Cisco Security Advisory
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CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N/E:X/RL:X/RC:X
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A vulnerability in the Secure Shell (SSH) authentication function of Cisco IOS XR Software could allow an authenticated, remote attacker to successfully log in to an affected device using two distinct usernames.
The vulnerability is due to a logic error that may occur when certain sequences of actions are processed during an SSH login event on the affected device. An attacker could exploit this vulnerability by initiating an SSH session to the device with a specific sequence that presents the two usernames. A successful exploit could result in logging data misrepresentation, user enumeration, or, in certain circumstances, a command authorization bypass.See the Details section for more information.
There are no workarounds that address this vulnerability.
This advisory is available at the following link:
https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190605-iosxr-ssh
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Vulnerable Products
This vulnerability affects Cisco IOS XR Software when the SSH server is enabled.
For information about affected software releases, see the Fixed Software section in this advisory.
Products Confirmed Not Vulnerable
Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.
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The impacts of this vulnerability when the SSH server is enabled are as follows:
- Some logged events will be attributed to the impersonated user.
Example: Authenticated Attacker A who is assigned the operator task group exploits the vulnerability to impersonate Admin B who is assigned the sysadmin task group. In this case, the attacker could execute all commands that are assigned to the operator task group, but the commands would be reported as being issued by Admin B.
- Username and user task group enumeration can be performed.
Example: Authenticated Attacker A exploits the vulnerability to impersonate Admin B. If Attacker A issues the CLI command show user <options>, relevant information for Admin B will be returned. If the Admin B username does not exist, the command will not be executed.
Additionally, if the authentication, authorization, and accounting (AAA) command authorization feature is enabled with the configuration command aaa authorization commands <options>, AAA command authorization bypass can occur.
The vulnerability is due to a logic error that results in the authenticated username being tied to the correct task groups; however, the alternate username is tied to the command authorization requests. The attacker will always retain the task groups that they were assigned as part of the successful authenticated username.
The impact of this vulnerability if the command being executed by the authenticated attacker is allowed as part of their task group configuration and AAA command authorization is configured on the device, is as follows:
- The command will be sent to the AAA server to validate that the command is authorized. This authorization request will be sent as the impersonated username, which may allow the authenticated attacker to execute unauthorized commands.
Example: Authenticated Attacker A who is assigned the operator task group exploits the vulnerability to impersonate Admin B who is assigned the sysadmin task group. If Attacker A tries to execute a command that is not allowed by the operator task group, but is authorized to the sysadmin group, the command will not be authorized and is not sent to the AAA server for authorization.
Example: Authenticated Attacker A who is assigned the operator task group, but with command restrictions imposed via their AAA profile, exploits the vulnerability to impersonate Admin B who is assigned the operator task group, but with no command restrictions imposed via their AAA profile. If Attacker A tries to execute a command that is allowed by the operator task group, but not allowed in their AAA profile, the command will be authorized because the command authorization check is performed using the Admin B username.
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Cisco will release free software updates that address the vulnerability described in this advisory. Customers may only install and expect support for software versions and feature sets for which they have purchased a license. By installing, downloading, accessing, or otherwise using such software upgrades, customers agree to follow the terms of the Cisco software license: https://www.cisco.com/c/en/us/products/end-user-license-agreement.html
Additionally, customers may only download software for which they have a valid license, procured from Cisco directly, or through a Cisco authorized reseller or partner. In most cases this will be a maintenance upgrade to software that was previously purchased. Free security software updates do not entitle customers to a new software license, additional software feature sets, or major revision upgrades.
When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories and Alerts page, to determine exposure and a complete upgrade solution.
In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.
Customers Without Service Contracts
Customers who purchase directly from Cisco but do not hold a Cisco service contract and customers who make purchases through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain upgrades by contacting the Cisco TAC: https://www.cisco.com/c/en/us/support/web/tsd-cisco-worldwide-contacts.html
Customers should have the product serial number available and be prepared to provide the URL of this advisory as evidence of entitlement to a free upgrade.
Fixed Releases
Cisco is releasing the following software maintenance upgrades (SMUs) that address this vulnerability:
SMU ID Platform Release AA15541 ASR9K-PX 6.1.4 AA15542 CRS-PX 6.1.4 AA15514 ASR9K-PX 6.3.3 AA15515 NCS5500 6.3.3 AA15516 NCS6K 6.3.3 AA15534 NCS6K 6.4.2 AA15526 ASR9K-PX
6.5.2 AA15527 ASR9K-X64
6.5.2 AA15528 NCS5500 6.5.2 This vulnerability is first fixed in Cisco IOS XR Software Releases 6.5.3, 6.6.2, and 7.0.0.
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The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.
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Cisco would like to thank Adam Haleen of Cellcom Wireless for finding and reporting this vulnerability.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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Show LessVersion Description Section Status Date 1.0 Initial public release. — Final 2019-June-05
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