CVE-2026-99077 (GCVE-0-2026-99077)

Vulnerability from cvelistv5 – Published: 2026-10-23 12:23 – Updated: 2026-10-26 15:13
VLAI
Title
ipv4: udp: Create exceptions before socket matching
Summary
In the Linux kernel, the following vulnerability has been resolved: ipv4: udp: Create exceptions before socket matching Currently, when ICMP Fragmentation Needed and Redirect Message packets are locally delivered and quote a UDP packet, a FIB nexthop exception (FNHE) is only created if the kernel can match the UDP packet to an existing socket. This behavior allows off-path attackers to conduct a side-channel attack on the FNHE cache in order to discover the ephemeral port used by a connected UDP socket. Commit 6457378fe796 ("ipv4: use siphash instead of Jenkins in fnhe_hashfun()") and commit 67d6d681e15b ("ipv4: make exception cache less predictible") tried to mitigate such attacks by making it harder for attackers to discover hash collisions in the FNHE cache and by randomizing the number of exceptions a hash bucket can hold, respectively. Unfortunately, both of the mitigations can be bypassed. Instead, mitigate such attacks by always creating a FNHE, even before trying to find a matching socket. Do that by calling ipv4_update_pmtu() and ipv4_redirect(), the helpers used when the quoted packet did not originate from a socket. This means that guesses (right or wrong) from an off-path attacker will always result in a FNHE being created or updated in the cache that the attacker can observe. Pass an oif of 0, in a similar fashion to icmp_err(). This is also the oif used by the socket path for sockets that are not bound to a device. Note that this does not allow attackers to create FNHEs that they could not create before, as both helpers can already be reached with little to no validation. For example, by sending an ICMP error that quotes an ICMP Echo Reply or one that quotes a UDP source port that matches a wildcard socket. Also note that in the good case (matched socket) the above scheme comes at the cost of an extra route lookup, as the no socket helpers perform their own lookup before the one performed by ipv4_sk_update_pmtu() / ipv4_sk_redirect(). When the two resolve to different nexthops, it also results in two exceptions being created for the same destination IP. One in the FNHE cache of the nexthop resolved by the no socket helpers and another in the FNHE cache of the nexthop used by the socket.
Severity
7.4 (High)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
Assigner
Linux CNA under the mitre root
CNA scorecard D 53/100 over 7063 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 4895c771c7f006b4b90f9d6b1d2210939ba57b38 , < ecd7ead4bf05e221e90f18a29aec570a50fd1592 (git)
Affected: 4895c771c7f006b4b90f9d6b1d2210939ba57b38 , < be4fd05398f2b5e42029be70ee875fc749b8ce3a (git)
Affected: 4895c771c7f006b4b90f9d6b1d2210939ba57b38 , < 4c3499f79f8c7e8561266bcc220a18538cec0458 (git)
guessed Create a notification for this product.
Linux Linux Affected: 3.6
Unaffected: 0 , < 3.6 (semver)
Unaffected: 6.18.59 , ≤ 6.18.* (semver)
Unaffected: 7.2.11 , ≤ 7.2.* (semver)
Unaffected: 7.3 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
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              "operator": "OR"
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      "descriptions": [
        {
          "lang": "en",
          "value": "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."
        }
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      "metrics": [
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            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
            "version": "3.1"
          },
          "scenarios": [
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              "lang": "en",
              "value": "AV:N - A remote IPv4 ICMP Fragmentation Needed or Redirect message carries the attacker-controlled inner IPv4/UDP quote through ip_rcv(), icmp_rcv(), icmp_socket_deliver(), and udp_err(), where its guessed four-tuple is matched.\nAC:H - Exploiting a connected socket requires turning fnhe_hashfun() bucket occupancy and update_or_create_fnhe() eviction into an oracle while the UDP tuple remains live; keyed hashing, randomized bucket depth, cache noise, and the short DNS response window are conditions the attacker cannot fully determine, although the mitigations can be bypassed.\nPR:N - icmp_rcv() checks packet structure, checksum, and policy but has no authentication or capability gate; icmp_unreach() passes a valid Fragmentation Needed quote to udp_err() without verifying that its sender is an on-path router.\nUI:N - The attacker can induce a resolver\u0027s connected UDP socket with a DNS query and then send the ICMP probes and cache-verification traffic; no victim user action is required.\nS:U - The FNHE oracle in udp_err() compromises networking and application data governed by the same host security authority; it does not cross a VM, IOMMU, or sandbox boundary.\nC:H - The __udp4_lib_lookup() result controls whether ipv4_sk_update_pmtu() or ipv4_sk_redirect() leaves an observable FNHE, disclosing the complete ephemeral source port. On a recursive DNS resolver, removing that entropy enables cache poisoning and redirection of client traffic, permitting broad disclosure.\nI:H - After learning the port, the same attacker can forge UDP DNS replies and brute-force the remaining transaction ID; an accepted reply can poison the resolver cache and map chosen names to attacker-controlled addresses.\nA:N - The fixed defect is the match-dependent FNHE oracle, not memory corruption or a crash. update_or_create_fnhe() already accepts other minimally validated ICMP cases, and the fix deliberately preserves exception creation, so route-cache manipulation is not an availability impact removed by this patch."
            }
          ]
        }
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        "dateUpdated": "2026-10-26T15:13:01.358Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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        },
        {
          "url": "https://git.kernel.org/stable/c/4c3499f79f8c7e8561266bcc220a18538cec0458"
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      "title": "ipv4: udp: Create exceptions before socket matching",
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-99077",
    "datePublished": "2026-10-23T12:23:22.773Z",
    "dateReserved": "2026-09-25T10:25:14.389Z",
    "dateUpdated": "2026-10-26T15:13:01.358Z",
    "state": "PUBLISHED"
  },
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      "date": "2026-10-27",
      "epss": "0.00374",
      "percentile": "0.3122"
    },
    "microsoft_vex": {
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      "cve": "CVE-2026-99077",
      "id": "msrc_CVE-2026-99077",
      "initial_release_date": "2026-10-25T01:05:13.000Z",
      "product_status:known_affected": "1",
      "product_status:under_investigation": "1",
      "source": "Microsoft CSAF VEX",
      "status": "final",
      "title": "ipv4: udp: Create exceptions before socket matching",
      "url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-99077.json",
      "version": "3"
    },
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                "product": "Linux",
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        "descriptions": [
          {
            "lang": "en",
            "value": "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."
          }
        ],
        "id": "CVE-2026-99077",
        "lastModified": "2026-10-26T16:17:24.160",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "HIGH",
                "attackVector": "NETWORK",
                "availabilityImpact": "NONE",
                "baseScore": 7.4,
                "baseSeverity": "HIGH",
                "confidentialityImpact": "HIGH",
                "integrityImpact": "HIGH",
                "privilegesRequired": "NONE",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
                "version": "3.1"
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              "impactScore": 5.2,
              "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
              "type": "Secondary"
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        "published": "2026-10-23T13:17:31.787",
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        "vulnStatus": "Received"
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}



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