{"id":"CVE-2026-99077","kind":"vulnerability","aliases":["CVE-2026-99077"],"related":[],"title":"Linux kernel: ICMP errors can reveal UDP ephemeral ports to off-path attackers","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nipv4: udp: Create exceptions before socket matching\n\nCurrently, when ICMP Fragmentation Needed and Redirect Message packets\nare locally delivered and quote a UDP packet, a FIB nexthop exception\n(FNHE) is only created if the kernel can match the UDP packet to an\nexisting socket.\n\nThis behavior allows off-path attackers to conduct a side-channel attack\non the FNHE cache in order to discover the ephemeral port used by a\nconnected UDP socket.\n\nCommit 6457378fe796 (\"ipv4: use siphash instead of Jenkins in\nfnhe_hashfun()\") and commit 67d6d681e15b (\"ipv4: make exception cache\nless predictible\") tried to mitigate such attacks by making it harder\nfor attackers to discover hash collisions in the FNHE cache and by\nrandomizing the number of exceptions a hash bucket can hold,\nrespectively. Unfortunately, both of the mitigations can be bypassed.\n\nInstead, mitigate such attacks by always creating a FNHE, even before\ntrying to find a matching socket. Do that by calling ipv4_update_pmtu()\nand ipv4_redirect(), the helpers used when the quoted packet did not\noriginate from a socket.\n\nThis means that guesses (right or wrong) from an off-path attacker will\nalways result in a FNHE being created or updated in the cache that the\nattacker can observe.\n\nPass an oif of 0, in a similar fashion to icmp_err(). This is also the\noif used by the socket path for sockets that are not bound to a device.\n\nNote that this does not allow attackers to create FNHEs that they could\nnot create before, as both helpers can already be reached with little to\nno validation. For example, by sending an ICMP error that quotes an ICMP\nEcho Reply or one that quotes a UDP source port that matches a wildcard\nsocket.\n\nAlso note that in the good case (matched socket) the above scheme comes\nat the cost of an extra route lookup, as the no socket helpers perform\ntheir own lookup before the one performed by ipv4_sk_update_pmtu() /\nipv4_sk_redirect(). When the two resolve to different nexthops, it also\nresults in two exceptions being created for the same destination IP. One\nin the FNHE cache of the nexthop resolved by the no socket helpers and\nanother in the FNHE cache of the nexthop used by the socket.","summary":"When the Linux kernel receives an ICMP Fragmentation Needed or Redirect message quoting a UDP packet, it only caches a route exception if the quoted packet matches a socket. An off-path attacker can watch that cache to learn the ephemeral port of a connected UDP socket.","severity":"low","cvss":null,"epss":0.0021,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["ipv4","kernel","linux","runtime","udp"],"not_affected_if":[],"how_to_check":null,"fix":null,"status":"open","published_at":"2026-10-23T13:17:31+00:00","first_seen_at":"2026-10-23T21:57:34+00:00","triaged_at":"2026-10-25T04:53:39+00:00","updated_at":"2026-10-25T12:38:50+00:00","category":"runtime","confidence":0.6,"triage":{"at":"2026-10-25T04:53:39Z","model":"claude-haiku-5-5","output":{"fix":"unknown","reach":"network","title":"Linux kernel: ICMP errors can reveal UDP ephemeral ports to off-path attackers","impact":"low","summary":"When the Linux kernel receives an ICMP Fragmentation Needed or Redirect message quoting a UDP packet, it only caches a route exception if the quoted packet matches a socket. An off-path attacker can watch that cache to learn the ephemeral port of a connected UDP socket.","category":"runtime","nicknames":[],"who_cares":["linux","kernel","ipv4","udp"],"confidence":0.6,"needs_auth":"no","worst_case":"off-path attacker learns UDP ephemeral ports via cache side channel","analyst_note":"Sources give no affected kernel versions or fixed commit; check the NVD entry and the upstream kernel stable commit for the fix version.