{"id":"CVE-2026-98684","kind":"vulnerability","aliases":["CVE-2026-98684"],"related":[],"title":"Linux kernel SMB client: fallocate could overwrite live data with zeroes","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: flush and commit data before querying allocated ranges\n\nThe mode 0 / FALLOC_FL_KEEP_SIZE fallocate emulation for small interior\nranges in smb3_simple_fallocate_range() issues\nFSCTL_QUERY_ALLOCATED_RANGES and zero-fills any sub-range the server\nreports as an unallocated hole, to force block allocation without\nchanging file contents.\n\nThis is unsafe against Windows servers.  Per the Win32 documentation,\nFSCTL_QUERY_ALLOCATED_RANGES only reports ranges that *may* contain\nnonzero data and is explicitly not coherent with recently written data:\n\n  \"a call to FSCTL_QUERY_ALLOCATED_RANGES [after writing to a network\n   file] would not necessarily return a correct list of allocated\n   regions.  To ensure coherency [...] flush the data to the file\"\n\nfsx (generic/363) reproduces the resulting corruption against Windows\nServer 2022: a range is written, an uncached read returns the data, yet\nthe next FSCTL_QUERY_ALLOCATED_RANGES on that range reports it as a whole\nhole, so the emulation overwrites live data with zeroes.  A network trace\nconfirmed the WRITE, the READ returning data and the query returning an\nempty range list within microseconds.  Samba does not exhibit this; its\nquery is coherent with the written data.\n\nWrite back the dirty pages, drain any outstanding I/O and issue an SMB2\nFLUSH to force the server to commit the data before querying the\nallocated ranges, so the query reflects the data actually present.\n\nWith this, generic/363 passes against both Windows Server 2022 and Samba\nover 100000 fsx operations.","summary":"The Linux kernel's SMB client emulated fallocate on small interior ranges by querying allocated ranges from the server, but Windows servers can report written data as a hole. The client then zero-filled that range, overwriting live file data.","severity":"unknown","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["file-servers","linux-kernel","package","smb3-client","windows-server"],"not_affected_if":["no-smb-mounts","samba-server"],"how_to_check":null,"fix":"- Apply the Linux kernel stable fix that flushes and commits data before querying allocated ranges in smb3_simple_fallocate_range() (smb: client: flush and commit data before querying allocated ranges).\n- Check the fixed kernel version for your branch in the NVD entry: `https://nvd.nist.gov/vuln/detail/CVE-2026-98684`","status":"open","published_at":"2026-10-18T16:17:21+00:00","first_seen_at":"2026-10-23T21:57:16+00:00","triaged_at":"2026-10-25T05:19:11+00:00","updated_at":"2026-10-25T05:19:11+00:00","category":"package","confidence":0.5,"triage":{"at":"2026-10-25T05:19:11Z","model":"claude-haiku-5-5","output":{"fix":"- Apply the Linux kernel stable fix that flushes and commits data before querying allocated ranges in smb3_simple_fallocate_range() (smb: client: flush and commit data before querying allocated ranges).\n- Check the fixed kernel version for your branch in the NVD entry: `https://nvd.nist.gov/vuln/detail/CVE-2026-98684`","reach":"network","title":"Linux kernel SMB client: fallocate could overwrite live data with zeroes","impact":"medium","summary":"The Linux kernel's SMB client emulated fallocate on small interior ranges by querying allocated ranges from the server, but Windows servers can report written data as a hole. The client then zero-filled that range, overwriting live file data.","category":"package","nicknames":[],"who_cares":["linux-kernel","smb3-client","windows-server","file-servers"],"confidence":0.5,"needs_auth":"unknown","worst_case":"Live file data silently overwritten with zeroes on SMB shares","analyst_note":"Sources give no fixed kernel version or exploit path; check the NVD entry and the linux-stable commit for affected and fixed versions.