Products & Services

Cisco IOS XR Software SSHv1 Denial of Service Vulnerability

Advisory ID: cisco-sa-20110525-iosxr-ssh

Revision 1.0

For Public Release 2011 May 25 16:00  UTC (GMT)


Cisco IOS XR Software contains a vulnerability in the SSH application that may result in a denial of service condition when the SSH version 1 (SSHv1) protocol is used. The vulnerability is a result of unremoved sshd_lock files consuming all available space in the /tmp filesystem.

Cisco has released software updates that address this vulnerability.

This advisory is posted at

Affected Products

This vulnerability affects all unfixed versions of Cisco IOS XR Software devices configured to accept SSHv1 connections. Details on the affected versions can be found in the Software Versions and Fixes section of this advisory.

Vulnerable Products

To determine the Cisco IOS XR Software release that is running on a Cisco product, administrators can log in to the device and issue the show version command to display the system banner. The system banner confirms that the device is running Cisco IOS XR Software by displaying text similar to "Cisco IOS XR Software". The software version is displayed after the text "Cisco IOS XR Software".

The following example identifies a Cisco CRS-1 that is running Cisco IOS XR Software Release 3.6.2:

RP/0/RP0/CPU0:CRS#show version
Tue Aug 18 14:25:17.407 AEST

Cisco IOS XR Software, Version 3.6.2[00]
Copyright (c) 2008 by Cisco Systems, Inc.

ROM: System Bootstrap, Version 1.49(20080319:195807) [CRS-1 ROMMON],

CRS uptime is 4 weeks, 4 days, 1 minute
System image file is "disk0:hfr-os-mbi-3.6.2/mbihfr-rp.vm"

cisco CRS-8/S (7457) processor with 4194304K bytes of memory.
7457 processor at 1197Mhz, Revision 1.2

17 Packet over SONET/SDH network interface(s)
1 DWDM controller(s)
17 SONET/SDH Port controller(s)
8 TenGigabitEthernet/IEEE 802.3 interface(s)
2 Ethernet/IEEE 802.3 interface(s)
1019k bytes of non-volatile configuration memory.
38079M bytes of hard disk.
981440k bytes of ATA PCMCIA card at disk 0 (Sector size 512 bytes).

Configuration register on node 0/0/CPU0 is 0x102
Boot device on node 0/0/CPU0 is mem:

!--- output truncated

The following example identifies a Cisco 12404 router that is running Cisco IOS XR Software Release 3.7.1:

RP/0/0/CPU0:GSR#show version

Cisco IOS XR Software, Version 3.7.1[00]
Copyright (c) 2008 by Cisco Systems, Inc.

ROM: System Bootstrap, Version 12.0(20051020:160303) SOFTWARE
Copyright (c) 1994-2005 by cisco Systems,  Inc.

GSR uptime is 3 weeks, 6 days, 3 hours, 20 minutes
System image file is "disk0:c12k-os-mbi-3.7.1/mbiprp-rp.vm"

cisco 12404/PRP (7457) processor with 2097152K bytes of memory.
7457 processor at 1266Mhz, Revision 1.2

1 Cisco 12000 Series Performance Route Processor
1 Cisco 12000 Series - Multi-Service Blade Controller
1 1 Port ISE Packet Over SONET OC-48c/STM-16 Controller (1 POS)
1 Cisco 12000 Series SPA Interface Processor-601/501/401
3 Ethernet/IEEE 802.3 interface(s)
1 SONET/SDH Port controller(s)
1 Packet over SONET/SDH network interface(s)
4 PLIM QoS controller(s)
8 FastEthernet/IEEE 802.3 interface(s)
1016k bytes of non-volatile configuration memory.
1000496k bytes of disk0: (Sector size 512 bytes).
65536k bytes of Flash internal SIMM (Sector size 256k).

Configuration register on node 0/0/CPU0 is 0x2102
Boot device on node 0/0/CPU0 is disk0:

!--- output truncated

Additional information about Cisco IOS XR Software release naming conventions is available in "White Paper: Cisco IOS Reference Guide" at the following link:

Additional information about Cisco IOS XR Software time-based release model is available in "White Paper: Guidelines for Cisco IOS XR Software" at the following link:

SSHv1 is configured in Cisco IOS XR Software with the configuration command ssh server enable. The device is vulnerable if it is running an affected Cisco IOS XR Software release and has SSHv1 enabled.

