{"id":"CVE-2026-99126","kind":"vulnerability","aliases":["CVE-2026-99126"],"related":[],"title":"Linux kernel s390 CTCM driver crashes on NULL XID pointer","description":"In the Linux kernel, the following vulnerability has been resolved:\n\ns390/ctcm: Prevent XID null dereference\n\nThe mpc_validate_xid() function sets grp->saved_xid2->xid2_flag2 to 0x40\nto signal XID validation error. If peer XID is NULL or r/w channel\npairing mismatch happens, grp->saved_xid2 is never initialized. An\nattempt to set the flag in such case leads to NULL dereference.\n\nFix this by using the always available priv->xid->xid2_flag2 instead of\ngrp->saved_xid2->xid2_flag2 for validation errors.","summary":"The s390 CTCM channel-to-channel network driver in the Linux kernel can dereference a NULL pointer during XID validation when the peer XID is missing or read/write channel pairing mismatches. The sources describe a kernel null dereference, which typically leads to a crash or denial of service rather than code execution.","severity":"unknown","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["ibm-z","infrastructure","linux-kernel","mainframe","s390"],"not_affected_if":["ctcm-not-loaded","non-s390-architecture"],"how_to_check":"- `uname -r` to confirm the running kernel version, then compare against the fixed kernel release in the vendor advisory\n- Check whether the ctcm module is loaded with `lsmod | grep ctcm` on IBM Z (s390) systems\n- unknown for any other specific check, as the sources do not say","fix":"- Update the Linux kernel to a release containing the s390/ctcm XID null dereference fix; the exact version is unknown from the sources\n- unknown for any other action, as the sources do not say","status":"open","published_at":"2026-10-28T11:17:32+00:00","first_seen_at":"2026-10-28T11:32:35+00:00","triaged_at":"2026-10-28T11:37:29+00:00","updated_at":"2026-10-28T11:37:29+00:00","category":"infrastructure","confidence":0.5,"triage":{"at":"2026-10-28T11:37:29Z","model":"claude-haiku-5-5","output":{"fix":"- Update the Linux kernel to a release containing the s390/ctcm XID null dereference fix; the exact version is unknown from the sources\n- unknown for any other action, as the sources do not say","reach":"local","title":"Linux kernel s390 CTCM driver crashes on NULL XID pointer","impact":"low","summary":"The s390 CTCM channel-to-channel network driver in the Linux kernel can dereference a NULL pointer during XID validation when the peer XID is missing or read/write channel pairing mismatches. The sources describe a kernel null dereference, which typically leads to a crash or denial of service rather than code execution.","category":"infrastructure","nicknames":[],"who_cares":["linux-kernel","s390","ibm-z","mainframe"],"confidence":0.5,"needs_auth":"unknown","worst_case":"Kernel null dereference crashes the s390 host or its network channel","analyst_note":"","how_to_check":"- `uname -r` to confirm the running kernel version, then compare against the fixed kernel release in the vendor advisory\n- Check whether the ctcm module is loaded with `lsmod | grep ctcm` on IBM Z (s390) systems\n- unknown for any other specific check, as the sources do not say","needs_analyst":false,"not_affected_if":["non-s390-architecture","ctcm-not-loaded"],"who_is_affected":"Only systems running the Linux kernel on IBM Z (s390) hardware that use the CTCM channel-to-channel network driver are exposed. Systems on x86, ARM or other architectures, and s390 hosts without the ctcm driver loaded, can skip it.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"unknown"},"cost_usd":0.000643,"input_tokens":3420,"output_tokens":601,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':12A,15A '-99126':13A,16A '0x40':106C 'alway':147C 'attempt':130C 'avail':148C 'case':137C 'channel':30B,32B,55B,119C 'channel-to-channel':29B 'code':77B 'crash':6A,70B 'ctcm':4A,28B 'cve':11A,14A 'denial':72B 'derefer':40B,64B,93C,141C 'describ':60B 'driver':5A,34B 'error':111C,162C 'execut':78B 'fix':142C 'flag':134C 'flag2':104C,152C,159C 'follow':84C 'function':98C 'grp':100C,123C,155C 'happen':122C 'ibm':18B 'ibm-z':17B 'infrastructur':20B 'initi':128C 'instead':153C 'kernel':2A,23B,38B,62B,82C 'lead':67B,138C 'linux':1A,22B,37B,81C 'linux-kernel':21B 'mainfram':24B 'mismatch':57B,121C 'miss':52B 'mpc':95C 'network':33B 'never':127C 'null':8A,42B,63B,92C,116C,140C 'pair':56B,120C 'peer':49B,113C 'pointer':10A,43B 'prevent':90C 'priv':149C 'r/w':118C 'rather':75B 'read/write':54B 'resolv':88C 's390':3A,25B,27B 's390/ctcm':89C 'save':101C,124C,156C 'servic':74B 'set':99C,132C 'signal':108C 'sourc':59B 'typic':66B 'use':145C 'valid':46B,96C,110C,161C 'vulner':85C 'xid':9A,45B,50B,91C,97C,109C,114C,150C 'xid2':102C,103C,125C,151C,157C,158C 'z':19B","nicknames":[],"mitigations":[],"who_affected":"Only systems running the Linux kernel on IBM Z (s390) hardware that use the CTCM channel-to-channel network driver are exposed. Systems on x86, ARM or other architectures, and s390 hosts without the ctcm driver loaded, can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-28T11:17:32+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-99126","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99126","severity":null,"published_at":"2026-10-28T11:17:32+00:00","modified_at":"2026-10-28T11:17:32+00:00","first_seen_at":"2026-10-28T11:32:35+00:00","fetched_at":"2026-10-28T11:32:35+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"}