CVE-2025-10735: WordPress Mailchimp Plugin SSRF Vulnerability

CVE-2025-10735 Overview

CVE-2025-10735 is a Blind Server-Side Request Forgery (SSRF) vulnerability [CWE-918] affecting the Block For Mailchimp – Easy Mailchimp Form Integration plugin for WordPress. The flaw exists in the mcbSubmit_Form_Data() function and impacts all versions up to and including 1.1.12. Unauthenticated attackers can coerce the WordPress server to issue arbitrary web requests to attacker-controlled or internal destinations. This exposure enables adversaries to probe internal services, interact with cloud metadata endpoints, and modify information reachable from the host.

Critical Impact
Unauthenticated remote attackers can pivot through vulnerable WordPress hosts to reach internal-only services normally shielded from the public internet.

Affected Products

Block For Mailchimp – Easy Mailchimp Form Integration plugin for WordPress
All versions up to and including 1.1.12
WordPress sites using the vulnerable mcbSubmit_Form_Data() handler

Discovery Timeline

2025-10-01 - CVE-2025-10735 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-10735

Vulnerability Analysis

The vulnerability resides in the mcbSubmit_Form_Data() function used by the Block For Mailchimp plugin to process form submissions. The function accepts user-supplied input and issues outbound HTTP requests without validating the destination against an allow-list of trusted hosts. Because no authentication is required to reach the submission endpoint, any remote attacker can influence the target URL of the server-initiated request.

The resulting SSRF is classified as blind because the plugin does not return response bodies to the caller. Attackers can still infer results through timing differences, response codes, and side effects on internal systems. The vulnerability class is tracked as CWE-918: Server-Side Request Forgery.

Root Cause

The root cause is missing validation of the destination URL used by the plugin's Mailchimp API client. The mcbSubmit_Form_Data() handler forwards attacker-controlled data into an outbound request without enforcing scheme restrictions, host allow-listing, or blocks against private IP ranges such as 127.0.0.0/8, 10.0.0.0/8, 169.254.0.0/16, and 172.16.0.0/12.

Attack Vector

An unauthenticated attacker sends a crafted HTTP request to the plugin's public form-submission endpoint. The plugin then issues an outbound request from the WordPress host to an attacker-specified location. Attackers commonly abuse this to reach cloud instance metadata services, internal admin interfaces, databases, and reverse-proxied applications that trust requests from the WordPress server. The Wordfence Vulnerability Report and the WordPress Plugin API File document the affected code path.

Detection Methods for CVE-2025-10735

Indicators of Compromise

Outbound HTTP or HTTPS connections from the WordPress PHP worker to non-Mailchimp destinations, particularly private IP ranges or cloud metadata endpoints like 169.254.169.254.
Unexpected POST requests to the plugin's AJAX action associated with mcbSubmit_Form_Data() originating from unfamiliar client IPs.
Web server access logs showing high volumes of form submissions from a single source with abnormal parameter values.

Detection Strategies

Inspect web server and WAF logs for POST requests to admin-ajax.php invoking the Mailchimp submit action with URL-like values in unexpected fields.
Correlate inbound plugin requests with outbound connections initiated by the PHP process to identify SSRF-driven traffic.
Deploy egress monitoring that flags WordPress hosts reaching internal services, metadata IPs, or non-approved external hosts.

Monitoring Recommendations

Enable network flow logging on WordPress infrastructure and alert on traffic to RFC1918 destinations or cloud metadata addresses.
Monitor PHP-FPM and web server processes for anomalous outbound socket activity relative to a known baseline.
Track plugin version inventory across managed WordPress sites and alert when block-for-mailchimp versions at or below 1.1.12 are present.

How to Mitigate CVE-2025-10735

Immediate Actions Required

Update the Block For Mailchimp plugin to a version above 1.1.12 as soon as a patched release is available from the WordPress Plugin Overview.
Deactivate and remove the plugin if a fixed version is not yet installed and the Mailchimp form functionality is not business-critical.
Restrict outbound network access from the WordPress host so it can only reach approved Mailchimp API endpoints.

Patch Information

Refer to the WordPress Change Log Entry for the vendor's fix commit and the Wordfence Vulnerability Report for additional remediation guidance. Administrators should apply the newest available plugin release and verify the version reported in the WordPress plugins dashboard.

Workarounds

Place a Web Application Firewall (WAF) rule in front of the site to block requests to the vulnerable AJAX action containing URL parameters targeting internal or metadata addresses.
Configure host-based firewall rules that deny egress from the PHP process to private IP ranges and cloud metadata IPs such as 169.254.169.254.
Enforce outbound proxying through a filtering egress gateway that only permits requests to *.api.mailchimp.com and other explicitly required destinations.

bash

# Example iptables egress restriction for a WordPress host running as user www-data
iptables -A OUTPUT -m owner --uid-owner www-data -d 169.254.169.254 -j DROP
iptables -A OUTPUT -m owner --uid-owner www-data -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -m owner --uid-owner www-data -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -m owner --uid-owner www-data -d 192.168.0.0/16 -j DROP
iptables -A OUTPUT -m owner --uid-owner www-data -p tcp --dport 443 -d api.mailchimp.com -j ACCEPT