Mattermost versions prior to the listed fixes allow a bot-username validation gap during bot registration. An attacker who can create a user account with a predictable plugin bot username can masquerade as that bot and receive direct messages that are intended for the legitimate bot. This exposes the contents of private conversations between the bot and other users, thereby compromising confidentiality.

Mattermost Server versions 10.11.0 through 10.11.15, 10.11.16, 11.5.0 through 11.5.4, and 11.6.0 through 11.6.1 are vulnerable. These versions lack validation that a username returned during bot registration belongs to a bot account, which allows an attacker to intercept private messages sent by plugins via direct message channels.

The CVSS score of 5.3 indicates medium severity. The attacker only needs to create a non-privileged user account to exploit the flaw, so the barrier to exploitation is low and no elevated privileges are required. The EPSS score is not available, and the vulnerability is not listed in the CISA KEV catalog. Likely attack vectors include local or remote account creation on the Mattermost instance, with the attacker intercepting private messages intended for plugins. The impact is primarily confidentiality leakage with potential to disrupt plugin functionality.