CVE-2026-20113: Cisco IOS XE CRLF Injection Vulnerability

CVE-2026-20113 Overview

A CRLF (Carriage Return Line Feed) injection vulnerability exists in the web-based Cisco IOx application hosting environment management interface of Cisco IOS XE Software. This vulnerability allows an unauthenticated, remote attacker to inject arbitrary content into log files by exploiting insufficient validation of user input.

The vulnerability stems from improper sanitization of user-supplied data, enabling attackers to send specially crafted packets to affected devices. Successful exploitation can result in arbitrary log entry injection, manipulation of log file structure, and the ability to obscure legitimate log events—potentially hindering forensic analysis and incident response efforts.

Critical Impact

Attackers can inject arbitrary log entries, manipulate log file structures, and hide malicious activity by obscuring legitimate log events, compromising audit trail integrity.

Affected Products

• Cisco IOS XE Software with IOx application hosting environment enabled
• Cisco devices running the web-based IOx management interface
• Network infrastructure devices with IOx web management capabilities

Discovery Timeline

• 2026-03-25 - CVE-2026-20113 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-20113

Vulnerability Analysis

This vulnerability is classified under CWE-93 (Improper Neutralization of CRLF Sequences), commonly known as HTTP Response Splitting or CRLF Injection. The web-based IOx management interface fails to properly sanitize carriage return (\r or %0d) and line feed (\n or %0a) characters in user-supplied input before writing to log files.

When an attacker includes CRLF sequences in HTTP requests or other input fields processed by the IOx management interface, these characters are not stripped or escaped. This allows the attacker to terminate the current log entry prematurely and inject entirely new log entries that appear legitimate to administrators and security tools.

The network-accessible nature of the vulnerability means attackers can exploit it remotely without requiring authentication, making it particularly concerning for internet-facing Cisco IOS XE devices with IOx enabled.

Root Cause

The root cause is insufficient input validation in the Cisco IOx application hosting environment management interface. The affected code paths fail to sanitize or encode CRLF sequences before incorporating user input into log entries. This oversight allows attackers to break out of the intended log format and inject arbitrary content.

Proper mitigation would require the application to either strip CRLF characters entirely, encode them as safe representations (such as %0d%0a), or implement strict allow-list validation on all user-controllable input that reaches logging functions.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying Cisco IOS XE devices with the IOx web management interface exposed
2. Crafting HTTP requests containing CRLF sequences in headers, parameters, or other input fields
3. Sending these malicious requests to the target device
4. The injected CRLF characters cause the logging mechanism to create new log entries controlled by the attacker

This technique can be used to inject false log entries that implicate other systems or users, insert entries that match known-good patterns to mask malicious activity, corrupt log file structure to impede parsing by SIEM systems, or create confusion during incident response by polluting audit trails.

The vulnerability mechanism involves CRLF injection in HTTP parameters processed by the IOx management interface. For detailed technical information, refer to the Cisco Security Advisory.

Detection Methods for CVE-2026-20113

Indicators of Compromise

• Log entries containing unexpected CRLF sequences or URL-encoded variants (%0d, %0a, %0d%0a)
• Unusual log formatting anomalies or entries that break expected patterns
• Multiple log entries appearing with identical timestamps that seem artificially constructed
• Log entries that reference non-existent users, IP addresses, or activities

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block CRLF sequences in HTTP requests
• Monitor IOx management interface access logs for requests containing encoded newline characters
• Deploy network intrusion detection signatures targeting CRLF injection patterns in HTTP traffic
• Establish baseline log entry patterns and alert on structural deviations

Monitoring Recommendations

• Enable verbose logging on Cisco IOS XE devices and forward logs to a centralized SIEM for correlation
• Configure alerting for access to the IOx management interface from unexpected source IP addresses
• Implement log integrity monitoring to detect unauthorized modifications to log files
• Review IOx management interface access patterns regularly for anomalous activity

How to Mitigate CVE-2026-20113

Immediate Actions Required

• Review the Cisco Security Advisory for official patch information and affected version details
• Restrict network access to the IOx management interface using access control lists (ACLs)
• Implement network segmentation to limit exposure of management interfaces
• Enable additional logging and monitoring on affected devices pending patch deployment

Patch Information

Cisco has published a security advisory addressing this vulnerability. Administrators should consult the Cisco Security Advisory (cisco-sa-iox-crlf-NvgKTKJZ) for specific patch versions and upgrade guidance applicable to their deployment.

Workarounds

• Disable the IOx web management interface if not required for operations
• Implement strict IP-based access control lists limiting management interface access to trusted networks only
• Deploy a reverse proxy with input sanitization capabilities in front of the IOx management interface
• Use out-of-band management networks isolated from production traffic

# Example ACL configuration to restrict IOx management access
! Restrict access to IOx management interface to trusted management subnet
ip access-list extended IOX-MGMT-RESTRICT
 permit tcp 10.0.0.0 0.0.0.255 any eq 443
 deny tcp any any eq 443 log
!
interface GigabitEthernet0/0
 ip access-group IOX-MGMT-RESTRICT in