.1YE	Migrate to 12.3 or later
  12.1YF	Migrate to 12.3 or later
  12.1YH	Migrate to 12.3 or later
  12.1YI	Migrate to 12.2(2)YC or later
  12.1YJ	Migrate to 12.1(22)EA2 or later
  Affected 12.2-Based	Rebuild	Maintenance Release
  12.2		12.2(27)
  12.2B	Migrate to 12.3(4)T11 or later
  12.2BC	Vulnerable; contact TAC
  12.2BW	Migrate to 12.3 or later
  12.2BX	Migrate to 12.3(7)XI3 -- Available 2/15/05
  12.2BY	Migrate to 12.3(4)T11 or later
  12.2BZ	Migrate to 12.3(7)XI3 -- Available 2/15/05
  12.2CZ	Vulnerable; contact TAC
  12.2DA	Vulnerable; contact TAC
  12.2DD	Migrate to 12.3(4)T11 or later
  12.2DX	Migrate to 12.3(4)T11 or later
  12.2EW	12.2(18)EW2
  		12.2(25)EW
  12.2JK	12.2(15)JK2
  12.2MB	Migrate to 12.2(25)SW or later
  12.2MC	Migrate to 12.3(11)T or later
  12.2MX	Migrate to 12.3(8)T5 or later
  12.2S		12.2(25)S
  	12.2(14)S13
  	12.2(18)S8
  	12.2(20)S7
  12.2SE	12.2(20)SE3
  12.2SU	12.2(14)SU2
  12.2SW	Migrate to 12.2(25)SW
  12.2SX	Migrate to 12.2(17d)SXB5
  12.2SXA	Migrate to 12.2(17d)SXB5
  12.2SXB	12.2(17d)SXB5
  12.2SXD	12.2(18)SXD2
  12.2SY	Migrate to 12.2(17d)SXB5
  12.2SZ	Migrate to 12.2(25)S or later
  12.2T	12.2(15)T15
  12.2XA	Migrate to 12.3 or later
  12.2XB	Migrate to 12.3 or later
  12.2XC	Migrate to 12.3 or later
  12.2XD	Migrate to 12.3 or later
  12.2XE	Migrate to 12.3 or later
  12.2XF	Vulnerable; contact TAC
  12.2XG	Migrate to 12.3 or later
  12.2XH	Migrate to 12.3 or later
  12.2XI	Migrate to 12.3 or later
  12.2XJ	Migrate to 12.3 or later
  12.2XK	Migrate to 12.3 or later
  12.2XL	Migrate to 12.3 or later
  12.2XM	Migrate to 12.3 or later
  12.2XN	Migrate to 12.3 or later
  12.2XQ	Migrate to 12.3 or later
  12.2XS	Migrate to 12.3 or later
  12.2XT	Migrate to 12.3 or later
  12.2XU	Migrate to 12.3 or later
  12.2XW	Migrate to 12.3 or later
  12.2XZ	Migrate to 12.3 or later
  12.2YA	12.2(4)YA8
  12.2YB	Migrate to 12.3 or later
  12.2YC	Migrate to 12.3 or later
  12.2YE	Migrate to 12.2S or later
  12.2YF	Migrate to 12.3 or later
  12.2YG	Migrate to 12.3 or later
  12.2YH	Migrate to 12.3 or later
  12.2YJ	Migrate to 12.3 or later
  12.2YK	Migrate to 12.3T or later
  12.2YL	Migrate to 12.3T or later
  12.2YM	Migrate to 12.3T or later
  12.2YN	Migrate to 12.3T or later
  12.2YO	Migrate to 12.2(17d)SXB5
  12.2YP	Migrate to 12.3 or later
  12.2YQ	Migrate to 12.3(4)T11 or later
  12.2YR	Migrate to 12.3(4)T11 or later
  12.2YS	Migrate to 12.3T or later
  12.2YT	Migrate to 12.3 or later
  12.2YU	Migrate to 12.3T or later
  12.2YV	Migrate to 12.3(4)T11 or later
  12.2YW	Migrate to 12.3(4)T11 or later
  12.2YX	Migrate to 12.2(14)SU2 or later
  12.2YY	Migrate to 12.3T or later
