A critical weakness in Cisco’s IOS and IOS XE platforms allows an unauthenticated adversary to bypass TACACS+ authentication, potentially granting full administrative control of affected routers and switches.
Tracked as CVE-2025-20160 and carrying a CVSS 3.1 score of 8.1, the flaw stems from improper validation of the TACACS+ shared secret configuration.
Cisco has not observed active exploitation but strongly urges immediate patching.
Overview of the Vulnerability
The vulnerability arises when TACACS+ is enabled on a device without setting a shared secret.
In such cases, the software neglects to verify the presence of a secret before processing authentication messages.
An attacker positioned on the network path, whether through man-in-the-middle tactics or direct network access, can read unencrypted TACACS+ packets or impersonate the TACACS+ server by sending crafted responses.
Given the lack of secret verification, the malicious responses bypass the authentication mechanism entirely.
| CVE | Affected Products | CVSS 3.1 Score |
|---|---|---|
| CVE-2025-20160 | Cisco IOS and IOS XE Software with TACACS+ configured without a shared secret | 8.1 (High) |
By exploiting this flaw, an attacker can gain unauthorized visibility into credentials and configuration data or achieve full device control, posing severe risks to network security and operational continuity.
Impact and Risk
Any Cisco device running vulnerable releases of IOS or IOS XE with misconfigured TACACS+ is at risk.
The potential impacts include:
- Unauthorized Disclosure: Attackers can intercept clear-text TACACS+ authentication payloads containing usernames and passwords, exposing privileged credentials.
- Authentication Bypass: Crafted server responses grant full administrative access, enabling configuration modification, payload delivery, and pivoting within the network.
- Operational Disruption: With root-level control, adversaries can disrupt routing, disable security controls, or introduce backdoors, severely impacting availability and integrity.
Despite the high severity, no public exploits or widespread malicious use have been reported to date.
However, the combination of high-impact outcomes and ease of exploitation elevates CVE-2025-20160 to a top remediation priority.
Mitigation and Recommendations
Cisco has released fixed IOS and IOS XE versions—detailed in the advisory’s “Fixed Software” section and urges all customers to upgrade without delay.
As an interim measure, administrators must audit their TACACS+ configurations to ensure every server entry includes a valid shared secret.
Verify that each TACACS server line is immediately followed by a key directive.
After confirming proper secret configuration, schedule an upgrade to a fixed release in accordance with your maintenance window.
Regular configuration audits and adherence to secure default settings will help prevent similar issues in the future.
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