{"id":"CVE-2026-98988","kind":"vulnerability","aliases":["CVE-2026-98988"],"related":[],"title":"Linux kernel: open_tree namespace copy reveals mounts hidden under other mounts","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnamespace: keep covered mounts covered in OPEN_TREE_NAMESPACE\n\nopen_tree(OPEN_TREE_NAMESPACE) creates a new mount namespace from a\ncopy of the tree at the given path. The caller needs to be privileged\nover its current user namespace because the new mount namespace will be\nowned by it. It doesn't need to be privileged over the mount namespace\nthe tree is copied from. An unprivileged user just needs to create a\nuser namespace first.\n\nThe copy follows the rules of a detached bind mount. Without\nAT_RECURSIVE only the mount itself is cloned and its children are left\nout. With AT_RECURSIVE unbindable mounts are skipped. Both is fine when\nthe caller has privileges over the source mount namespace because it\ncould unmount those mounts anyway. Not so for an unprivileged user in\nthat mount namespace. For them a mount namespace copy is what unshare()\ndoes. copy_mnt_ns() copies everything including unbindable mounts and\nlock_mnt_tree() makes sure nothing can be unmounted in the copy. Nothing\nthat was covered gets revealed.\n\ncreate_new_namespace() only does the locking and so the covered content\nis right there in the new mount namespace:\n\n  # mount -t tmpfs none /tmp/otn; mkdir /tmp/otn/covered\n  # echo hidden > /tmp/otn/covered/under.txt\n  # mount -t tmpfs none /tmp/otn/covered\n\n  uid 1000, after unshare(CLONE_NEWUSER):\n  fd = open_tree(AT_FDCWD, \"/tmp/otn\", OPEN_TREE_NAMESPACE);\n  setns(fd, CLONE_NEWNS);\n  open(\"/covered/under.txt\", O_RDONLY)  -> \"hidden\"\n\nCovering paths with mounts is how container runtimes mask parts of\n/proc and /sys and how admins hide things.\n\nWhen the caller's user namespace doesn't own the source mount namespace\ncopy the way unshare() does and refuse a non-recursive copy of a mount\nthat has anything mounted below the requested directory and copy\nunbindable mounts in a recursive copy. A caller that is privileged over\nthe source mount namespace sees no change.","summary":"An unprivileged user can call open_tree with OPEN_TREE_NAMESPACE to copy a mount tree that includes mounts that were meant to cover directories, so the content under those covering mounts becomes readable. The copy ignores the covering mounts and unbindable mounts that the source namespace would have kept hidden from that user.","severity":"unknown","cvss":null,"epss":0.00198,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["containers","kernel","linux","runtime","self-hosted"],"not_affected_if":["unprivileged-userns-disabled"],"how_to_check":"- Run `uname -r` and compare against the fixed kernel builds listed in the NVD or distribution advisory for CVE-2026-98988\n- Check whether unprivileged user namespaces are enabled with `sysctl kernel.unprivileged_userns_clone` or `sysctl user.max_user_namespaces`","fix":"- Update the Linux kernel to a build that includes the upstream fix for CVE-2026-98988 (exact fixed version unknown from sources)\n- Until patched, disable unprivileged user namespaces if your workload does not need them, for example `sysctl -w user.max_user_namespaces=0`","status":"open","published_at":"2026-10-23T13:17:20+00:00","first_seen_at":"2026-10-23T21:57:33+00:00","triaged_at":"2026-10-25T04:56:10+00:00","updated_at":"2026-10-25T12:38:50+00:00","category":"runtime","confidence":0.5,"triage":{"at":"2026-10-25T04:56:10Z","model":"claude-haiku-5-5","output":{"fix":"- Update the Linux kernel to a build that includes the upstream fix for CVE-2026-98988 (exact fixed version unknown from sources)\n- Until patched, disable unprivileged user namespaces if your workload does not need them, for example `sysctl -w user.max_user_namespaces=0`","reach":"local","title":"Linux kernel: open_tree namespace copy reveals mounts hidden under other mounts","impact":"low","summary":"An unprivileged user can call open_tree with OPEN_TREE_NAMESPACE to copy a mount tree that includes mounts that were meant to cover directories, so the content under those covering mounts becomes readable. The