{"id":"CVE-2026-98977","kind":"vulnerability","aliases":["CVE-2026-98977"],"related":[],"title":"Linux kernel: automount lock handling lets a host mount's flags be locked","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnamespace: handle mount locking for automounts correctly\n\nIf mounts are propagated across user namespaces, attach_recursive_mnt()\nlocks every copy of the source mount to protect overmounts from\nvanishing and revealing the underlying files or directories.\n\nThe user namespace is taken from the caller's mount namespaces since\nthis is where the mounts end up. Except, that's not always true.\nAutomounts may legitimately get popped in by tasks located in a\ndifferent mount and user namespace during path lookup.\n\nThen check doesn't make sense at that point. The copy in the namespace\nof the parent - which may be the host's - is now locked and the host\ncannot change the flags of its own mounts anymore.\n\nHere's the reproducer:\n\n- host has debugfs mounted nosuid,nodev,noexec and shared\n- tracefs automount below it is not yet active\n- hand a child process a directory descriptor on that mount\n- child process enters auser and mount namespace\n- child process' namespace now holds a locked copy of the host's debugfs\n  mount that receives propagation from it\n- child process stats \"tracing/.\" through the descriptor\n- lookup runs on the host's mount so the automount lands below the host's mount\n- propagation puts a copy below the child's copy\n- both try to clear the flags on the mount they got, with a bind remount:\n\n    host, on its own automount:            MS_REMOUNT|MS_BIND = EPERM\n    child, on the copy in its namespace:   MS_REMOUNT|MS_BIND = 0\n\nSo the lock landed on the host's mount instead of the child's copy.\nCongrats. So we need to compare with the owner of the namespace the\nmount actually gets mounted on. For all regular cases that is the\ncaller's mount namespace and so nothing changes.\n\nDetached trees in anonymous mount namespaces by be handed over via\nSCM_RIGHTS or inherited in other ways on purpose so the attaching task's\nmount namespace is authoritative, not the creator of the detached tree.","summary":"When mounts propagate across user namespaces, the kernel can lock the wrong copy of an automounted mount, so a host mount can no longer have its flags changed. The sources describe a local reproducer, but do not state a confirmed privilege escalation or data exposure.","severity":"unknown","cvss":null,"epss":0.00209,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["containers","infrastructure","linux-kernel","self-hosted"],"not_affected_if":["default-config"],"how_to_check":"- `uname -r` and compare against the fixed kernel version in your distribution's advisory\n- unknown","fix":"- Update the Linux kernel to a release that includes the upstream commit for CVE-2026-98977 (fixed version unknown from the sources)\n- unknown","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":0.4,"triage":{"at":"2026-10-25T04:56:21Z","model":"claude-haiku-5-5","output":{"fix":"- Update the Linux kernel to a release that includes the upstream commit for CVE-2026-98977 (fixed version unknown from the sources)\n- unknown","reach":"local","title":"Linux kernel: automount lock handling lets a host mount's flags be locked","impact":"low","summary":"When mounts propagate across user namespaces, the kernel can lock the wrong copy of an automounted mount, so a host mount can no longer have its flags changed. The sources describe a local reproducer, but do not state a confirmed privilege escalation or data exposure.","category":"infrastructure","nicknames":[],"who_cares":["linux-kernel","self-hosted","containers"],"confidence":0.4,"needs_auth":"yes","worst_case":"unknown impact on mount flags; local user may bypass mount restrictions","analyst_note":"Find the fixed kernel versions and any distro advisory for CVE-2026-98977 at https://nvd.nist.gov/vuln/detail/CVE-2026-98977.","how_to_check":"- `uname -r` and compare against the fixed kernel version in your distribution's advisory\n- unknown","needs_analyst":true,"not_affected_if":["default-config"],"who_is_affected":"Systems running a Linux kernel that includes the affected mount code, especially hosts that use user namespaces with propagated mounts such as debugfs or tracefs automounts. Hosts that do not use user namespaces or shared mount propagation are less likely to be exposed. The sources do not say more.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"unknown"},"cost_usd":0.000664,"input_tokens":3877,"output_tokens":552,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':15A,18A '-98977':16A,19A '0':326C 'across':31B,94C 'activ':221C 'actual':356C 'alway':142C 'anonym':378C 'anymor':200C 'attach':97C,397C 'auser':235C 'authorit':403C 'automount':3A,43B,88C,144C,215C,274C,309C 'bind':303C,313C,325C 'caller':126C,367C 'cannot':192C 'case':363C 'chang':55B,193C,374C 'check':164C 'child':224C,232C,239C,258C,287C,315C,339C 'clear':293C 'compar':347C 'confirm':67B 'congrat':342C 'contain':20B 'copi':40B,102C,173C,246C,284C,289C,318C,341C 'correct':89C 'creator':406C 'cve':14A,17A 'data':71B 'debugf':207C,251C 'describ':58B 'descriptor':228C,264C 'detach':375C,409C 'differ':155C 'directori':118C,227C 'doesn':165C 'end':136C 'enter':234C 'eperm':314C 'escal':69B 'everi':101C 'except':138C 'exposur':72B 'file':116C 'flag':11A,54B,195C,295C 'follow':78C 'get':147C,357C 'got':300C 'hand':222C,383C 'handl':5A,84C 'hold':243C 'host':8A,27B,47B,184C,191C,205C,249C,269C,278C,305C,333C 'infrastructur':21B 'inherit':389C 'instead':336C 'kernel':2A,24B,35B,76C 'land':275C,330C 'legitim':146C 'let':6A 'linux':1A,23B,75C 'linux-kernel':22B 'local':60B 'locat':152C 'lock':4A,13A,37B,86C,100C,188C,245C,329C 'longer':51B 'lookup':162C,265C 'make':167C 'may':145C,181C 'mnt':99C 'mount':9A,29B,44B,48B,85C,91C,106C,128C,135C,156C,199C,208C,231C,237C,252C,271C,280C,298C,335C,355C,358C,369C,379C,400C 'ms':310C,312C,322C,324C 'namespac':33B,83C,96C,121C,129C,159C,176C,238C,241C,321C,353C,370C,380C,401C 'need':345C 'nodev':210C 'noexec':211C 'nosuid':209C 'noth':373C 'overmount':109C 'owner':350C 'parent':179C 'path':161C 'point':171C 'pop':148C 'privileg':68B 'process':225C,233C,240C,259C 'propag':30B,93C,255C,281C 'protect':108C 'purpos':394C 'put':282C 'receiv':254C 'recurs':98C 'regular':362C 'remount':304C,311C,323C 'reproduc':61B,204C 'resolv':82C 'reveal':113C 'right':387C 'run':266C 'scm':386C 'self':26B 'self-host':25B 'sens':168C 'share':213C 'sinc':130C 'sourc':57B,105C 'stat':260C 'state':65B 'taken':123C 'task':151C,398C 'trace':261C 'tracef':214C 'tree':376C,410C 'tri':291C 'true':143C 'under':115C 'user':32B,95C,120C,158C 'vanish':111C 'via':385C 'vulner':79C 'way':392C 'wrong':39B 'yet':220C","nicknames":[],"mitigations":[],"who_affected":"Systems running a Linux kernel that includes the affected mount code, especially hosts that use user namespaces with propagated mounts such as debugfs or tracefs automounts. Hosts that do not use user namespaces or shared mount propagation are less likely to be exposed. The sources do not say more.","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-2026-98977","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98977","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-2026-98977","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-98977","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"}