{"id":"CVE-2026-16251","kind":"vulnerability","aliases":["CVE-2026-16251"],"related":[],"title":"Mattermost: unbounded image proxy buffering lets remote servers exhaust memory","description":"Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to limit the size of remote image responses buffered in memory during local image proxy direct fetches, which allows a remote attacker to cause memory exhaustion on the server via a crafted oversized image response.. Mattermost Advisory ID: MMSA-2026-00726","summary":"Mattermost's local image proxy buffers remote image responses in memory without a size limit during direct fetches. A remote attacker who serves a crafted, oversized image can exhaust server memory and cause a denial of service.","severity":"medium","cvss":6.5,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["mattermost","other","self-hosted"],"not_affected_if":["managed-hosting","unaffected-version"],"how_to_check":"- Check the Mattermost server version with `mattermost version` or the System Console and compare it to the affected ranges: 11.10.0 and earlier in 11.10.x, 11.7.9 and earlier in 11.7.x, 11.9.1 and earlier in 11.9.x, 11.8.5 and earlier in 11.8.x.\n- Confirm whether the local image proxy direct fetch path is in use (unknown from the sources whether it is enabled by default).","fix":"- Upgrade Mattermost to a fixed release in the 11.10.x, 11.9.x, 11.8.x or 11.7.x line beyond the affected ranges listed above (exact fixed versions: unknown from the sources).\n- Review the MMSA-2026-00726 advisory from Mattermost for the patched version numbers.","status":"open","published_at":"2026-10-16T08:17:12+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","triaged_at":"2026-10-25T03:57:57+00:00","updated_at":"2026-10-25T03:57:57+00:00","category":"other","confidence":0.6,"triage":{"at":"2026-10-25T03:57:57Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade Mattermost to a fixed release in the 11.10.x, 11.9.x, 11.8.x or 11.7.x line beyond the affected ranges listed above (exact fixed versions: unknown from the sources).\n- Review the MMSA-2026-00726 advisory from Mattermost for the patched version numbers.","reach":"internet","title":"Mattermost: unbounded image proxy buffering lets remote servers exhaust memory","impact":"low","summary":"Mattermost's local image proxy buffers remote image responses in memory without a size limit during direct fetches. A remote attacker who serves a crafted, oversized image can exhaust server memory and cause a denial of service.","category":"other","nicknames":[],"who_cares":["mattermost","self-hosted"],"confidence":0.6,"needs_auth":"unknown","worst_case":"Server memory is exhausted, causing a denial of service.","analyst_note":"","how_to_check":"- Check the Mattermost server version with `mattermost version` or the System Console and compare it to the affected ranges: 11.10.0 and earlier in 11.10.x, 11.7.9 and earlier in 11.7.x, 11.9.1 and earlier in 11.9.x, 11.8.5 and earlier in 11.8.x.\n- Confirm whether the local image proxy direct fetch path is in use (unknown from the sources whether it is enabled by default).","needs_analyst":false,"not_affected_if":["managed-hosting","unaffected-version"],"who_is_affected":"Self-hosted Mattermost servers running one of the affected version ranges are exposed, since the flaw is in server-side image fetching. Teams on Mattermost Cloud or on versions outside the listed ranges are not affected according to the sources, which do not say more.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"medium"},"cost_usd":0.000665,"input_tokens":3409,"output_tokens":649,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-00726':115C '-16251':13A,16A '-2026':12A,15A,114C '..':109C '11.10':61C '11.10.0':63C '11.7':64C '11.7.9':66C '11.8':70C '11.8.5':72C '11.9':67C '11.9.1':69C 'advisori':111C 'allow':92C 'attack':42B,95C 'buffer':5A,27B,82C 'caus':54B,97C 'craft':46B,105C 'cve':11A,14A 'denial':56B 'direct':38B,89C 'exhaust':9A,50B,99C 'fail':73C 'fetch':39B,90C 'host':21B 'id':112C 'imag':3A,25B,29B,48B,80C,87C,107C 'let':6A 'limit':36B,75C 'local':24B,86C 'mattermost':1A,17B,22B,59C,110C 'memori':10A,32B,52B,84C,98C 'mmsa':113C 'overs':47B,106C 'proxi':4A,26B,88C 'remot':7A,28B,41B,79C,94C 'respons':30B,81C,108C 'self':20B 'self-host':19B 'serv':44B 'server':8A,51B,102C 'servic':58B 'size':35B,77C 'unbound':2A 'version':60C 'via':103C 'without':33B 'x':62C,65C,68C,71C","nicknames":[],"mitigations":[],"who_affected":"Self-hosted Mattermost servers running one of the affected version ranges are exposed, since the flaw is in server-side image fetching. Teams on Mattermost Cloud or on versions outside the listed ranges are not affected according to the sources, which do not say more.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-16T08:17:12+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","cvss_by":"responsibledisclosure@mattermost.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-16251","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-16251","severity":null,"published_at":"2026-10-16T08:17:12+00:00","modified_at":"2026-10-16T19:23:18+00:00","first_seen_at":"2026-10-23T21:57:12+00:00","fetched_at":"2026-10-23T22:28:55+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"}