{"id":"CVE-2026-98993","kind":"vulnerability","aliases":["CVE-2026-98993"],"related":[],"title":"Linux kernel: lazily unmounted mounts can be re-attached with a stale unmount flag","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnamespace: don't inherit MNT_UMOUNT in clone_mnt()\n\nMNT_UMOUNT gets raised on a mount that umount_tree() or umount_one() has\ntaken out of its namespace. It's never cleared. __detach_mounts()\nunhooks a mount that carries it instead of unmounting it.\n\ndisconnect_mount() lets an unmounted child stay attached only if its\nparent carries it. propagate_umount() takes it as the sign that a mount\nis going to be unmounted when it decides what else can be unmounted,\nwhere a chain of locked mounts ends and where a for reparenting an\novermount.\n\nclone_mnt() copies the flags of the source and only masks\nMNT_INTERNAL_FLAGS which doesn't include MNT_UMOUNT. A mount that was\nlazily unmounted but is kept alive by a file descriptor can still be\ncopied when its root is an nsfs or pidfs file because may_copy_tree()\naccepts those before checking whether the source is mounted at all:\n\n  mount --bind /proc/self/ns/net /tmp/x\n  exec 3< /tmp/x\n  umount -l /tmp/x\n  mount --bind /proc/self/fd/3 /tmp/y\n\nThe mount on /tmp/y is attached to the caller's namespace with\nMNT_UMOUNT already set and every copy of it inherits the flag again.\n\nThat causes a number of bugs:\n\n- Unlink its mountpoint from a mount namespace where it isn't one.\n  __detach_mounts() takes it for a mount that was unmounted earlier,\n  unhooks it from its parent and drops the reference the namespace\n  holds on it. It stays in the namespace with one reference too few\n  and as its own parent, so it can't be unmounted through the file\n  descriptor anymore and the namespace never finds it again. When the\n  namespace goes away it's leaked and points to a freed struct\n  mnt_namespace that may_change_propagation() and anon_ns_root() read\n  on the next propagation change or mount_setattr() through the file\n  descriptor.\n\n- Make it shared and unmount a peer. trace_transfers() follows the\n  flag from the peer into it and strips it from its peer group.\n\n- Let a propagated umount reach it. gather_candidates() marks it with\n  T_UMOUNT_CANDIDATE but never puts it on the candidate list, so the\n  mark is never cleared and every later propagated umount skips it. If\n  it's the parent of a locked mount handle_locked() takes it as the\n  cutoff and commits the locked child that should have stayed and\n  disconnect_mount() leaves that child attached to it, unmounted under\n  a mounted parent, which is what disconnect_mount() exists to\n  prevent. When the namespace goes away umount_tree() visits the child\n  a second time. And once __detach_mounts() has made it its own parent\n  reparent() never finds the end of the chain of unmounted parents.\n\nA clone is a new mount that has never been unmounted. Add MNT_UMOUNT\nto MNT_INTERNAL_FLAGS so clone_mnt() never copies it. And warn when\numount_tree() reaches a mount that already has it set.","summary":"A mount that was lazily unmounted but is still held open by a file descriptor can be copied into the caller's mount namespace with its unmount flag still set. This corrupts mount namespace bookkeeping, causing leaks, use-after-free reads and propagation errors.","severity":"unknown","cvss":null,"epss":0.00209,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["containers","linux-kernel","runtime","self-hosted"],"not_affected_if":["default-config"],"how_to_check":"- `uname -r` and compare against the kernel version that carries the upstream fix, which the sources do not name\n- unknown","fix":"- Install the kernel release that includes the upstream fix for CVE-2026-98993 (fixed version not stated in the sources)\n- unknown","status":"open","published_at":"2026-10-23T13:17:21+00:00","first_seen_at":"2026-10-23T21:57:33+00:00","triaged_at":"2026-10-25T04:56:04+00:00","updated_at":"2026-10-25T12:38:50+00:00","category":"runtime","confidence":0.5,"triage":{"at":"2026-10-25T04:56:04Z","model":"claude-haiku-5-5","output":{"fix":"- Install the kernel release that includes the upstream fix for CVE-2026-98993 (fixed version not stated in the sources)\n- unknown","reach":"local","title":"Linux kernel: lazily unmounted mounts can be re-attached with a stale unmount flag","impact":"medium","summary":"A mount that was lazily unmounted but is still held open by a file descriptor can be copied into the caller's mount namespace with its unmount flag still set. This corrupts mount namespace bookkeeping, causing leaks, use-after-free reads and propagation