The following example shows a device that is running Cisco IOS XR Software that is configured with SSHv1:

(Router)# show running-config | inc ssh

ssh server vrf default

If the command returns "ssh server v2", then the SSH server is not configured to accept SSHv1 connections and the device is not vulnerable.

Products Confirmed Not Vulnerable

The following Cisco products are confirmed not vulnerable:

  • Cisco IOS Software
  • Cisco IOS XR Software without SSH enabled
  • Cisco IOS XR Software configured to accept only SSHv2 connections

No other Cisco products are currently known to be affected by this vulnerability.


This vulnerability affects Cisco IOS XR devices that are running affected software releases and are configured to accept SSHv1 connections. When an SSHv1 connection is made to the SSH server that is running on a Cisco IOS XR device, a file is created in the /tmp directory. This file begins with the text "sshd_lock" and may not be properly removed when the session ends. Multiple connections may consume all available space in the /tmp filesystem and cause the system to crash, leading to a denial of service condition.

This vulnerability is documented in Cisco Bug ID CSCtd64417 ( registered customers only) and has been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2011-0949.

Vulnerability Scoring Details

Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.

CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.

Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.

Cisco has provided an FAQ to answer additional questions regarding CVSS at .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at .

SSHv1 may leave /tmp/sshd_lock files

Calculate the environmental score of CSCtd64417

CVSS Base Score - 7.8

Access Vector

Access Complexity


Confidentiality Impact

Integrity Impact

Availability Impact







CVSS Temporal Score - 6.4


Remediation Level

Report Confidence





Successful exploitation of this vulnerability may cause the Cisco IOS XR device to crash, resulting in a denial of service condition.

Software Versions and Fixes

When considering software upgrades, also consult 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) or your contracted maintenance provider for assistance.

This bug was introduced in Cisco IOS XR Software release 3.6.2 and is fixed with SMU hfr-k9sec-3.6.2.CSCtd74795. The SMU ID for this fix in 3.6.2 is AA03656. This vulnerability has been fixed in 3.8.3, 3.9.1, and 4.0.0 for customers running later software versions. Software version 3.7 is not affected by this vulnerability.


SSHv1 can be disabled by configuring the SSH server to only accept SSHv2 connections. In order to configure a device to only accept SSHv2 connections, administrators can issue the command ssh server v2. Administrators should manually remove lock files after disabling SSHv1 or after the server is upgraded to a non-vulnerable version. The command run rm /tmp/sshd_lock* will delete any sshd_lock files on the system.

Obtaining Fixed Software

Cisco has released software updates that address these vulnerabilities. Prior to deploying software, customers should consult their maintenance provider or check the software for feature set compatibility and known issues specific to their environment.

Customers may only install and expect support for the feature sets they have purchased. By installing, downloading, accessing or otherwise using such software upgrades, customers agree to be bound by the terms of Cisco's software license terms found at , or as otherwise set forth at Downloads at .

Do not contact or for software upgrades

Customers with Service Contracts

Customers with contracts should obtain software through their regular update channels. For most customers, software patches and bug fixes should be obtained through the Software Center on Cisco's worldwide website at

Customers using Third Party Support Organizations

Customers whose Cisco products are provided or maintained through prior or existing agreements with third-party support organizations, such as Cisco Partners, authorized resellers, or service providers should contact that support organization for guidance and assistance with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix 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 or fix is the most appropriate for use in the intended network before it is deployed.

Customers without Service Contracts

Customers who purchase direct from Cisco but do not hold a Cisco service contract, and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain software patches and bug fixes by contacting the Cisco Technical Assistance Center (TAC). TAC contacts are as follows.

  • +1 800 553 2447 (toll free from within North America)
  • +1 408 526 7209 (toll call from anywhere in the world)
  • e-mail:

Customers should have the product serial number available and be prepared to provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.

Refer to for additional TAC contact information, including localized telephone numbers, and instructions and e-mail addresses for use in various languages.

Exploitation and Public Announcements

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

Customers encountering device crashes during normal network operations reported this vulnerability to Cisco.

Status of this Notice: Final


A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.


This advisory is posted on Cisco's worldwide website at:

In addition to worldwide web posting, a text version of this notice is clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail and Usenet news recipients.


Future updates of this advisory, if any, will be placed on Cisco's worldwide website, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the above URL for any updates.

Revision History

Revision 1.0


Initial public release.

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at