CVE-2026-34090 Overview
CVE-2026-34090 is an information disclosure vulnerability in the Wikimedia Foundation CheckUser extension for MediaWiki. The flaw exposes sensitive information to an unauthorized actor [CWE-200] and affects CheckUser versions from 1.45.0 before 1.45.2. The CheckUser extension is a privileged tool used by trusted MediaWiki users to investigate IP addresses and user agents tied to accounts, making any data exposure within it a meaningful operational risk. Exploitation requires high privileges and some user interaction, which limits the practical attack surface but does not eliminate it.
Critical Impact
An authenticated actor with elevated privileges can access sensitive CheckUser data not intended for their role, undermining the confidentiality controls that protect investigator workflows on MediaWiki installations.
Affected Products
- Wikimedia Foundation CheckUser extension 1.45.0
- Wikimedia Foundation CheckUser versions before 1.45.2
- MediaWiki installations running the affected CheckUser releases
Discovery Timeline
- 2026-05-11 - CVE-2026-34090 published to NVD
- 2026-05-14 - Last updated in NVD database
Technical Details for CVE-2026-34090
Vulnerability Analysis
The vulnerability is classified under [CWE-200] Exposure of Sensitive Information to an Unauthorized Actor. In affected CheckUser releases, data managed by the extension can be reached by an actor who should not be able to read it. The CheckUser extension processes IP address history, user agent strings, and related metadata for MediaWiki accounts. Any unintended exposure of this dataset weakens the privacy guarantees that MediaWiki communities depend on when granting CheckUser rights.
The issue is network-reachable and requires user interaction along with high privileges to trigger. The scope of impact is limited to confidentiality, with no reported integrity or availability impact. The EPSS data indicates a low probability of opportunistic exploitation in the near term, consistent with the privilege and interaction requirements.
Root Cause
The root cause is improper enforcement of access controls on sensitive data inside the CheckUser extension code path. Vendor tracking is maintained in the Wikimedia Task Discussion, which serves as the authoritative reference for the defect and its remediation in version 1.45.2.
Attack Vector
An authenticated user with high privileges interacts with the affected CheckUser workflow over the network. The interaction causes the extension to return data that should be restricted, allowing the actor to obtain information outside their authorized view. No public proof-of-concept or exploit code is available at the time of publication.
No verified exploitation code is available. Refer to the Wikimedia Task Discussion for technical specifics tied to the fix.
Detection Methods for CVE-2026-34090
Indicators of Compromise
- Unexpected access to CheckUser query endpoints by accounts that do not normally perform investigations.
- Spikes in CheckUser log entries that do not correspond to known cases or community requests.
- MediaWiki instances still reporting CheckUser version 1.45.0 or 1.45.1 after the patch window.
Detection Strategies
- Audit the MediaWiki cu_log and cu_private_event tables for queries that fall outside documented investigator activity.
- Compare installed CheckUser extension version against 1.45.2 across all MediaWiki nodes using extension.json or the Special:Version page.
- Correlate authentication logs with CheckUser access events to identify privileged sessions that retrieved data without an associated case ticket.
Monitoring Recommendations
- Forward MediaWiki application logs and CheckUser audit logs to a centralized log platform for retention and review.
- Alert on any new account being added to the checkuser group, and review historical group membership changes.
- Track outbound responses from CheckUser endpoints for anomalous response sizes that may indicate bulk data retrieval.
How to Mitigate CVE-2026-34090
Immediate Actions Required
- Upgrade the CheckUser extension to version 1.45.2 or later on all MediaWiki installations.
- Review the checkuser user group membership and remove accounts that no longer require the privilege.
- Rotate credentials for any privileged account suspected of misuse during the exposure window.
Patch Information
The vendor has resolved the issue in CheckUser1.45.2. Track the remediation and any follow-up notes in the Wikimedia Task Discussion. Apply the upgrade through the standard MediaWiki extension update process and verify the running version after deployment.
Workarounds
- Temporarily restrict the checkuser group to a minimal set of trusted administrators until the patch is applied.
- Disable the CheckUser extension on non-essential MediaWiki instances until they are upgraded to 1.45.2.
- Increase audit log review cadence for CheckUser actions during the remediation window.
# Verify the installed CheckUser extension version on a MediaWiki host
php maintenance/run.php CheckExtension --extension=CheckUser
# Example upgrade workflow using git for the extension directory
cd extensions/CheckUser
git fetch --tags
git checkout 1.45.2
php ../../maintenance/run.php update.php --quick
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