{"id":"CVE-2026-99110","kind":"vulnerability","aliases":["CVE-2026-99110"],"related":[],"title":"Linux kernel: CIFS ACL write overflows heap buffer on oversized ACLs","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: fix heap overflow in cifs_do_set_acl()\n\ncifs_set_acl() validates ACL size using posix_acl_xattr_size():\n\n        4 + (count * 8)  // 4-byte header + 8 bytes per ACE\n\ncifs_do_set_acl() then calls posix_acl_to_cifs() to write the CIFS\nwire format into the same buffer:\n\n        6 + (count * 10)  // 6-byte header + 10 bytes per ACE\n\nAn ACL that passes the xattr-based check in cifs_set_acl() can\noverflow the heap when posix_acl_to_cifs() writes the larger CIFS\nformat.\n\nValidate the CIFS format size against the remaining buffer space and\nUSHRT_MAX before converting--data_count is __u16, so sizes above\nUSHRT_MAX truncate the on-wire packet length, causing the server to\napply a partial ACL.  Replace MaxDataCount = 1000 with\nmin(CIFSMaxBufSize, USHRT_MAX).","summary":"The Linux kernel's SMB client (cifs) can write a CIFS ACL larger than the buffer it allocated when setting file ACLs, because the size check uses the POSIX xattr format rather than the larger CIFS wire format. The overflow is a kernel heap write, which can corrupt memory and may allow privilege escalation, and oversized ACLs can also truncate the packet so the server applies a partial ACL.","severity":"unknown","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["cifs","kernel","linux","runtime","self-hosted","smb"],"not_affected_if":["cifs-module-not-loaded","no-cifs-mounts"],"how_to_check":"- Check the running kernel version with `uname -r` and compare it to the fixed release in the kernel stable branch.\n- Check whether the cifs module is loaded with `lsmod | grep cifs`; systems that never mount SMB shares are not exposed.\n- unknown for a fixed version number, since the sources do not list one.","fix":"- Update the Linux kernel to a release that includes the cifs_do_set_acl() fix (version unknown from the sources).\n- Until patched, unload or avoid mounting SMB/CIFS shares with ACL support on untrusted servers.","status":"open","published_at":"2026-10-28T11:17:30+00:00","first_seen_at":"2026-10-28T11:32:34+00:00","triaged_at":"2026-10-28T11:37:45+00:00","updated_at":"2026-10-28T11:37:45+00:00","category":"runtime","confidence":0.5,"triage":{"at":"2026-10-28T11:37:45Z","model":"claude-haiku-5-5","output":{"fix":"- Update the Linux kernel to a release that includes the cifs_do_set_acl() fix (version unknown from the sources).\n- Until patched, unload or avoid mounting SMB/CIFS shares with ACL support on untrusted servers.","reach":"unknown","title":"Linux kernel: CIFS ACL write overflows heap buffer on oversized ACLs","impact":"high","summary":"The Linux kernel's SMB client (cifs) can write a CIFS ACL larger than the buffer it allocated when setting file ACLs, because the size check uses the POSIX xattr format rather than the larger CIFS wire format. The overflow is a kernel heap write, which can corrupt memory and may allow privilege escalation, and oversized ACLs can also truncate the packet so the server applies a partial ACL.","category":"runtime","nicknames":[],"who_cares":["linux","kernel","cifs","smb","self-hosted"],"confidence":0.5,"needs_auth":"unknown","worst_case":"Kernel heap memory is overwritten, possibly enabling code execution or privilege escalation","analyst_note":"Find the upstream kernel commit and fixed stable versions for CVE-2026-99110, and confirm whether an attacker needs share access or local privileges.","how_to_check":"- Check the running kernel version with `uname -r` and compare it to the fixed release in the kernel stable branch.\n- Check whether the cifs module is loaded with `lsmod | grep cifs`; systems that never mount SMB shares are not exposed.\n- unknown for a fixed version number, since the sources do not list one.","needs_analyst":true,"not_affected_if":["no-cifs-mounts","cifs-module-not-loaded"],"who_is_affected":"Systems that mount SMB/CIFS shares with the kernel cifs client and set ACLs on those shares are the ones that call the vulnerable path. Machines that do not mount CIFS shares, or that run kernels without the cifs module, can skip it. The sources do not say whether an attacker needs access to the share or a local account to trigger it.","user_interaction":"unknown","needs_special_setup":"unknown","severity_if_unknown":"unknown"},"cost_usd":0.000732,"input_tokens":3571,"output_tokens":750,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':13A,16A '-99110':14A,17A '10':158C,162C '1000':234C '4':126C,129C '6':156C,159C '8':128C,132C 'ace':135C,165C 'acl':4A,11A,37B,47B,82B,94B,114C,117C,119C,123C,139C,143C,167C,178C,185C,231C 'alloc':43B 'allow':77B 'also':84B 'appli':91B,228C 'base':173C 'buffer':8A,41B,155C,201C 'byte':130C,133C,160C,163C 'call':141C 'caus':224C 'check':51B,174C 'cif':3A,18B,32B,36B,61B,111C,115C,136C,145C,149C,176C,187C,191C,195C 'cifsmaxbufs':237C 'client':31B,106C 'convert':207C 'corrupt':73B 'count':127C,157C,209C 'cve':12A,15A 'data':208C 'escal':79B 'file':46B 'fix':107C 'follow':100C 'format':56B,63B,151C,192C,196C 'header':131C,161C 'heap':7A,69B,108C,182C 'host':24B 'kernel':2A,19B,28B,68B,98C 'larger':38B,60B,190C 'length':223C 'linux':1A,20B,27B,97C 'max':205C,216C,239C 'maxdatacount':233C 'may':76B 'memori':74B 'min':236C 'on-wir':219C 'overflow':6A,65B,109C,180C 'overs':10A,81B 'packet':87B,222C 'partial':93B,230C 'pass':169C 'per':134C,164C 'posix':54B,122C,142C,184C 'privileg':78B 'rather':57B 'remain':200C 'replac':232C 'resolv':104C 'runtim':21B 'self':23B 'self-host':22B 'server':90B,226C 'set':45B,113C,116C,138C,177C 'size':50B,120C,125C,197C,213C 'smb':25B,30B,105C 'space':202C 'truncat':85B,217C 'u16':211C 'use':52B,121C 'ushrt':204C,215C,238C 'valid':118C,193C 'vulner':101C 'wire':62B,150C,221C 'write':5A,34B,70B,147C,188C 'xattr':55B,124C,172C 'xattr-bas':171C","nicknames":[],"mitigations":[],"who_affected":"Systems that mount SMB/CIFS shares with the kernel cifs client and set ACLs on those shares are the ones that call the vulnerable path. Machines that do not mount CIFS shares, or that run kernels without the cifs module, can skip it. The sources do not say whether an attacker needs access to the share or a local account to trigger it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-28T11:17:30+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-99110","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99110","severity":null,"published_at":"2026-10-28T11:17:30+00:00","modified_at":"2026-10-28T11:17:30+00:00","first_seen_at":"2026-10-28T11:32:34+00:00","fetched_at":"2026-10-28T11:32:34+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"}