CVE-2018-0179

What is Cisco IOS Login Enhancements (Login Block)?

Login Enhancements, commonly called Login Block, is a Cisco IOS feature that protects device management access by detecting patterns consistent with dictionary or brute-force login attempts and temporarily blocking further login attempts from offending sources. It is a security control in its own right, making a vulnerability in the feature itself a case of the defense becoming an attack surface.

Overview

CVE-2018-0179 is a medium-severity denial-of-service vulnerability in the Login Block feature of Cisco IOS Software. An unauthenticated, remote attacker can send a sequence of crafted login attempts to an affected device that triggers a fault in the Login Block logic, causing the device to reload and resulting in a denial-of-service condition.

Technical Details

The vulnerability is classified under CWE-399 (resource management errors). Because Login Block's job is to actively track and act on incoming authentication attempts, it processes attacker-controlled input (usernames, attempt timing, source addresses) before those attempts are ever validated — an unauthenticated attacker can shape that input specifically to trigger the flaw, without needing valid credentials, only the ability to attempt logins against a management-facing service.

Discovery

Cisco identified and fixed the issue internally as part of the March 2018 semiannual IOS security advisory bundle. This CVE was published together with an essentially identical companion vulnerability, CVE-2018-0180, affecting the same feature.

Exploitation Context

CISA's KEV catalog listing confirms this vulnerability has been exploited in real-world attacks. A denial-of-service bug in a brute-force protection feature is a particularly convenient target for attackers: crashing the very mechanism meant to slow down credential-guessing attempts can both disrupt the device and, depending on configuration, potentially clear existing login-attempt tracking state.

Remediation

1. Identify Cisco IOS devices with Login Block/Login Enhancements configured and confirm their current software version.
2. Apply the Cisco-published fixed software release from the corresponding security advisory.
3. Restrict management-plane access (SSH, Telnet, HTTP/HTTPS) to trusted administrative networks via management-plane protection or ACLs, reducing exposure regardless of this specific bug.
4. Monitor for unexpected device reloads correlated with login attempt activity.
5. Patch CVE-2018-0180 at the same time, since it affects the same feature and was fixed in the same release.