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AUSCERT External Security Bulletin Redistribution
ESB-2019.2024
Cisco IOS XR Software Secure Shell Authentication Vulnerability
6 June 2019
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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:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190605-iosxr-ssh
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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
CVE-2019-1842
CWE-285
CVSS Score:
5.4 AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N/E:X/RL:X/RC:X
Summary
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:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
cisco-sa-20190605-iosxr-ssh
Affected Products
o 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.
Details
o 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.
Workarounds
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/
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.
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.
Source
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.
URL
o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
cisco-sa-20190605-iosxr-ssh
Revision History
o +---------+---------------------------+---------+--------+---------------+
| Version | Description | Section | Status | Date |
+---------+---------------------------+---------+--------+---------------+
| 1.0 | Initial public release. | - | Final | 2019-June-05 |
+---------+---------------------------+---------+--------+---------------+
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