CVE-2025-61766

Executive Summary

CVE-2025-61766 is a denial-of-service (DoS) vulnerability in the Bucket extension for MediaWiki. The flaw allows an unauthenticated attacker to trigger infinite recursion by crafting a query that uses the != comparator. This leads to exhaustion of PHP's call stack and increased memory consumption, potentially crashing the web server or making the wiki unavailable. The vulnerability affects all versions prior to 1.0.0. No public exploit code or proof-of-concept is currently available, but the attack vector is straightforward and requires no authentication.

Supply Chain & Package Security

The Bucket extension is a third-party MediaWiki extension, not part of the core MediaWiki distribution. Administrators who have manually installed this extension from sources like GitHub or the MediaWiki extension repository are at risk. The fix is available in version 1.0.0, which should be obtained from the official repository. There is no known compromise of the package distribution channel.

CVSS Breakdown & Threat Level

• CVSS Score: 6.5 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: High

The vulnerability is easy to exploit remotely without authentication, making it a significant availability risk for any MediaWiki instance using the vulnerable extension.

Deep Dive & Mechanics

The Bucket extension provides a structured data storage and retrieval mechanism for MediaWiki articles. It allows users to define "buckets" and query them using various comparators (e.g., =, !=, <, >). The vulnerability lies in the query parser's handling of the != (not equal) comparator.

When a user submits a query like bucket_field != value, the extension's code recursively processes the condition. Due to a missing base case or improper termination condition, the recursion does not stop when the comparison is evaluated. Instead, it repeatedly calls itself, each time consuming a stack frame and allocating memory for intermediate results. This leads to:

1. PHP Call Stack Exhaustion: PHP has a configurable maximum nesting level (xdebug.max_nesting_level or zend.max_input_nesting_level). Once exceeded, the script terminates with a fatal error.
2. Memory Exhaustion: Each recursive call may allocate memory for temporary data structures, leading to increased memory consumption. If the recursion is deep enough, the server may run out of memory, causing the web server process to be killed.

The attack can be triggered by a simple HTTP request to a MediaWiki page that processes a bucket query with the != comparator. No authentication is required if the wiki allows public access to pages using the Bucket extension.

Vulnerable vs. Patched Code

The vulnerable code likely resides in the query evaluation function of the Bucket extension. A simplified representation:

Vulnerable Code (Pre-1.0.0):

function evaluateCondition($field, $comparator, $value) {
    if ($comparator == '!=') {
        // Missing base case - recursively calls itself
        return !evaluateCondition($field, '=', $value);
    }
    // ... other comparators
}

Patched Code (1.0.0):

function evaluateCondition($field, $comparator, $value) {
    if ($comparator == '!=') {
        // Properly evaluate the not-equal condition without recursion
        $actualValue = getFieldValue($field);
        return $actualValue != $value;
    }
    // ... other comparators
}

The patch replaces the recursive call with a direct comparison, eliminating the infinite recursion.

Exploitation & Lab Setup

Red Team (Attack Simulation)

1. Identify Target: Find a MediaWiki instance running the Bucket extension. This can be done by checking the page source for references to extensions/Bucket or by probing known endpoints.
2. Craft Payload: Send a GET or POST request to a page that uses a bucket query with the != comparator. For example:

   https://target-wiki/index.php?title=Special:BucketQuery&field=test&comparator=!=&value=anything
   

3. Trigger DoS: The server will enter infinite recursion, consuming resources. Multiple concurrent requests can accelerate the denial of service.

Example curl command:

curl -v "https://target-wiki/index.php?title=Special:BucketQuery&field=test&comparator=!=&value=anything"

Blue Team (Defense & Detection)

1. Monitor Resource Usage: Watch for spikes in PHP-FPM or Apache worker memory and CPU usage.
2. Web Server Logs: Look for repeated requests to Special:BucketQuery with comparator=!= in the query string.
3. PHP Error Logs: Check for "Maximum function nesting level" or "Allowed memory size exhausted" errors.
4. Rate Limiting: Implement rate limiting on the Special:BucketQuery endpoint to prevent abuse.
5. WAF Rules: Create a rule to block requests containing comparator=!= in the query string.

Indicators of Compromise (IOCs)

• Network: HTTP requests to Special:BucketQuery with comparator=!= parameter.
• Logs: PHP error logs containing "Maximum function nesting level" or "Allowed memory size exhausted".
• System: High CPU and memory usage by PHP-FPM or Apache processes.
• File: Presence of the Bucket extension directory (extensions/Bucket) in the MediaWiki installation.

Detection Engineering

YARA Rule

rule CVE_2025_61766_Bucket_Extension {
    meta:
        description = "Detects vulnerable Bucket extension files for MediaWiki"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "unknown"
    strings:
        $vuln_func = "evaluateCondition" ascii wide nocase
        $vuln_recursion = "!evaluateCondition" ascii wide nocase
    condition:
        $vuln_func and $vuln_recursion
}

Sigma Rule

title: CVE-2025-61766 Bucket Extension DoS Attempt
id: 8f7a3b2c-1d4e-5f6a-7b8c-9d0e1f2a3b4c
status: experimental
description: Detects attempts to exploit infinite recursion in MediaWiki Bucket extension
author: Valters IT
date: 2025/04/10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-uri-query|contains: "comparator=!="
    condition: selection
falsepositives:
    - Legitimate use of the `!=` comparator in non-recursive contexts (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-61766 MediaWiki Bucket Extension DoS Attempt"; flow:to_server; content:"comparator=!="; http_uri; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade the Bucket extension to version 1.0.0 or later.
2. Workaround: If upgrading is not possible, disable the Bucket extension by removing or renaming the extensions/Bucket directory.
3. Mitigation: Implement a Web Application Firewall (WAF) rule to block requests containing comparator=!= in the query string.
4. Monitoring: Enable detailed logging for PHP errors and web server access logs to detect exploitation attempts.
5. Rate Limiting: Apply rate limiting to the Special:BucketQuery endpoint to reduce the impact of a DoS attack.

Valters IT
DevSecOps & Cyber Threat Intelligence