{"id":"CVE-2026-98976","kind":"vulnerability","aliases":["CVE-2026-98976"],"related":[],"title":"Linux kernel: mount propagation lets a user namespace reveal covered files","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnamespace: keep the lock on a mount that a propagated copy is moved beneath\n\nattach_recursive_mnt() transfers MNT_LOCKED from the top mount to the\nmount that is moved beneath it with MOVE_MOUNT_BENEATH. This allows the\nowner of a user namespace to replace its locked /proc or its root. The\nmount beneath takes on the job of covering the underlying mountpoint\nallowing the top mount to be unmounted.\n\nConsider two mount namespaces:\n\n(H) The host H has a shared mount P with a secret in P/d\n(Z) Z is a user namespace made from M. Its copy of P receives\n    propagation from the host's P and its copy of M's cover on P/d is\n    locked Z's owner must not get to see P/d.\n\nNow (H) mounts X on P/d. This propagates into (Z). The copy of X lands\nbeneath (Z)'s locked cover. The locked property is transfered from (Z)'s\ncover to the copy of X propagated beneath it. The cover is now unlocked.\n\nNow (H) unmounts X again. The copy of X in (Z) gets unmounted and the\ncovering mount is left unlocked on top of P/d. (Z) can now unmount it:\n\n    Z: umount2(P/d)             = EINVAL            /* the cover is locked */\n    H: mount X on P/d, umount X                     /* both propagate into Z /*\n    Z: umount2(P/d)             = 0                 /* the cover is now unlocked */\n    Z: read P/d/secret          = \"covered-by-root\" /* secret revealed */\n\nSo only transfer the locked property to the mount beneath for mounts the\ncaller has placed. A propagated copy that lands beneath a locked mount\nis locked as well so that the mount at the bottom of the stack carries a\nlock the way every check expects. The mount on top of it remains locked\nto ensure that it keeps covering even if the propagated mount is\nunmounted again.","summary":"In the Linux kernel, mount propagation can unlock a locked mount covering a directory, so a user namespace owner can unmount the cover and read files underneath, such as a secret in P/d. The fix keeps the lock on the mount beneath a propagated copy so the cover stays locked.","severity":"medium","cvss":null,"epss":0.00198,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["containers","infrastructure","linux-kernel","user-namespaces"],"not_affected_if":["default-config"],"how_to_check":null,"fix":null,"status":"open","published_at":"2026-10-23T13:17:19+00:00","first_seen_at":"2026-10-23T21:57:33+00:00","triaged_at":"2026-10-25T04:56:21+00:00","updated_at":"2026-10-25T12:38:50+00:00","category":"infrastructure","confidence":0.6,"triage":{"at":"2026-10-25T04:56:21Z","model":"claude-haiku-5-5","output":{"fix":"unknown","reach":"local","title":"Linux kernel: mount propagation lets a user namespace reveal covered files","impact":"medium","summary":"In the Linux kernel, mount propagation can unlock a locked mount covering a directory, so a user namespace owner can unmount the cover and read files underneath, such as a secret in P/d. The fix keeps the lock on the mount beneath a propagated copy so the cover stays locked.","category":"infrastructure","nicknames":[],"who_cares":["linux-kernel","user-namespaces","containers"],"confidence":0.6,"needs_auth":"yes","worst_case":"namespace owner reads host files hidden under locked mounts","analyst_note":"Find the fixed kernel versions and affected ranges from the upstream kernel commit or the NVD entry.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["default-config"],"who_is_affected":"Systems that run Linux kernels with user namespaces and shared mount propagation, where a host mounts or unmounts filesystems that propagate into unprivileged user namespaces. Kernel versions and patched releases are not stated in the sources. Systems without user namespaces or without shared mount propagation are likely not exposed, but the sources do not confirm this.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000623,"input_tokens":3870,"output_tokens":472,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':13A,16A '-98976':14A,17A '/proc':134C '0':308C 'allow':123C,150C 'attach':100C 'beneath':67B,99C,116C,121C,140C,230C,250C,332C,344C 'bottom':358C 'caller':336C 'carri':362C 'check':368C 'consid':157C 'contain':18B 'copi':70B,96C,185C,197C,226C,246C,263C,341C 'cover':10A,37B,48B,73B,146C,201C,234C,243C,253C,272C,291C,310C,318C,383C 'covered-by-root':317C 'cve':12A,15A 'directori':39B 'einval':289C 'ensur':379C 'even':384C 'everi':367C 'expect':369C 'file':11A,51B 'fix':60B 'follow':81C 'get':211C,268C 'h':161C,164C,216C,258C,294C 'host':163C,192C 'infrastructur':19B 'job':144C 'keep':61B,87C,382C 'kernel':2A,22B,29B,79C 'land':229C,343C 'left':275C 'let':5A 'linux':1A,21B,28B,78C 'linux-kernel':20B 'lock':35B,63B,75B,89C,105C,133C,205C,233C,236C,293C,327C,346C,349C,364C,377C 'm':183C,199C 'made':181C 'mnt':102C,104C 'mount':3A,30B,36B,66B,92C,109C,112C,120C,139C,153C,159C,168C,217C,273C,295C,331C,334C,347C,355C,371C,388C 'mountpoint':149C 'move':98C,115C,119C 'must':209C 'namespac':8A,25B,43B,86C,129C,160C,180C 'owner':44B,125C,208C 'p':169C,187C,194C 'p/d':58B,174C,203C,214C,220C,280C,288C,298C,307C 'p/d/secret':316C 'place':338C 'propag':4A,31B,69B,95C,189C,222C,249C,302C,340C,387C 'properti':237C,328C 'read':50B,315C 'receiv':188C 'recurs':101C 'remain':376C 'replac':131C 'resolv':85C 'reveal':9A,322C 'root':137C,320C 'secret':56B,172C,321C 'see':213C 'share':167C 'stack':361C 'stay':74B 'take':141C 'top':108C,152C,278C,373C 'transfer':103C,239C,325C 'two':158C 'umount':299C 'umount2':287C,306C 'under':148C 'underneath':52B 'unlock':33B,256C,276C,313C 'unmount':46B,156C,259C,269C,284C,390C 'user':7A,24B,42B,128C,179C 'user-namespac':23B 'vulner':82C 'way':366C 'well':351C 'x':218C,228C,248C,260C,265C,296C,300C 'z':175C,176C,206C,224C,231C,241C,267C,281C,286C,304C,305C,314C","nicknames":[],"mitigations":[],"who_affected":"Systems that run Linux kernels with user namespaces and shared mount propagation, where a host mounts or unmounts filesystems that propagate into unprivileged user namespaces. Kernel versions and patched releases are not stated in the sources. Systems without user namespaces or without shared mount propagation are likely not exposed, but the sources do not confirm this.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-23T13:17:19+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98976","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98976","severity":null,"published_at":"2026-10-23T13:17:19+00:00","modified_at":"2026-10-23T13:17:19+00:00","first_seen_at":"2026-10-23T21:57:33+00:00","fetched_at":"2026-10-23T22:29:03+00:00","note":null},{"source":"epss","external_id":"CVE-2026-98976","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-98976","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-25T12:38:50+00:00","fetched_at":"2026-10-25T12:38:50+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"}