CVE-2026-20125: Cisco IOS HTTP Server DoS Vulnerability

CVE-2026-20125 Overview

A vulnerability exists in the HTTP Server feature of Cisco IOS Software and Cisco IOS XE Software Release 3E that could allow an authenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to improper validation of user-supplied input when processing HTTP requests.

Critical Impact

An attacker with valid credentials can send malformed HTTP requests to trigger a watchdog timer expiration, causing the device to reload and disrupting network availability.

Affected Products

• Cisco IOS Software with HTTP Server feature enabled
• Cisco IOS XE Software Release 3E with HTTP Server feature enabled

Discovery Timeline

• 2026-03-25 - CVE CVE-2026-20125 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-20125

Vulnerability Analysis

This denial of service vulnerability stems from improper handling of malformed HTTP requests in the HTTP Server component of Cisco IOS and IOS XE Software. When a specially crafted HTTP request is sent to an affected device, the input validation routines fail to properly sanitize user-supplied data, leading to unexpected behavior in the device's watchdog timer mechanism.

The watchdog timer is a hardware or software mechanism designed to reset the device if the system becomes unresponsive. In this case, the malformed input triggers a condition that causes the watchdog timer to expire prematurely, forcing the device to reload. This reload results in a temporary loss of all services handled by the affected device, impacting network connectivity and availability.

The vulnerability is classified under CWE-228 (Improper Handling of Syntactically Invalid Structure), indicating that the HTTP Server fails to properly process structurally malformed input.

Root Cause

The root cause of this vulnerability is improper validation of user-supplied input in the HTTP Server feature. The HTTP request parsing logic does not adequately verify the structure and content of incoming requests, allowing malformed data to reach internal processing routines that cannot handle syntactically invalid structures. This causes system instability that triggers the watchdog timer mechanism.

Attack Vector

The attack vector is network-based, requiring the attacker to have authenticated access to the target device. The attacker must possess a valid user account on the affected Cisco device and be able to send HTTP requests to the HTTP Server feature. By crafting malformed HTTP requests with invalid structural elements, the attacker can repeatedly cause the device to reload, effectively creating a persistent denial of service condition.

The exploitation flow involves:

1. Authenticating to the target Cisco device using valid credentials
2. Sending specially crafted malformed HTTP requests to the HTTP Server
3. The improper input validation allows the malformed request to trigger internal errors
4. The watchdog timer expires due to the resulting system instability
5. The device reloads, causing service disruption

For technical details on the vulnerability mechanism, refer to the Cisco Security Advisory.

Detection Methods for CVE-2026-20125

Indicators of Compromise

• Unexpected device reloads or reboots on Cisco IOS or IOS XE devices with HTTP Server enabled
• Watchdog timer expiration events recorded in device logs
• Unusual patterns of HTTP requests from authenticated users targeting the management interface
• Multiple reload events in short succession indicating potential exploitation attempts

Detection Strategies

• Monitor device logs for watchdog timer expiration events and unexpected reload patterns
• Implement network intrusion detection rules to identify anomalous HTTP traffic patterns targeting Cisco management interfaces
• Review authentication logs for unusual access patterns from legitimate user accounts
• Deploy SentinelOne Singularity platform for network visibility and anomaly detection across infrastructure devices

Monitoring Recommendations

• Enable detailed HTTP Server logging on affected Cisco devices to capture request details
• Configure SNMP traps or syslog alerts for device reload events
• Establish baseline metrics for normal HTTP traffic to management interfaces to identify deviations
• Implement centralized log aggregation for correlation of events across multiple devices

How to Mitigate CVE-2026-20125

Immediate Actions Required

• Review the Cisco Security Advisory for specific patching guidance
• Restrict HTTP Server access to trusted management networks and IP addresses only
• Audit user accounts with HTTP Server access and remove unnecessary privileges
• Consider disabling the HTTP Server feature if not required for operations
• Implement network segmentation to limit access to device management interfaces

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific software version information and upgrade paths. Apply the appropriate fixed software release for your device platform and configuration.

Workarounds

• Disable the HTTP Server feature on affected devices if not operationally required using no ip http server and no ip http secure-server commands
• Implement access control lists (ACLs) to restrict HTTP/HTTPS access to management interfaces from trusted IP addresses only
• Use out-of-band management networks to isolate device management traffic from production networks
• Enable Control Plane Policing (CoPP) to rate-limit HTTP traffic to the device

# Configuration example - Restrict HTTP Server access via ACL
access-list 10 permit 10.10.10.0 0.0.0.255
ip http access-class 10
ip http secure-server
no ip http server