CVE-2026-75025 Overview
CVE-2026-75025 affects Mattermost Desktop App versions 6.2 and earlier. The desktop client does not sufficiently restrict server-rendered content from accessing local or private network resources. An attacker who can influence server-rendered content can trigger requests from the client to internal endpoints that would otherwise be unreachable. The issue maps to [CWE-346] (Origin Validation Error) and is tracked under Mattermost Advisory ID MMSA-2026-00698. Mattermost fixed the issue in version 6.2.2.0.
Critical Impact
The flaw enables server-side request forgery style access to local and private network resources from the desktop client, potentially exposing internal services during user interaction with malicious content.
Affected Products
- Mattermost Desktop App versions <= 6.2
- Fixed in Mattermost Desktop App 6.2.2.0
- Mattermost Advisory MMSA-2026-00698
Discovery Timeline
- 2026-09-16 - CVE-2026-75025 published to the National Vulnerability Database (NVD)
- 2026-09-16 - Last updated in NVD database
- Reported by game0v3r under the Mattermost responsible disclosure policy
Technical Details for CVE-2026-75025
Vulnerability Analysis
The Mattermost Desktop App renders content served by connected Mattermost servers. In vulnerable releases, the client does not enforce sufficient restrictions on where that server-rendered content can send subsequent requests. As a result, embedded resources or scripts can reach hosts on the loopback interface or the user's private network segment. This behavior expands the attack surface from the Mattermost server itself to any reachable internal service from the endpoint running the desktop client. Because user interaction is required, exploitation typically depends on convincing a user to load or interact with attacker-influenced content inside the client.
Root Cause
The root cause is an origin validation failure [CWE-346]. The desktop client did not enforce network egress restrictions on server-rendered content, allowing it to address resources on local (127.0.0.1, ::1) or RFC1918 private ranges that were never intended to be reachable through the client's rendering context.
Attack Vector
An attacker with the ability to place content on a Mattermost server, or to control a server the victim connects to, can craft rendered elements that issue requests to internal addresses. Because the client executes these requests with the endpoint's network position, responses may reveal internal services, metadata endpoints, or admin interfaces that are not exposed externally. Confidentiality is the primary impacted property; integrity and availability are not directly affected.
No verified public exploit code is available. See the Mattermost Security Updates advisory for further technical detail.
Detection Methods for CVE-2026-75025
Indicators of Compromise
- Outbound connections from the Mattermost Desktop process to loopback addresses (127.0.0.1, ::1) or RFC1918 ranges that do not correspond to normal client behavior.
- Unexpected HTTP requests originating from endpoints running the Mattermost Desktop App toward internal management interfaces or cloud metadata services.
- Server-side logs on internal services showing requests with User-Agent strings associated with the Electron-based Mattermost client.
Detection Strategies
- Monitor endpoint process telemetry for the Mattermost Desktop binary initiating network flows to private or link-local ranges.
- Correlate desktop client network activity with Mattermost server post events to identify content-driven request patterns.
- Alert on requests from user endpoints to cloud instance metadata endpoints such as 169.254.169.254.
Monitoring Recommendations
- Baseline normal Mattermost Desktop network destinations and flag deviations toward internal subnets.
- Ingest desktop endpoint DNS and connection telemetry into a centralized data lake for retrospective hunting.
- Track installed Mattermost Desktop versions across the fleet to identify hosts still running versions at or below 6.2.
How to Mitigate CVE-2026-75025
Immediate Actions Required
- Upgrade all Mattermost Desktop App installations to version 6.2.2.0 or later.
- Inventory endpoints running the Mattermost Desktop client and prioritize hosts with access to sensitive internal networks.
- Review Mattermost server audit logs for suspicious posts or attachments containing embedded external resources.
Patch Information
Mattermost addressed the issue in Desktop App version 6.2.2.0 under advisory MMSA-2026-00698. Details are available in the Mattermost Security Updates portal. Apply the vendor-supplied update through your normal software distribution channels.
Workarounds
- Restrict endpoint egress so that the Mattermost Desktop process cannot reach sensitive internal management interfaces or cloud metadata endpoints.
- Limit which Mattermost servers users may connect to, reducing exposure to untrusted server-rendered content.
- Educate users to avoid interacting with unexpected embedded content until the client is patched.
# Example host firewall rule to block Mattermost Desktop egress to cloud metadata
# (Linux nftables illustrative example)
nft add rule inet filter output meta skuid mattermost ip daddr 169.254.169.254 drop
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