copy ignores the covering mounts and unbindable mounts that the source namespace would have kept hidden from that user.","category":"runtime","nicknames":[],"who_cares":["linux","kernel","containers","self-hosted"],"confidence":0.5,"needs_auth":"no","worst_case":"local user reads files hidden under covering mounts","analyst_note":"Find the fixed kernel versions and affected ranges for CVE-2026-98988 in the NVD entry or the upstream kernel stable commit.","how_to_check":"- Run `uname -r` and compare against the fixed kernel builds listed in the NVD or distribution advisory for CVE-2026-98988\n- Check whether unprivileged user namespaces are enabled with `sysctl kernel.unprivileged_userns_clone` or `sysctl user.max_user_namespaces`","needs_analyst":true,"not_affected_if":["unprivileged-userns-disabled"],"who_is_affected":"Hosts running a vulnerable Linux kernel where unprivileged users can create user namespaces, and where mounts are used to mask paths such as in container runtimes or hardened sandboxes. Systems without unprivileged user namespaces or without covered mounts relying on hiding are less exposed. Sources do not give the affected version range.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000746,"input_tokens":3978,"output_tokens":696,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':14A,17A '-98988':15A,18A '/covered/under.txt':321C '/proc':336C '/sys':338C '/tmp/otn':290C,312C '/tmp/otn/covered':292C,300C '/tmp/otn/covered/under.txt':295C '1000':302C 'admin':341C 'anyth':374C 'anyway':218C 'becom':58B 'bind':175C 'call':30B 'caller':120C,204C,346C,389C 'chang':400C 'children':188C 'clone':185C,305C,318C 'contain':19B,331C 'content':53B,277C 'copi':6A,38B,61B,111C,154C,168C,234C,239C,242C,259C,357C,368C,381C,387C 'could':214C 'cover':49B,56B,64B,92C,94C,263C,276C,325C 'creat':104C,162C,266C 'current':127C 'cve':13A,16A 'detach':174C 'directori':50B,379C 'doesn':141C,350C 'echo':293C 'everyth':243C 'fd':307C,317C 'fdcwd':311C 'fine':201C 'first':166C 'follow':85C,169C 'get':264C 'given':117C 'hidden':9A,76B,294C,324C 'hide':342C 'host':25B 'ignor':62B 'includ':43B,244C 'keep':91C 'kept':75B 'kernel':2A,20B,83C 'left':190C 'linux':1A,21B,82C 'lock':248C,272C 'make':251C 'mask':333C 'meant':47B 'mkdir':291C 'mnt':240C,249C 'mount':8A,12A,40B,44B,57B,65B,68B,93C,107C,133C,149C,176C,182C,196C,210C,217C,227C,232C,246C,284C,286C,296C,328C,355C,371C,375C,383C,396C 'namespac':5A,36B,72B,90C,98C,103C,108C,129C,134C,150C,165C,211C,228C,233C,268C,285C,315C,349C,356C,397C 'need':121C,143C,160C 'new':106C,132C,267C,283C 'newn':319C 'newus':306C 'non':366C 'non-recurs':365C 'none':289C,299C 'noth':253C,260C 'ns':241C 'o':322C 'open':3A,31B,34B,96C,99C,101C,308C,313C,320C 'own':137C 'part':334C 'path':118C,326C 'privileg':124C,146C,206C,392C 'rdon':323C 'readabl':59B 'recurs':179C,194C,367C,386C 'refus':363C 'request':378C 'resolv':89C 'reveal':7A,265C 'right':279C 'rule':171C 'runtim':22B,332C 'see':398C 'self':24B 'self-host':23B 'setn':316C 'skip':198C 'sourc':71B,209C,354C,395C 'sure':252C 'thing':343C 'tmpfs':288C,298C 'tree':4A,32B,35B,41B,97C,100C,102C,114C,152C,250C,309C,314C 'uid':301C 'unbind':67B,195C,245C,382C 'unmount':215C,256C 'unprivileg':27B,157C,223C 'unshar':237C,304C,360C 'user':28B,79B,128C,158C,164C,224C,348C 'vulner':86C 'way':359C 'without':177C 'would':73B","nicknames":[],"mitigations":[],"who_affected":"Hosts running a vulnerable Linux kernel where unprivileged users can create user namespaces, and where mounts are used to mask paths such as in container runtimes or hardened sandboxes. Systems without unprivileged user namespaces or without covered mounts relying on hiding are less exposed. Sources do not give the affected version range.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-23T13:17:20+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98988","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98988","severity":null,"published_at":"2026-10-23T13:17:20+00:00","modified_at":"2026-10-23T13:17:20+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-2026-98988","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-98988","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"}