{"id":"CVE-2026-93851","kind":"vulnerability","aliases":["CVE-2026-93851"],"related":[],"title":"Mattermost: forged mention reminder lets a post trigger plugin replacement and server code execution","description":"Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10 fail to validate the target post ID in the Agents plugin's mention reminder component, which allows an authenticated user who can post in a channel visible to a system administrator to cause the administrator's browser to send an unintended API request, resulting in replacement of an installed plugin and code execution on the server, via a crafted post containing a forged mention reminder that the administrator clicks. Mattermost Advisory ID: MMSA-2026-00742","summary":"Mattermost's Agents plugin does not validate the target post ID in its mention reminder component. A user who can post in a channel an administrator can see can plant a crafted post that, when the administrator clicks it, makes the administrator's browser replace an installed plugin, which leads to code execution on the server.","severity":"high","cvss":8.0,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["agents-plugin","mattermost","package","plugins","self-hosted"],"not_affected_if":["default-config"],"how_to_check":"- Check the Mattermost server version with `mattermost version` and compare it to the affected lines: 11.9.x up to 11.9.1, 11.8.x up to 11.8.5, 11.7.x up to 11.7.10.\n- Check whether the Agents plugin is installed and its version in System Console > Plugins > Plugin Management.","fix":"- Upgrade Mattermost to a release that includes the MMSA-2026-00742 fix: a fixed build for 11.9.x, 11.8.x or 11.7.x beyond the listed affected versions (exact fixed version unknown from the sources).\n- Update the Agents plugin to the fixed release that ships with that server build (exact version unknown from the sources).","status":"open","published_at":"2026-10-28T22:17:17+00:00","first_seen_at":"2026-10-28T22:35:35+00:00","triaged_at":"2026-10-28T22:36:42+00:00","updated_at":"2026-10-28T22:36:42+00:00","category":"package","confidence":0.6,"triage":{"at":"2026-10-28T22:36:42Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Mattermost to a release that includes the MMSA-2026-00742 fix: a fixed build for 11.9.x, 11.8.x or 11.7.x beyond the listed affected versions (exact fixed version unknown from the sources).\n- Update the Agents plugin to the fixed release that ships with that server build (exact version unknown from the sources).","reach":"internet","title":"Mattermost: forged mention reminder lets a post trigger plugin replacement and server code execution","impact":"high","summary":"Mattermost's Agents plugin does not validate the target post ID in its mention reminder component. A user who can post in a channel an administrator can see can plant a crafted post that, when the administrator clicks it, makes the administrator's browser replace an installed plugin, which leads to code execution on the server.","category":"package","nicknames":[],"who_cares":["mattermost","self-hosted","plugins","agents-plugin"],"confidence":0.6,"needs_auth":"yes","worst_case":"Attacker runs code on the Mattermost server via plugin replacement.","analyst_note":"Confirm the fixed versions for each affected line and the Agents plugin fixed release at the Mattermost advisory MMSA-2026-00742; the NVD entry lists no fix details.","how_to_check":"- Check the Mattermost server version with `mattermost version` and compare it to the affected lines: 11.9.x up to 11.9.1, 11.8.x up to 11.8.5, 11.7.x up to 11.7.10.\n- Check whether the Agents plugin is installed and its version in System Console > Plugins > Plugin Management.","needs_analyst":true,"not_affected_if":["default-config"],"who_is_affected":"Self-hosted or managed Mattermost servers on the affected 11.7, 11.8 and 11.9 lines that have the Agents plugin installed. The exposure depends on an authenticated user being able to post in a channel an administrator can see, and on an administrator clicking the crafted post. Instances on other version lines, or without the Agents plugin, are not listed as affected.","user_interaction":"yes","needs_special_setup":"yes","severity_if_unknown":"high"},"cost_usd":0.000715,"input_tokens":3434,"output_tokens":744,"severity_before":"high"},"analyst_status":null,"analyst_note":null,"search":"'-00742':171C '-2026':16A,19A,170C '-93851':17A,20A '11.7':94C '11.7.10':96C '11.8':91C '11.8.5':93C '11.9':88C '11.9.1':90C 'administr':55B,66B,71B,127C,131C,164C 'advisori':167C 'agent':22B,32B,106C 'agents-plugin':21B 'allow':113C 'api':138C 'authent':115C 'browser':73B,133C 'caus':129C 'channel':53B,122C 'click':67B,165C 'code':13A,81B,148C 'compon':45B,111C 'contain':157C 'craft':61B,155C 'cve':15A,18A 'execut':14A,82B,149C 'fail':97C 'forg':2A,159C 'host':29B 'id':40B,103C,168C 'instal':76B,145C 'lead':79B 'let':5A 'make':69B 'mattermost':1A,24B,30B,86C,166C 'mention':3A,43B,109C,160C 'mmsa':169C 'packag':25B 'plant':59B 'plugin':9A,23B,26B,33B,77B,107C,146C 'post':7A,39B,50B,62B,102C,119C,156C 'remind':4A,44B,110C,161C 'replac':10A,74B,142C 'request':139C 'result':140C 'see':57B 'self':28B 'self-host':27B 'send':135C 'server':12A,85B,152C 'system':126C 'target':38B,101C 'trigger':8A 'unintend':137C 'user':47B,116C 'valid':36B,99C 'version':87C 'via':153C 'visibl':123C 'x':89C,92C,95C","nicknames":[],"mitigations":[],"who_affected":"Self-hosted or managed Mattermost servers on the affected 11.7, 11.8 and 11.9 lines that have the Agents plugin installed. The exposure depends on an authenticated user being able to post in a channel an administrator can see, and on an administrator clicking the crafted post. Instances on other version lines, or without the Agents plugin, are not listed as affected.","tier":"B","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-28T22:17:17+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:H/A:H","cvss_by":"responsibledisclosure@mattermost.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-93851","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-93851","severity":null,"published_at":"2026-10-28T22:17:17+00:00","modified_at":"2026-10-28T22:17:17+00:00","first_seen_at":"2026-10-28T22:35:35+00:00","fetched_at":"2026-10-28T22:35:35+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}