Cisco IOS Software contains a vulnerability that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition.
The vulnerability is due to errors in the reverse SSH feature. An unauthenticated, remote attacker could exploit this vulnerability to cause the targeted device to reset, resulting in a DoS condition.
Cisco confirmed the vulnerability in a security advisory and released updated software.
Indicators of Compromise
Cisco has published a list of affected Cisco IOS Software releases in the security advisory. The Vendor Announcements section of this alert contains a link to the advisory.
The vulnerability is due to errors in the SSH version 2 reverse SSH feature. This feature is often used for dial-out modems.
An unauthenticated, remote attacker could exploit this vulnerability by attempting to access the device via a reverse SSH login with a crafted username. Processing the crafted username could cause the device to reset. Sustained attacks could result in a DoS condition.
To exploit this vulnerability, the attacker would need to attempt to authenticate to the targeted system using a crafted login. To achieve this objective, the attacker may need access to trusted, internal network resources. This access requirement reduces the exposure of this vulnerability.
Cisco indicates through the CVSS score that functional exploit code exists; however, the code is not known to be publicly available.
Administrators are advised to apply the appropriate updates.
Administrators may consider using IP-based access control lists (ACLs) to allow only trusted systems to access the affected devices.
Administrators may consider enabling SSH version 1 on affected devices. Customers should be aware of the limitations and vulnerabilities of SSH version 1 protocol before applying this workaround.
Administrators may consider using a vty access class to allow only trusted systems to access SSH on the affected devices.
Administrators may consider using the Control Plane Policing (CoPP) feature to allow only trusted systems reach the route processor (RP) on the affected devices.
Administrators are advised to allow only trusted users to have network access.
Administrators may consider disabling the SSH server feature on trusted networks.
Administrators are advised to monitor affected systems.
Cisco customers with active contracts can obtain updates through the Software Center at the following link: Cisco. Cisco customers without contracts can obtain upgrades by contacting the Cisco Technical Assistance Center at 1-800-553-2447 or 1-408-526-7209 or via e-mail at firstname.lastname@example.org.
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Cisco Multivendor Vulnerability Alerts respond to vulnerabilities identified in third-party vendors' products. These alerts contain information compiled from diverse sources and provide comprehensive technical descriptions, objective analytical assessments, workarounds and practical safeguards, and links to vendor advisories and patches.