{"id":"CVE-2026-99155","kind":"vulnerability","aliases":["CVE-2026-99155"],"related":[],"title":"Linux Xen SCSI frontend crashes guest on unpopulated backend response","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nscsi: xen/scsifront: Check for a NULL shadow entry on a backend response\n\nscsifront_do_response() validates the rqid in a backend response against\nVSCSIIF_MAX_REQS before using it to index info->shadow[], but then\ndereferences the entry to test ->inflight without checking that the slot\nis populated.  Shadow slots are NULL before a command is submitted (the\nhost private area is zeroed at allocation) and are reset to NULL in\n_scsifront_put_rqid() once a request completes, so an in-range rqid that\ndoes not correspond to an outstanding request makes the frontend\ndereference a NULL pointer.\n\nA malicious or buggy backend can thus crash the guest by returning a\nresponse whose rqid is in range but not in flight -- for example a\nspurious response before any command has been issued, or a duplicate of\none already completed.  The ring has only VSCSIIF_MAX_REQS (16) slots,\nso before the first command every in-range rqid selects a NULL slot.\n\nReject a response whose shadow slot is not populated.","summary":"The Xen SCSI frontend in the Linux kernel dereferences a NULL shadow entry when the backend sends a response whose request ID has no outstanding request. A malicious or buggy Xen backend can crash the guest kernel with this response.","severity":"low","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["guest-vm","linux-kernel","other","xen"],"not_affected_if":["no-xen-scsi-frontend"],"how_to_check":null,"fix":null,"status":"open","published_at":"2026-10-28T11:17:35+00:00","first_seen_at":"2026-10-28T11:32:36+00:00","triaged_at":"2026-10-28T11:35:50+00:00","updated_at":"2026-10-28T11:35:50+00:00","category":"other","confidence":0.6,"triage":{"at":"2026-10-28T11:35:50Z","model":"claude-haiku-5-5","output":{"fix":"unknown","reach":"local","title":"Linux Xen SCSI frontend crashes guest on unpopulated backend response","impact":"low","summary":"The Xen SCSI frontend in the Linux kernel dereferences a NULL shadow entry when the backend sends a response whose request ID has no outstanding request. A malicious or buggy Xen backend can crash the guest kernel with this response.","category":"other","nicknames":[],"who_cares":["linux-kernel","xen","guest-vm"],"confidence":0.6,"needs_auth":"unknown","worst_case":"Guest kernel crashes via NULL pointer dereference from backend","analyst_note":"","how_to_check":"unknown","needs_analyst":false,"not_affected_if":["no-xen-scsi-frontend"],"who_is_affected":"unknown","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"low"},"cost_usd":0.00052,"input_tokens":3586,"output_tokens":322,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':12A,15A '-99155':13A,16A '16':222C 'alloc':139C 'alreadi':213C 'area':135C 'backend':9A,40B,56B,85C,95C,178C 'buggi':54B,177C 'check':77C,117C 'command':129C,204C,228C 'complet':152C,214C 'correspond':162C 'crash':5A,58B,181C 'cve':11A,14A 'derefer':33B,110C,170C 'duplic':210C 'entri':37B,82C,112C 'everi':229C 'exampl':198C 'first':227C 'flight':196C 'follow':70C 'frontend':4A,28B,169C 'guest':6A,18B,60B,183C 'guest-vm':17B 'host':133C 'id':46B 'in-rang':155C,230C 'index':105C 'inflight':115C 'info':106C 'issu':207C 'kernel':22B,32B,61B,68C 'linux':1A,21B,31B,67C 'linux-kernel':20B 'make':167C 'malici':52B,175C 'max':99C,220C 'null':35B,80C,126C,144C,172C,236C 'one':212C 'outstand':49B,165C 'pointer':173C 'popul':122C,246C 'privat':134C 'put':147C 'rang':157C,192C,232C 'reject':238C 'req':100C,221C 'request':45B,50B,151C,166C 'reset':142C 'resolv':74C 'respons':10A,43B,64B,86C,89C,96C,187C,201C,240C 'return':185C 'ring':216C 'rqid':92C,148C,158C,189C,233C 'scsi':3A,27B,75C 'scsifront':87C,146C 'select':234C 'send':41B 'shadow':36B,81C,107C,123C,242C 'slot':120C,124C,223C,237C,243C 'spurious':200C 'submit':131C 'test':114C 'thus':180C 'unpopul':8A 'use':102C 'valid':90C 'vm':19B 'vscsiif':98C,219C 'vulner':71C 'whose':44B,188C,241C 'without':116C 'xen':2A,24B,26B,55B 'xen/scsifront':76C 'zero':137C","nicknames":[],"mitigations":[],"who_affected":null,"tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-28T11:17:35+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-99155","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99155","severity":null,"published_at":"2026-10-28T11:17:35+00:00","modified_at":"2026-10-28T11:17:35+00:00","first_seen_at":"2026-10-28T11:32:36+00:00","fetched_at":"2026-10-28T11:32:36+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"}