CVE-2026-10103 Overview
CVE-2026-10103 affects Mattermost Server across multiple release trains. The vulnerability resides in the shared channel inbound sync handler, which fails to verify post ownership before processing modification and deletion requests. An authenticated remote cluster can send crafted sync messages referencing arbitrary post IDs in channels shared with that remote. This allows the remote cluster to modify or delete posts authored by local users or by other remote clusters. Mattermost tracks this issue under advisory MMSA-2026-00689 and classifies it under CWE-639: Authorization Bypass Through User-Controlled Key.
Critical Impact
A federated remote cluster can tamper with or delete posts it does not own within any channel shared with that remote, undermining message integrity across federated Mattermost deployments.
Affected Products
- Mattermost Server 11.7.x through 11.7.2
- Mattermost Server 11.6.x through 11.6.4
- Mattermost Server 10.11.x through 10.11.19
Discovery Timeline
- 2026-07-13 - CVE-2026-10103 published to NVD
- 2026-07-14 - Last updated in NVD database
Technical Details for CVE-2026-10103
Vulnerability Analysis
Mattermost supports shared channels, a federation feature that synchronizes messages between independent Mattermost clusters. When a remote cluster pushes updates through the inbound sync handler, the server processes post modification and deletion events using the post identifiers supplied in the sync payload. The handler does not validate that the referenced post actually belongs to the remote sending the request. Any authenticated remote paired with the local server can therefore target posts authored by local users or by unrelated remote clusters. The exploitation path requires only that the target post reside in a channel shared with the malicious remote.
Root Cause
The root cause is a missing authorization check on a user-controlled identifier, consistent with [CWE-639]. The inbound sync code path trusts the post ID supplied by the remote cluster and applies the requested mutation without cross-referencing the post's original author or origin remote. Ownership metadata exists on each post record but is not consulted before executing the write operation.
Attack Vector
Exploitation requires an attacker to control an authenticated remote cluster that shares at least one channel with the target Mattermost server. The attacker enumerates or guesses post IDs within that shared channel, then issues sync messages containing modification or deletion instructions for those IDs. The vulnerable handler accepts these instructions and applies them against the local database. No end-user interaction is required, and no privileges beyond an established remote cluster trust relationship are needed. The impact is limited to integrity of post content within shared channels; confidentiality and availability of the wider system are not directly affected.
Detection Methods for CVE-2026-10103
Indicators of Compromise
- Unexpected post edits or deletions in shared channels where the acting user is a remote system account.
- Sync events in Mattermost audit logs referencing post IDs whose original author does not belong to the sending remote cluster.
- User complaints about missing or altered messages that they authored in federated channels.
Detection Strategies
- Review the SharedChannelRemotes and post history tables for post mutations where the modifying remote does not match the original RemoteId of the post.
- Correlate inbound shared channel sync events with post ownership metadata to flag mismatches.
- Baseline the volume of post updates and deletions per remote cluster and alert on statistical deviations.
Monitoring Recommendations
- Forward Mattermost audit and application logs to a centralized analytics platform for retention and query.
- Enable verbose logging on the shared channel service to capture inbound sync payloads and the associated remote identifiers.
- Monitor the mattermost.com/security-updates feed for follow-up advisories tied to MMSA-2026-00689.
How to Mitigate CVE-2026-10103
Immediate Actions Required
- Upgrade Mattermost Server to a fixed release above 11.7.2, 11.6.4, or 10.11.19 as published in the Mattermost Security Updates advisory.
- Inventory all remote cluster trust relationships and remove any that are no longer required.
- Audit recent post modifications and deletions in shared channels to identify tampering that may have occurred before patching.
Patch Information
Mattermost has published fixed builds through advisory MMSA-2026-00689. Administrators should consult the Mattermost Security Updates page for the specific patched version aligned to their release train and apply the upgrade following the vendor's standard upgrade procedure.
Workarounds
- Temporarily unshare sensitive channels with untrusted remote clusters until the server is upgraded.
- Restrict shared channel federation to remotes operated by trusted administrators.
- Preserve database backups and audit logs prior to upgrade to support forensic review of any suspected tampering.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