  12.2YZ	Migrate to 12.2(25)S or later
  12.2ZA	Migrates to 12.2(17d)SXB5 or 12.2(18)SXD2
  12.2ZB	Migrate to 12.3T or later
  12.2ZC	Migrate to 12.3T or later
  12.2ZD	Migrate to 12.3 or later
  12.2ZE	Migrate to 12.3 or later
  12.2ZF	Migrate to 12.3(4)T11 or later
  12.2ZG	Migrate to 12.3(4)T11 or later
  12.2ZH	Migrate to 12.3(4)T11 or later
  12.2ZI	Migrate to 12.2(25)S or later
  12.2ZJ	Migrate to 12.3T or later
  12.2ZK	12.2(15)ZK6
  12.2ZL	Migrate to 12.3(7)T7 or later
  12.2ZN	Migrate to 12.3T or later
  12.2ZO	Migrate to 12.3 or later
  12.2ZP	Vulnerable; contact TAC
  Affected 12.3-Based	Rebuild	Maintenance Release
  12.3	12.3(6d)
  	12.3(9c)
  	12.3(10a)
  		12.3(12)
  12.3B	12.3(5a)B3
  12.3BC	12.3(9a)BC1
  12.3BW	Migrate to 12.3(7)T7 or later
  12.3T	12.3(4)T11
  	12.3(7)T7
  	12.3(8)T5
  		12.3(11)T
  12.3XA	Migrate to 12.3(7)T7 or later
  12.3XB	Migrate to 12.3(8)T5 or later
  12.3XC	12.3(2)XC3
  12.3XD	12.3(4)XD4
  12.3XE	12.3(2)XE1
  12.3XF	Migrate to 12.3(11)T or later
  12.3XG	Migrate to 12.3(11)T or later
  12.3XH	Migrate to 12.3(11)T or later
  12.3XI	12.3(7)XI3 - Available 2/15/05
  12.3XJ	Vulnerable; contact TAC
  12.3XK	Vulnerable; contact TAC
  12.3XL	Vulnerable; contact TAC
  12.3XN	Vulnerable; contact TAC
  12.3XQ	12.3(4)XQ1 Release date not yet determined
  12.3XR	Vulnerable; contact TAC
  12.3XS	12.3(7)XS2
  12.3XU	12.3(8)XU4
  12.3XV	Migrate to 12.3(11)T or later
  12.3XX	12.3(8)XX1
  12.3YA	12.3(8)YA1
  12.3YC	Vulnerable; contact TAC
  12.3YD	Vulnerable; contact TAC
  12.3YE	Migrate to 12.3(4)T11 or later
  12.3YF	Vulnerable; contact TAC
  12.3YH	Vulnerable; contact TAC
  12.3YJ	Vulnerable; contact TAC
  12.3YL	Vulnerable; contact TAC

Exploitation and Public Announcements

• The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability described in this advisory.

  This vulnerability was discovered by Cisco during internal testing.

URL

• http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050126-bgp

Revision History

• Revision 1.5	2005-March-21	Updated the IOS Software 12.2T available repaired release information in the IOS release table in the Software Versions and Fixes section.
  Revision 1.4	2005-February-09	Modifications and additions to the Details section.
  Revision 1.3	2005-February-04	Modifications to Software Versions and Fixes Table, Summary, Affected Products, and Details sections.
  Revision 1.2	2005-February-01	Additional rebuilds added to 12.2S
  Revision 1.1	2005-January-29	Added IOS XR as affected. Minor syntax changes. Modified 12.2ZA migration path.
  Revision 1.0	2005-January-26	Initial public release.

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