CVE-2026-58027: AbuseFilter Information Disclosure Flaw

CVE-2026-58027 Overview

CVE-2026-58027 is an information disclosure vulnerability in the Wikimedia Foundation AbuseFilter extension for MediaWiki. The flaw resides in the includes/Api/QueryAbuseFilters.php component and exposes sensitive information to unauthorized actors [CWE-200]. An authenticated attacker with low privileges can query the AbuseFilter API and retrieve data that should remain restricted to privileged users.

The vulnerability affects AbuseFilter versions prior to 1.46.0, 1.45.4, 1.44.6, and 1.43.9. AbuseFilter is a widely deployed MediaWiki extension used across Wikipedia and other Wikimedia projects to detect and prevent abusive editing behavior.

Critical Impact
Authenticated users can retrieve confidential AbuseFilter data through the API, potentially disclosing filter logic intended to remain hidden from non-privileged accounts.

Affected Products

Wikimedia Foundation AbuseFilter versions before 1.46.0
Wikimedia Foundation AbuseFilter versions before 1.45.4 (1.45.x branch)
Wikimedia Foundation AbuseFilter versions before 1.44.6 (1.44.x branch) and 1.43.9 (1.43.x branch)

Discovery Timeline

2026-07-01 - CVE-2026-58027 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-58027

Vulnerability Analysis

The vulnerability originates in the AbuseFilter API query handler, specifically includes/Api/QueryAbuseFilters.php. This module responds to MediaWiki API requests for AbuseFilter metadata and filter definitions. The handler fails to enforce appropriate access controls before returning filter data to the requester.

AbuseFilter allows administrators to mark filters as private, hiding filter logic from ordinary users. Private filters often contain patterns that reveal spam-detection heuristics, sockpuppet identification rules, or other sensitive moderation logic. Disclosure of these patterns enables attackers to craft edits that evade detection.

Exploitation requires network access to the MediaWiki API and low-privilege authenticated credentials. No user interaction is required. The impact is limited to confidentiality; the flaw does not permit modification of data or denial of service.

Root Cause

The root cause is missing or insufficient authorization checks within the QueryAbuseFilters API module. The code returns AbuseFilter records without verifying that the requesting user holds the permissions required to view protected filter attributes. This design flaw falls under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor.

Attack Vector

An attacker authenticates to the target MediaWiki instance with a standard user account. The attacker then issues a query to the AbuseFilter API endpoint. The vulnerable handler returns filter data that should be restricted, allowing the attacker to enumerate filter definitions or metadata that administrators intended to keep confidential.

Refer to the Wikimedia Task Discussion for technical details on the affected code path.

Detection Methods for CVE-2026-58027

Indicators of Compromise

Unexpected volume of API requests to the list=abusefilters endpoint from non-administrator accounts.
API access patterns showing enumeration of filter IDs sequentially by low-privilege users.
Log entries from QueryAbuseFilters.php indicating access to filter records outside the caller's normal activity profile.

Detection Strategies

Review MediaWiki API access logs for authenticated requests to AbuseFilter query endpoints originating from accounts without moderator or administrator rights.
Correlate AbuseFilter API queries with subsequent edit activity to identify accounts probing filter logic before attempting evasion.
Baseline normal AbuseFilter API usage and alert on deviations, particularly bulk enumeration patterns.

Monitoring Recommendations

Enable verbose logging on the MediaWiki API layer for all AbuseFilter-related modules.
Forward web server and MediaWiki application logs to a centralized platform for retention and analysis.
Monitor authentication events for new accounts that immediately query AbuseFilter endpoints, which may indicate reconnaissance.

How to Mitigate CVE-2026-58027

Immediate Actions Required

Upgrade AbuseFilter to version 1.46.0, 1.45.4, 1.44.6, or 1.43.9 depending on the deployed branch.
Audit AbuseFilter API access logs for prior unauthorized queries against private filters.
Review private filter definitions and rotate any detection logic that may have been exposed.

Patch Information

Wikimedia Foundation has released fixed versions of the AbuseFilter extension. Administrators should install AbuseFilter 1.46.0 on current MediaWiki deployments, or apply 1.45.4, 1.44.6, or 1.43.9 for supported maintenance branches. See the Wikimedia Task Discussion for patch references.

Workarounds

Restrict access to the MediaWiki API using web server access controls where feasible until the patch is applied.
Temporarily disable the AbuseFilter extension if private filter confidentiality is critical and patching cannot occur immediately.
Limit account creation and require elevated verification for accounts that need API access on sensitive wikis.

bash

# Upgrade AbuseFilter via git for MediaWiki extensions directory
cd /var/www/mediawiki/extensions/AbuseFilter
git fetch --tags
git checkout 1.46.0
php /var/www/mediawiki/maintenance/update.php