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["no-smb-mounts","samba-server"],"who_is_affected":"Systems running a Linux kernel with the SMB3 client mounting shares from Windows Server, where applications use fallocate with mode 0 / FALLOC_FL_KEEP_SIZE on files. The sources say Samba is not affected by this behaviour. Hosts that do not mount SMB shares can skip it.","user_interaction":"unknown","needs_special_setup":"unknown","severity_if_unknown":"unknown"},"cost_usd":0.000678,"input_tokens":3809,"output_tokens":595,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':13A,16A '-98684':14A,17A '0':94C '100000':325C '2022':210C,321C 'actual':311C 'alloc':45B,90C,113C,133C,150C,174C,190C,226C,303C 'back':279C 'block':132C 'call':170C 'chang':135C 'client':4A,27B,36B,61B,83C 'coher':164C,194C,273C 'commit':86C,297C 'confirm':248C 'contain':157C 'content':137C 'correct':187C 'corrupt':206C 'could':6A 'cve':12A,15A 'data':9A,56B,71B,87C,159C,168C,197C,220C,242C,254C,277C,299C,310C 'dirti':281C 'document':147C 'drain':283C 'empti':260C 'emul':37B,100C,239C 'ensur':193C 'exhibit':268C 'explicit':162C 'falloc':5A,38B,95C,99C,108C 'file':19B,70B,136C,181C,200C 'file-serv':18B 'fill':65B,118C 'fl':96C 'flush':84C,195C,291C 'follow':77C 'forc':131C,293C 'fsctl':111C,148C,172C,224C 'fsx':201C,326C 'generic/363':202C,315C 'hole':59B,129C,236C 'i/o':286C 'interior':41B,103C 'issu':110C,288C 'keep':97C 'kernel':2A,23B,33B,75C 'linux':1A,22B,32B,74C 'linux-kernel':21B 'list':188C,262C 'live':8A,69B,241C 'may':156C 'microsecond':264C 'mode':93C 'necessarili':184C 'network':180C,246C 'next':223C 'nonzero':158C 'oper':327C 'outstand':285C 'overwrit':7A,68B,240C 'packag':24B 'page':282C 'pass':316C 'per':144C 'present':312C 'queri':44B,89C,112C,149C,173C,225C,257C,271C,301C,307C 'rang':42B,46B,67B,91C,104C,109C,114C,122C,151C,154C,175C,212C,227C,230C,261C,304C 'read':217C,252C 'recent':166C 'reflect':308C 'region':191C 'report':54B,125C,153C,231C 'reproduc':203C 'resolv':81C 'result':205C 'return':185C,218C,253C,258C 'samba':265C,323C 'server':20B,30B,49B,52B,124C,143C,209C,295C,320C 'simpl':107C 'size':98C 'small':40B,102C 'smb':3A,35B,82C 'smb2':290C 'smb3':26B,106C 'smb3-client':25B 'sub':121C 'sub-rang':120C 'trace':247C 'unalloc':128C 'uncach':216C 'unsaf':140C 'vulner':78C 'whole':235C 'win32':146C 'window':29B,51B,142C,208C,319C 'windows-serv':28B 'within':263C 'without':134C 'would':182C 'write':177C,250C,278C 'written':55B,167C,214C,276C 'yet':221C 'zero':11A,64B,117C,244C 'zero-fil':63B,116C","nicknames":[],"mitigations":[],"who_affected":"Systems running a Linux kernel with the SMB3 client mounting shares from Windows Server, where applications use fallocate with mode 0 / FALLOC_FL_KEEP_SIZE on files. The sources say Samba is not affected by this behaviour. Hosts that do not mount SMB shares can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-18T16:17:21+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98684","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98684","severity":null,"published_at":"2026-10-18T16:17:21+00:00","modified_at":"2026-10-19T10:19:32+00:00","first_seen_at":"2026-10-23T21:57:16+00:00","fetched_at":"2026-10-23T22:28:56+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"}