Mattermost CVE-2026-96259

Requires an authenticated System Administrator (PR:H) on the Mattermost server, plus a deployment where OAuth is used such that the token and userinfo endpoints are configurable/reachable. The specific defect is that Mattermost's internal-connection (SSRF) filter is not applied to OAuth endpoint requests, so an admin who sets the OAuth token or userinfo endpoint to an internal address causes the server to make and return those internal requests. Limiting factors: exploitation is confined to the already-highly-trusted System Administrator role, impact is limited to reading responses from internal addresses the server can route to (C:L/I:L, no availability impact), and it only matters where the server has network reachability to sensitive internal services worth targeting.

This is a genuinely LOW-priority issue despite the CWE-918 SSRF classification, and the vendor CVSS of 5.5 (MEDIUM) is well-calibrated. The single most important signal is PR:H in the CVSS vector (AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N): exploitation requires an authenticated System Administrator, which the description explicitly confirms ('allows a System Administrator to make the server issue requests'). A System Administrator is already the most-trusted principal in a Mattermost deployment and can typically alter configuration, plugins, and outbound integrations directly, so an SSRF primitive adds only marginal capability - this is closer to a defense-in-depth/hardening gap than a privilege-escalation or unauthenticated-RCE class flaw. No EPSS score, KEV listing, POC, or SSVC decision data was provided; per precision rules the exploitation status is 'no public exploit identified at time of analysis'. There is no conflict between the CVSS vector and the description - both agree on network vector, low complexity, and high-privilege prerequisite. The C:L/I:L/S:C impact reflects limited read of internal-service responses and limited ability to trigger internal requests across a scope boundary, not full compromise. Net: real-world urgency is low, driven almost entirely by the trusted-admin prerequisite; patch during normal maintenance rather than as an emergency.

A malicious or compromised Mattermost System Administrator configures OAuth token/userinfo endpoint URLs pointing at internal-only network addresses (e.g., cloud metadata services or internal admin APIs). Because the internal-connection filter is not applied to OAuth endpoint requests, the Mattermost server issues those requests and returns the responses, letting the admin read internal resources the server can reach but they otherwise could not. No public exploit code was identified; exploitation is straightforward (AC:L) but strictly gated behind System Administrator access (PR:H).