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":[],"who_is_affected":"unknown","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"low"},"cost_usd":0.00059,"input_tokens":4017,"output_tokens":377,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':16A,19A '-99077':17A,20A '0':294C '6457378fe796':160C '67d6d681e15b':172C 'allow':130C,326C 'alreadi':341C 'also':304C,375C,425C 'alway':224C,273C 'anoth':452C 'attack':14A,59B,134C,141C,183C,189C,222C,271C,287C,327C 'behavior':129C 'bound':317C 'bucket':207C 'bypass':218C 'cach':44B,63B,145C,176C,197C,284C,441C,456C 'call':239C 'case':381C 'channel':140C 'collis':193C 'come':387C 'commit':159C,171C 'conduct':136C 'connect':71B,156C 'cost':390C 'could':333C 'creat':86C,115C,225C,279C,329C,335C,431C 'current':91C 'cve':15A,18A 'deliv':102C 'destin':435C 'devic':320C 'differ':422C 'discov':149C,191C 'echo':360C 'ephemer':8A,67B,151C 'err':301C 'error':4A,355C 'even':228C 'exampl':350C 'except':47B,87C,111C,175C,204C,429C 'exist':126C 'extra':393C 'fashion':298C 'fib':109C 'find':232C 'fnhe':112C,144C,168C,196C,227C,277C,440C,455C 'fnhes':330C 'follow':79C 'fragment':33B,94C 'good':380C 'guess':262C 'harder':187C 'hash':192C,206C 'hashfun':169C 'helper':247C,339C,400C,450C 'hold':209C 'icmp':3A,32B,93C,300C,354C,359C 'instead':164C,219C 'ip':436C 'ipv4':21B,84C,161C,173C,240C,244C,410C,414C 'jenkin':166C 'kernel':2A,22B,29B,77C,118C 'learn':65B 'less':177C 'linux':1A,23B,28B,76C 'littl':345C 'local':101C 'lookup':395C,404C 'make':174C,185C 'match':52B,90C,120C,234C,371C,382C 'mean':260C 'messag':37B,98C 'mitig':181C,215C,220C 'need':34B,95C 'nexthop':110C,423C,444C,459C 'note':321C,376C 'number':202C 'observ':289C 'off-path':11A,56B,131C,268C 'oif':292C,306C 'one':363C,407C,437C 'order':147C 'origin':255C 'packet':41B,51B,99C,107C,123C,252C 'pass':290C 'path':13A,58B,133C,270C,311C 'perform':401C,408C 'pmtu':242C,413C 'port':9A,68B,152C,369C 'predict':178C 'quot':38B,50B,104C,251C,357C,365C 'random':200C 'reach':343C 'receiv':30B 'redirect':36B,97C,245C,416C 'repli':361C 'resolv':83C,420C,445C 'respect':210C 'result':274C,426C 'reveal':6A 'right':263C 'rout':46B,394C 'runtim':24B 'scheme':386C 'send':352C 'side':139C 'side-channel':138C 'similar':297C 'siphash':163C 'sk':411C,415C 'socket':54B,73B,89C,127C,158C,235C,258C,310C,313C,374C,383C,399C,449C,463C 'sourc':368C 'tri':179C,230C 'two':419C,428C 'udp':7A,25B,40B,72B,85C,106C,122C,157C,367C 'unfortun':211C 'updat':241C,281C,412C 'use':153C,162C,248C,307C,460C 'valid':348C 'vulner':80C 'watch':61B 'wildcard':373C 'wrong':265C","nicknames":[],"mitigations":[],"who_affected":null,"tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-23T13:17:31+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-99077","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99077","severity":null,"published_at":"2026-10-23T13:17:31+00:00","modified_at":"2026-10-23T13:17:31+00:00","first_seen_at":"2026-10-23T21:57:34+00:00","fetched_at":"2026-10-23T22:29:03+00:00","note":null},{"source":"epss","external_id":"CVE-2026-99077","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-99077","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"}