CVE-2025-9076: Mattermost Server Information Disclosure Flaw

CVE-2025-9076 is an information disclosure vulnerability in Mattermost Server affecting shared channel synchronization. Malicious remote clusters can access sensitive user data through unsanitized objects. This article covers technical details, affected versions, impact assessment, and mitigation strategies.

CVE-2025-9076 Overview

CVE-2025-9076 is an information disclosure vulnerability in Mattermost Server versions 10.10.x through 10.10.1. The flaw resides in the shared channel membership synchronization logic, which fails to sanitize user objects before transmission to remote clusters. A malicious or compromised remote cluster can retrieve sensitive user data through unsanitized user fields. The vulnerability is classified as Missing Authorization [CWE-862] and affects deployments where shared channels are enabled.

Critical Impact
Remote clusters participating in shared channels can access sensitive user information they should not be authorized to view, breaching data confidentiality across federated Mattermost instances.

Affected Products

Mattermost Server versions 10.10.0 through 10.10.1
Mattermost Server instances with shared channels feature enabled
Deployments federating with remote Mattermost clusters

Discovery Timeline

2025-09-15 - CVE-2025-9076 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-9076

Vulnerability Analysis

Mattermost supports shared channels, a federation feature that allows users on separate Mattermost Server instances to collaborate in the same channel. Membership synchronization propagates user objects between the local cluster and remote clusters that participate in the shared channel.

In affected versions, the synchronization routine transmits user objects without stripping fields that should remain private to the local instance. A remote cluster receiving these objects can read data beyond what the shared channel context requires. This constitutes an authorization failure at the data-serialization layer rather than at the API boundary.

Exploitation requires authenticated participation as a remote cluster peer. An attacker who controls or compromises a federated Mattermost instance can passively harvest user data whenever membership synchronization events occur.

Root Cause

The root cause is missing authorization enforcement during object serialization [CWE-862]. The shared channel service does not apply a sanitization filter to user structures before they leave the local trust boundary. Fields intended for internal use are serialized alongside the fields required for federation, exposing them to any remote peer subscribed to the shared channel.

Attack Vector

The attack vector is network-based and requires low privileges, specifically an authenticated remote cluster relationship. No user interaction is required. Exploitation is limited to confidentiality impact; integrity and availability of the target server are not affected. See the Mattermost Security Updates advisory for vendor-provided technical detail.

Detection Methods for CVE-2025-9076

Indicators of Compromise

Unexpected shared channel synchronization events from unfamiliar or newly established remote clusters.
Elevated volumes of membership sync API calls from a specific remote cluster identifier.
Remote cluster peers registered without an approved federation change ticket.

Detection Strategies

Audit the RemoteClusters table in the Mattermost database to inventory all federated peers and validate each entry against approved partners.
Review Mattermost server logs for SharedChannel and remotecluster service entries, correlating sync events with expected federation activity.
Compare running Mattermost Server versions across the fleet against the vulnerable range 10.10.0 to 10.10.1.

Monitoring Recommendations

Forward Mattermost application logs to a centralized logging or SIEM platform and alert on new remote cluster registrations.
Monitor outbound network connections from Mattermost Server hosts to remote cluster endpoints and baseline expected peers.
Track user object payload sizes in federation traffic; unexplained increases can indicate over-disclosure of user fields.

How to Mitigate CVE-2025-9076

Immediate Actions Required

Upgrade Mattermost Server to a fixed version as listed on the Mattermost Security Updates page.
Inventory all shared channel remote cluster relationships and remove any peer that is not explicitly required.
Rotate remote cluster invitation tokens after patching to invalidate any previously issued credentials.

Patch Information

Mattermost has published fixes through its security update process. Administrators should consult the Mattermost Security Updates advisory to identify the specific fixed release and apply the upgrade to all affected instances running 10.10.0 through 10.10.1.

Workarounds

Disable the shared channels feature in the Mattermost System Console until the server is patched.
Terminate active remote cluster relationships that are not strictly required for business operations.
Restrict inbound and outbound network access for the Mattermost remote cluster service to a known allowlist of federated peer IP addresses.

bash

# Configuration example: disable shared channels via config.json
# Set ExperimentalSettings.EnableSharedChannels to false, then restart Mattermost
jq '.ExperimentalSettings.EnableSharedChannels = false | .ExperimentalSettings.EnableRemoteClusterService = false' \
  /opt/mattermost/config/config.json > /tmp/config.json.new \
  && mv /tmp/config.json.new /opt/mattermost/config/config.json \
  && systemctl restart mattermost