CVE-2026-2717: HTTP Headers Plugin DoS Vulnerability

CVE-2026-2717 Overview

The HTTP Headers plugin for WordPress contains a CRLF Injection vulnerability affecting all versions up to and including 1.19.2. This vulnerability arises from insufficient sanitization of custom header name and value fields before they are written to the Apache .htaccess file via the insert_with_markers() function. Authenticated attackers with Administrator-level access can inject arbitrary newline characters and additional Apache directives into the .htaccess configuration file through the 'Custom Headers' settings, potentially causing Apache configuration parse errors and site-wide denial of service.

Critical Impact

Successful exploitation enables malicious administrators to corrupt the .htaccess configuration, leading to Apache configuration parse errors and potential site-wide denial of service affecting all website visitors.

Affected Products

• HTTP Headers Plugin for WordPress versions up to and including 1.19.2
• WordPress sites using Apache web server with .htaccess support
• WordPress installations with the HTTP Headers plugin Custom Headers feature enabled

Discovery Timeline

• April 22, 2026 - CVE-2026-2717 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-2717

Vulnerability Analysis

This CRLF (Carriage Return Line Feed) Injection vulnerability exists in the HTTP Headers WordPress plugin's handling of custom header configurations. The plugin fails to properly sanitize user-supplied input in the custom header name and value fields before incorporating them into Apache .htaccess directives. When an authenticated administrator configures custom HTTP headers through the plugin's settings interface, the unsanitized values are passed to WordPress's insert_with_markers() function and written directly to the .htaccess file.

The vulnerability is classified under CWE-93 (Improper Neutralization of CRLF Sequences). The attack requires network access and high-level privileges (Administrator role), limiting the attack surface to compromised admin accounts or malicious insiders. However, the impact can be severe—a corrupted .htaccess file can render the entire website inaccessible, effectively causing a denial of service condition.

Root Cause

The root cause of this vulnerability lies in the plugin's failure to properly validate and sanitize newline characters (\r\n, \r, or \n) from user-controlled input fields before writing them to the .htaccess configuration file. The vulnerable code paths exist at lines 745 and 1098 of the http-headers.php file in version 1.19.2. Without proper input sanitization, CRLF sequences embedded in header names or values break out of the intended context and allow injection of arbitrary Apache configuration directives.

Attack Vector

The attack vector requires an authenticated user with Administrator privileges to access the HTTP Headers plugin settings within the WordPress admin dashboard. By navigating to the 'Custom Headers' configuration and entering specially crafted values containing CRLF sequences, an attacker can inject arbitrary newlines into the .htaccess file. This can introduce malformed Apache directives or additional configuration statements that cause the web server to fail when parsing the configuration, resulting in HTTP 500 errors across the entire site.

The attack exploits insufficient input validation in the custom header configuration feature. When malicious input containing CRLF characters is saved, the insert_with_markers() function writes these characters verbatim to .htaccess, corrupting the file structure and potentially introducing unintended Apache directives.

Detection Methods for CVE-2026-2717

Indicators of Compromise

• Unexpected newline characters or malformed entries in the WordPress .htaccess file
• Apache error logs showing configuration parse errors or syntax errors related to .htaccess
• Sudden HTTP 500 errors affecting the entire WordPress site
• Unusual modifications to .htaccess entries between the HTTP Headers plugin markers

Detection Strategies

• Monitor .htaccess file integrity using file integrity monitoring (FIM) solutions
• Review WordPress admin activity logs for suspicious changes to HTTP Headers plugin settings
• Implement web application firewall (WAF) rules to detect CRLF sequences in HTTP Header plugin configuration requests
• Configure alerts for Apache configuration errors in server logs

Monitoring Recommendations

• Enable detailed WordPress audit logging to track administrator actions in the HTTP Headers plugin
• Set up real-time alerts for changes to .htaccess files on WordPress installations
• Monitor for sudden increases in HTTP 500 errors that could indicate configuration corruption
• Review server access logs for administrator login activity from unusual IP addresses

How to Mitigate CVE-2026-2717

Immediate Actions Required

• Update the HTTP Headers plugin to the latest patched version when available
• Review current custom header configurations for suspicious or unexpected entries
• Audit administrator account access and remove unnecessary admin privileges
• Implement strong authentication mechanisms such as two-factor authentication for admin accounts

Patch Information

Security researchers have identified the vulnerable code at lines 745 and 1098 in the HTTP Headers plugin. Administrators should monitor the WordPress plugin repository for an updated version that addresses this CRLF injection issue. Additional details are available in the Wordfence Vulnerability Report.

Workarounds

• Temporarily disable the HTTP Headers plugin Custom Headers feature until a patch is available
• Restrict WordPress administrator access to trusted users only
• Implement a Web Application Firewall (WAF) to filter requests containing CRLF characters in plugin configuration endpoints
• Create regular backups of the .htaccess file to enable quick recovery if corruption occurs

# Configuration example - backup and restore .htaccess
# Create a backup of the current .htaccess file
cp /var/www/html/.htaccess /var/www/html/.htaccess.backup.$(date +%Y%m%d)

# Set restrictive permissions to prevent unauthorized modifications
chmod 644 /var/www/html/.htaccess

# Monitor for changes to .htaccess
inotifywait -m -e modify /var/www/html/.htaccess