errors.","category":"runtime","nicknames":[],"who_cares":["linux-kernel","self-hosted","containers"],"confidence":0.5,"needs_auth":"unknown","worst_case":"Kernel mount state corruption; possible use-after-free and unmount of peer mounts","analyst_note":"Find the fixed kernel versions and stable backport commits from the NVD entry or the upstream kernel git log for the MNT_UMOUNT clone_mnt fix.","how_to_check":"- `uname -r` and compare against the kernel version that carries the upstream fix, which the sources do not name\n- unknown","needs_analyst":true,"not_affected_if":["default-config"],"who_is_affected":"Systems running a Linux kernel without the upstream fix that allow unprivileged or container workloads to bind-mount nsfs or pidfs file descriptors; the sources do not say which configurations are exposed. Hosts with no untrusted users or containers may have lower exposure, but the sources do not confirm that.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"unknown"},"cost_usd":0.000713,"input_tokens":4288,"output_tokens":569,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':17A,20A '-98993':18A,21A '/proc/self/fd/3':254C '/proc/self/ns/net':244C '/tmp/x':245C,248C,251C '/tmp/y':255C,259C '3':247C 'accept':231C 'add':545C 'aliv':209C 'alreadi':270C,567C 'anon':378C 'anymor':349C 'attach':10A,136C,261C,484C 'away':361C,504C 'bind':243C,253C 'bookkeep':64B 'bug':286C 'caller':50B,264C 'candid':425C,431C,438C 'carri':123C,141C 'caus':65B,282C 'chain':168C,530C 'chang':375C,386C 'check':234C 'child':134C,473C,483C,509C 'clear':116C,445C 'clone':92C,180C,535C,553C 'commit':470C 'contain':22B 'copi':47B,182C,217C,229C,274C,556C 'corrupt':61B 'cutoff':468C 'cve':16A,19A 'decid':160C 'descriptor':44B,213C,348C,393C 'detach':117C,299C,515C 'disconnect':129C,479C,495C 'doesn':195C 'drop':316C 'earlier':309C 'els':162C 'end':172C,527C 'error':74B 'everi':273C,447C 'exec':246C 'exist':497C 'file':43B,212C,226C,347C,392C 'find':354C,525C 'flag':15A,57B,184C,193C,279C,405C,551C 'follow':80C,403C 'free':70B 'freed':369C 'gather':424C 'get':96C 'go':154C 'goe':360C,503C 'group':417C 'handl':462C 'held':39B 'hold':321C 'host':29B 'includ':197C 'inherit':88C,277C 'instead':125C 'intern':192C,550C 'isn':296C 'kept':208C 'kernel':2A,25B,78C 'l':250C 'later':448C 'lazili':3A,34B,204C 'leak':66B,364C 'leav':481C 'let':131C,418C 'linux':1A,24B,77C 'linux-kernel':23B 'list':439C 'lock':170C,460C,463C,472C 'made':518C 'make':394C 'mark':426C,442C 'mask':190C 'may':228C,374C 'mnt':89C,93C,94C,181C,191C,198C,268C,371C,546C,549C,554C 'mount':5A,31B,52B,62B,100C,118C,121C,130C,152C,171C,201C,239C,242C,252C,257C,292C,300C,305C,388C,461C,480C,490C,496C,516C,539C,565C 'mountpoint':289C 'namespac':53B,63B,85C,112C,266C,293C,320C,328C,352C,359C,372C,502C 'never':115C,353C,433C,444C,524C,542C,555C 'new':538C 'next':384C 'ns':379C 'nsfs':223C 'number':284C 'one':106C,298C,330C 'open':40B 'overmount':179C 'parent':140C,314C,338C,457C,491C,522C,533C 'peer':400C,408C,416C 'pidf':225C 'point':366C 'prevent':499C 'propag':73B,143C,376C,385C,420C,449C 'put':434C 'rais':97C 're':9A 're-attach':8A 'reach':422C,563C 'read':71B,381C 'refer':318C,331C 'repar':177C,523C 'resolv':84C 'root':220C,380C 'runtim':26B 'second':511C 'self':28B 'self-host':27B 'set':59B,271C,570C 'setattr':389C 'share':396C 'sign':149C 'skip':451C 'sourc':187C,237C 'stale':13A 'stay':135C,325C,477C 'still':38B,58B,215C 'strip':412C 'struct':370C 'take':145C,301C,464C 'taken':108C 'time':512C 'trace':401C 'transfer':402C 'tree':103C,230C,506C,562C 'umount':90C,95C,102C,105C,144C,199C,249C,269C,421C,430C,450C,505C,547C,561C 'unhook':119C,310C 'unlink':287C 'unmount':4A,14A,35B,56B,127C,133C,157C,165C,205C,308C,344C,398C,487C,532C,544C 'use':68B 'use-after-fre':67B 'visit':507C 'vulner':81C 'warn':559C 'whether':235C","nicknames":[],"mitigations":[],"who_affected":"Systems running a Linux kernel without the upstream fix that allow unprivileged or container workloads to bind-mount nsfs or pidfs file descriptors; the sources do not say which configurations are exposed. Hosts with no untrusted users or containers may have lower exposure, but the sources do not confirm that.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-23T13:17:21+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98993","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98993","severity":null,"published_at":"2026-10-23T13:17:21+00:00","modified_at":"2026-10-23T13:17:21+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-2026-98993","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-98993","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"}