CVE-2026-25863: Contact Form 7 WordPress Plugin DoS Flaw

CVE-2026-25863 Overview

CVE-2026-25863 is a denial-of-service vulnerability in the Conditional Fields for Contact Form 7 WordPress plugin through version 2.6.7. The flaw resides in the Wpcf7cfMailParser class, where the hide_hidden_mail_fields_regex_callback() method reads an iteration count directly from user-supplied POST parameters. The plugin does not validate the value or enforce an upper bound. Unauthenticated attackers can submit an arbitrarily large integer through the REST API endpoint to trigger an unbounded loop executing multiple preg_replace() operations. The result is server memory exhaustion and PHP process termination. The issue is tracked under [CWE-1284] (Improper Validation of Specified Quantity in Input).

Critical Impact

Unauthenticated remote attackers can crash the PHP process and exhaust server memory by sending a single crafted REST API request, causing site-wide denial of service.

Affected Products

• Conditional Fields for Contact Form 7 WordPress plugin, all versions through 2.6.7
• WordPress installations exposing the plugin's REST API endpoint
• PHP backends serving Contact Form 7 forms with conditional logic enabled

Discovery Timeline

• 2026-05-04 - CVE-2026-25863 published to NVD
• 2026-05-05 - Last updated in NVD database

Technical Details for CVE-2026-25863

Vulnerability Analysis

The vulnerability resides in the Wpcf7cfMailParser class shipped with the Conditional Fields for Contact Form 7 plugin. The hide_hidden_mail_fields_regex_callback() method reads an iteration count from POST data submitted by the client. The plugin treats this attacker-controlled integer as a loop bound and uses it to drive repeated preg_replace() calls against email field content.

Because the value is neither sanitized, capped, nor type-checked beyond integer coercion, an attacker can submit an arbitrarily large number. Each loop iteration allocates memory and consumes CPU cycles for regular expression compilation and replacement. Sufficiently large values exhaust the PHP memory_limit and cause the worker process to terminate. Repeated requests starve the web server of available PHP-FPM workers and deny service to legitimate users.

The REST API endpoint that invokes this code path is reachable without authentication. No form submission state, nonce verification, or rate limiting prevents repeated abuse.

Root Cause

The root cause is missing input validation on a quantity field used as a loop counter. The plugin trusts a client-supplied value to control server-side iteration. Secure handling requires casting the value to a bounded integer, rejecting values above a sane maximum, and applying timeout or memory ceilings around the regex operations.

Attack Vector

The attack vector is network-based. An unauthenticated attacker sends an HTTP POST request to the plugin's REST API endpoint with a crafted parameter containing a large integer iteration count. The vulnerable callback enters the unbounded loop, and the PHP process consumes memory until it crashes. Repeated requests sustain the denial of service.

No special privileges, user interaction, or prior reconnaissance beyond identifying a WordPress site running the plugin are required. See the VulnCheck advisory on this DoS for additional context.

Detection Methods for CVE-2026-25863

Indicators of Compromise

• Spikes in PHP process memory usage correlated with POST requests to the Contact Form 7 conditional fields REST endpoint
• PHP fatal errors in web server logs referencing Allowed memory size exhausted inside Wpcf7cfMailParser
• Repeated 5xx responses from /wp-json/ routes associated with the plugin
• Unusually large integer values in POST parameters submitted to form-handling endpoints

Detection Strategies

• Inspect web server access logs for repeated POST requests to plugin REST endpoints originating from a small set of source IPs
• Alert on PHP-FPM worker crashes and memory_limit errors that coincide with traffic to WordPress endpoints
• Apply WAF rules that flag oversized numeric values in POST body parameters destined for /wp-json/contact-form-7/ and conditional-fields routes

Monitoring Recommendations

• Monitor PHP process memory and CPU consumption per request and baseline normal form submission profiles
• Track HTTP request rates per source IP against WordPress REST endpoints and rate-limit outliers
• Forward WordPress, PHP, and web server logs to a centralized logging platform for correlation and retention

How to Mitigate CVE-2026-25863

Immediate Actions Required

• Update the Conditional Fields for Contact Form 7 plugin to a version newer than 2.6.7 once the maintainers release a fix
• Restrict access to the plugin's REST API endpoint to authenticated users where business requirements allow
• Deploy WAF rules to reject POST requests containing oversized numeric values for the affected parameters
• Apply per-IP rate limiting to WordPress REST API routes used by the plugin

Patch Information

A fixed version had not been listed in the NVD entry at the time of publication. Administrators should consult the WordPress plugin page for release notes and install the first patched version as soon as it becomes available. Until a patch is available, treat the workarounds below as the primary control.

Workarounds

• Disable the Conditional Fields for Contact Form 7 plugin if conditional logic in forms is not required
• Lower PHP memory_limit and max_execution_time for the WordPress front-end to bound the impact of a single malicious request
• Place the WordPress site behind a reverse proxy or CDN with request size and rate-limit policies enforced at the edge
• Block public access to plugin REST routes at the web server layer when forms are only used by authenticated audiences

# Example NGINX configuration to rate-limit and size-cap requests to plugin REST routes
limit_req_zone $binary_remote_addr zone=cf7cf:10m rate=5r/s;

server {
    client_max_body_size 64k;

    location ~* /wp-json/contact-form-7/ {
        limit_req zone=cf7cf burst=10 nodelay;
        client_body_buffer_size 16k;
        proxy_pass http://php_backend;
    }
}