Mattermost EUVD-2026-43307

Exploitation requires the attacker to control or compromise a Mattermost remote cluster that has an established, administratively configured shared channel relationship with the target instance - this is a non-default feature requiring explicit setup by system administrators on both sides. The shared channels feature must be enabled and at least one channel must be actively shared with the attacker-controlled remote. Limiting factors: requires a pre-existing federation trust relationship and authenticated remote cluster credentials; unauthenticated external attackers cannot reach the sync handler; organizations that do not use shared channels are entirely unaffected. The attack is constrained to posts within channels shared with the malicious remote - posts in non-shared channels cannot be targeted.

The vendor-assigned and NVD-recorded CVSS 3.1 vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N yields a score of 4.3 (Medium), which accurately reflects the constrained blast radius: exploitation is network-reachable (AV:N), requires no special conditions beyond established federation (AC:L), but demands an authenticated remote cluster credential (PR:L). There is no confidentiality or availability impact - only limited integrity loss (I:L) confined to posts within shared channels. No EPSS data was provided; however, the requirement for a pre-existing, administratively configured remote cluster trust relationship substantially lowers real-world risk compared to the theoretical CVSS ceiling. No CISA KEV listing and no public POC further reduce urgency for organizations that do not use the shared channels feature at all. For organizations that do operate federated Mattermost deployments, the risk is concrete: a malicious or compromised peer instance could silently tamper with message history.

An attacker who controls or has compromised an authenticated Mattermost remote cluster federated with a target organization sends crafted inbound sync messages to the target's shared channel sync handler, embedding arbitrary post IDs belonging to local users or other remotes. Because the handler does not verify post ownership, the target instance processes the modification or deletion as legitimate, silently altering or erasing message history in the shared channel without any action required from the victim users.