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             AUSCERT External Security Bulletin Redistribution

      Cisco IOS XR Software Secure Shell Authentication Vulnerability
                                6 June 2019


        AusCERT Security Bulletin Summary

Product:           Cisco IOS XR Software Secure Shell
Publisher:         Cisco Systems
Operating System:  Cisco
Impact/Access:     Unauthorised Access            -- Existing Account
                   Provide Misleading Information -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2019-1842  

Original Bulletin: 

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Cisco IOS XR Software Secure Shell Authentication Vulnerability

Priority:        Medium

Advisory ID:     cisco-sa-20190605-iosxr-ssh

First Published: 2019 June 5 16:00 GMT

Version 1.0:     Final

Workarounds:     No workarounds availableCisco Bug IDs:   CSCvo03672



CVSS Score:


  o 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:

Affected Products

  o Vulnerable Products

    This vulnerability affects Cisco IOS XR Software when the SSH server is

    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.


  o The impacts of this vulnerability when the SSH server is enabled are as

       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

    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.


  o There are no workarounds that address this vulnerability.

Fixed Software

  o 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/

    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

    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

    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

    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.

Exploitation and Public Announcements

  o 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.


  o Cisco would like to thank Adam Haleen of Cellcom Wireless for finding and
    reporting this vulnerability.

Cisco Security Vulnerability Policy

  o 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.


  o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/

Revision History

  o +---------+---------------------------+---------+--------+---------------+
    | Version |        Description        | Section | Status |     Date      |
    | 1.0     | Initial public release.   | -       | Final  | 2019-June-05  |

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