Cisco Security Advisory
Cisco IOS Software Login Enhancements Login Block Denial of Service Vulnerabilities
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CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
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Multiple vulnerabilities in the Login Enhancements (Login Block) feature of Cisco IOS Software could allow an unauthenticated, remote attacker to trigger a reload of an affected system, resulting in a denial of service (DoS) condition.
For more information about these vulnerabilities, see the Details section of this security advisory.
There are no workarounds that address both vulnerabilities.
This advisory is available at the following link:
https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-slogin
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Vulnerable Products
These vulnerabilities affect Cisco devices that are running Cisco IOS Software Release 15.4(2)T, 15.4(3)M, or 15.4(2)CG and later.
Determining the Cisco IOS Software Release
To determine which Cisco IOS Software release is running on a device, administrators can log in to the device, use the show version command in the CLI, and then refer to the system banner that appears. If the device is running Cisco IOS Software, the system banner displays text similar to Cisco Internetwork Operating System Software or Cisco IOS Software. The banner also displays the installed image name in parentheses, followed by the Cisco IOS Software release number and release name. Some Cisco devices do not support the show version command or may provide different output.
The following example shows the output of the command for a device that is running Cisco IOS Software Release 15.5(2)T1 and has an installed image name of C2951-UNIVERSALK9-M:
Router> show version Cisco IOS Software, C2951 Software (C2951-UNIVERSALK9-M), Version 15.5(2)T1, RELEASE SOFTWARE (fc1) Technical Support: http://www.cisco.com/techsupport Copyright (c) 1986-2015 by Cisco Systems, Inc. Compiled Mon 22-Jun-15 09:32 by prod_rel_team . . .For information about the naming and numbering conventions for Cisco IOS Software releases, see the Cisco IOS and NX-OS Software Reference Guide.
Products Confirmed Not Vulnerable
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The Cisco IOS Login Enhancements (Login Block) Feature
The Cisco IOS Login Enhancements (Login Block) feature allows users to enhance the security of a router by configuring options to automatically block further login attempts when a possible DoS attack is detected.
The login block and login delay options introduced by this feature can be configured for Telnet or SSH virtual connections. By enabling this feature, you can slow down “dictionary attacks” by enforcing a “quiet period” if multiple failed connection attempts are detected, thereby protecting the routing device from a type of DoS attack.
Further information on the Login Enhancements (Login Block) feature is available in the Cisco IOS Security Configuration Guide.
Vulnerability Details
Two vulnerabilities in the Login Enhancements (Login Block) feature of Cisco IOS Software could allow an unauthenticated, remote attacker to trigger a reload of an affected device, resulting in a denial of service (DoS) condition.
The vulnerabilities are not dependent on one another; exploitation of one of the vulnerabilities is not required to exploit the other vulnerability. In addition, a software release that is affected by one of the vulnerabilities may not be affected by the other vulnerability.
Details about the vulnerabilities are as follows.
Cisco IOS Software Login Block Denial of Service Vulnerability
A vulnerability in the Login Enhancements (Login Block) feature of Cisco IOS Software could allow an unauthenticated, remote attacker to trigger a reload of an affected system, resulting in a denial of service (DoS) condition.
The vulnerability is due to an attempt to free an area of memory that has not been previously allocated. An attacker could exploit this vulnerability by attempting to log in to an affected device via Secure Shell (SSH) or Telnet with invalid credentials multiple times.
This vulnerability is exploitable only if both of the following conditions are met:
- login quiet-mode access-class has been configured prior to configuring login block-for. These features are not enabled by default.
- The device has not been reloaded since that configuration occurred.
The Common Vulnerabilities and Exposures (CVE) ID for this vulnerability is: CVE-2018-0179
The Security Impact Rating (SIR) for this vulnerability is: Medium
The Cisco bug ID for this vulnerability is: CSCuy32360
Cisco IOS Software Login Block Denial of Service Vulnerability
A vulnerability in the Login Enhancements (Login Block) feature of Cisco IOS Software could allow an unauthenticated, remote attacker to trigger a reload of an affected system, resulting in a denial of service (DoS) condition.
The vulnerability is due to an attempt to free an area of memory that has not been previously allocated. An attacker could exploit this vulnerability by attempting to log in to an affected device via SSH or Telnet with invalid credentials multiple times while the administrator modifies the login block-for configuration.
This vulnerability is exploitable only if login block-for has been configured on an affected device. This feature is not enabled by default.
The CVE ID for this vulnerability is: CVE-2018-0180
The SIR for this vulnerability is: Medium
The Cisco bug ID for this vulnerability is: CSCuz60599
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The following conditions can indicate that a compromise has occurred.
Cisco IOS Software Login Block Denial of Service Vulnerability - CSCuy32360
Immediately before the device crashes and reloads, the device will log messages similar to the following example at least twice:
%SEC_LOGIN-4-LOGIN_FAILED: Login failed [user: USERNAME] [Source: xx.xx.xx.xx [localport: 22] [Reason: Login Authentication Failed] at 03:37:06 UTC Sat Feb 13 2016
%SEC_LOGIN-1-QUIET_MODE_ON: Still timeleft for watching failures is 52 secs, [user: USERNAME] [Source: xx.xx.xx.xx] [localport: 22] [Reason: Login Authentication Failed] [ACL: LOGIN_ACL] at 03:37:06 UTC Sat Feb 13 2016Exploitation of this vulnerability will cause an affected device to generate a crashinfo file. Contact the Cisco Technical Assistance Center (TAC) to review the output of the show tech-support command and the crashinfo file to determine whether the device has been compromised by exploitation of this vulnerability.
Cisco IOS Software Login Block Denial of Service Vulnerability - CSCuz60599
After this vulnerability has been exploited on an affected system, the device will log an error message about a bad chunk reference count in process Secure Login similar to the following example:
%SYS-2-CHUNKBADREFCOUNT: Bad chunk reference count, chunk 3F57D864 data 3F589728 refcount FFFFFFFF alloc pc 323635BC. -Process= "Secure Login", ipl= 0, pid= 388
Exploitation of this vulnerability will cause an affected device to generate a crashinfo file. Contact the Cisco Technical Assistance Center (TAC) to review the output of the show tech-support command and the crashinfo file to determine whether the device has been compromised by exploitation of this vulnerability.
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As a workaround for the vulnerability described in CSCuy32360, administrators can take one of the following actions:
- Configure login block-for prior to configuring login quiet-mode access-class
- Ensure that the device is reloaded after login block-for and login quiet-mode access-class are configured
There are no workarounds that address the vulnerability described in CSCuz60599.
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For detailed information about fixed software releases, consult the Cisco bug ID(s) at the top of this advisory.
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.
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In March 2022, the Cisco Product Security Incident Response Team (PSIRT) became aware of additional attempted exploitation of this vulnerability in the wild. Cisco continues to strongly recommend that customers upgrade to a fixed software release to remediate this vulnerability.
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These vulnerabilities were found during the resolution of Cisco TAC support cases.
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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 1.1 Updated exploitation information. Exploitation and Public Announcements Final 2022-DEC-15 1.0 Initial public release. - Final 2018-MAR-28
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