CVE-2026-74656 (GCVE-0-2026-74656)

Vulnerability from cvelistv5 – Published: 2026-08-22 15:32 – Updated: 2026-08-25 05:41
VLAI
Title
ipv4: fix use-after-free in fib_nhc_update_mtu()
Summary
In the Linux kernel, the following vulnerability has been resolved: ipv4: fix use-after-free in fib_nhc_update_mtu() fib_nhc_update_mtu() walks the nexthop exception table under RTNL, but RTNL does not serialize this walk with PMTU exception updates. The walk uses rcu_dereference_protected() with a constant true condition without holding fnhe_lock. The following interleaving can therefore occur: CPU 0 CPU 1 fib_nhc_update_mtu() update_or_create_fnhe() load fnhe spin_lock_bh(&fnhe_lock) fnhe_remove_oldest() unlink fnhe kfree_rcu(fnhe, rcu) <quiescent state> access fnhe after grace period KASAN reported: BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410 Read of size 8 at addr ffff888107d49000 by task poc/90 Call Trace: fib_nhc_update_mtu+0x3df/0x410 fib_sync_mtu+0x7a/0xd0 fib_netdev_event+0x229/0x3f0 netif_set_mtu_ext+0x33a/0x570 dev_set_mtu+0x88/0x120 The same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a pair and other writers serialize them with fnhe_lock. RCU alone prevents reclamation, but would still allow concurrent writers to leave a mixed pair. Walk the table under RCU and acquire fnhe_lock only while updating each exception. RCU keeps the current entry alive while the short critical section serializes its paired PMTU fields. This avoids holding the global lock while scanning all 2048 buckets for every nexthop.
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < fd39e711866498ae94fcf9acf6f422a4f045b681 (git)
Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < e1e602d6b22d5cb1641c4459c487eb18bf569e0a (git)
Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < e00f7d2b5f2540a3415a229c982af7a25ff6362e (git)
Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < dfe388da13aa784851e5ebbea90afbb099075761 (git)
Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < 5a28a4b22dde92f9d293b94236314b8d6181dc4a (git)
Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < 63996ffc594d128ccec8fc0983f91effd2d3adc4 (git)
Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < ed503eaad62f20cdd5122d7c3078a648a99c8f16 (git)
Affected: af7d6cce53694a88d6a1bb60c9a239a6a5144459 , < bc5bde9ce3cc36502839dfe98e068f7303a50982 (git)
Affected: b427832009b97a3ee412ef643a80b15372a7754d (git)
Affected: 2b7e4c735933be79882aba2bed9afa789e03c62f (git)
Affected: 8d59c3a6376bbc6dd3f7303968a719ffba75a4f1 (git)
Affected: 9b4869cf385aa16f89c0f019eed4ec4e36aa441c (git)
Affected: ff34695ced21e2dfa04d0fa1c5f6c35011fa8117 (git)
Affected: 3.16.62 , < 3.17 (semver)
Affected: 4.4.162 , < 4.5 (semver)
Affected: 4.9.134 , < 4.10 (semver)
Affected: 4.14.77 , < 4.15 (semver)
Affected: 4.18.15 , < 4.19 (semver)
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Linux Linux Affected: 4.19
Unaffected: 0 , < 4.19 (semver)
Unaffected: 5.10.265 , ≤ 5.10.* (semver)
Unaffected: 5.15.216 , ≤ 5.15.* (semver)
Unaffected: 6.1.183 , ≤ 6.1.* (semver)
Unaffected: 6.6.152 , ≤ 6.6.* (semver)
Unaffected: 6.12.104 , ≤ 6.12.* (semver)
Unaffected: 6.18.45 , ≤ 6.18.* (semver)
Unaffected: 7.1.9 , ≤ 7.1.* (semver)
Unaffected: 7.2 , ≤ * (original_commit_for_fix)
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nipv4: fix use-after-free in fib_nhc_update_mtu()\n\nfib_nhc_update_mtu() walks the nexthop exception table under RTNL, but\nRTNL does not serialize this walk with PMTU exception updates. The walk\nuses rcu_dereference_protected() with a constant true condition without\nholding fnhe_lock.\n\nThe following interleaving can therefore occur:\n\n  CPU 0                              CPU 1\n  fib_nhc_update_mtu()               update_or_create_fnhe()\n    load fnhe                          spin_lock_bh(\u0026fnhe_lock)\n                                       fnhe_remove_oldest()\n                                         unlink fnhe\n                                         kfree_rcu(fnhe, rcu)\n    \u003cquiescent state\u003e\n    access fnhe after grace period\n\nKASAN reported:\n\n  BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410\n  Read of size 8 at addr ffff888107d49000 by task poc/90\n  Call Trace:\n   fib_nhc_update_mtu+0x3df/0x410\n   fib_sync_mtu+0x7a/0xd0\n   fib_netdev_event+0x229/0x3f0\n   netif_set_mtu_ext+0x33a/0x570\n   dev_set_mtu+0x88/0x120\n\nThe same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a\npair and other writers serialize them with fnhe_lock. RCU alone prevents\nreclamation, but would still allow concurrent writers to leave a mixed\npair.\n\nWalk the table under RCU and acquire fnhe_lock only while updating each\nexception. RCU keeps the current entry alive while the short critical\nsection serializes its paired PMTU fields. This avoids holding the global\nlock while scanning all 2048 buckets for every nexthop."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
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          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The UAF is hit in fib_nhc_update_mtu(), entered only via NETDEV_CHANGEMTU from dev_set_mtu()/netif_set_mtu_ext() (SIOCSIFMTU or RTM_NEWLINK IFLA_MTU); per kernel CNA guidance this administrative netdev path is Local even though the racing update_or_create_fnhe() leg is driven by received ICMP PMTU packets.\nAC:L - KASAN reproduced this from task poc/90 by racing dev_set_mtu() against concurrent PMTU exception creation/eviction; an attacker controls both CPUs by running an MTU-change thread alongside ICMP or socket traffic that floods update_or_create_fnhe() and fnhe_remove_oldest(), without depending on uncontrollable victim state.\nPR:L - Changing interface MTU requires CAP_NET_ADMIN checked via ns_capable(net-\u003euser_ns, CAP_NET_ADMIN) on SIOCSIFMTU and RTM_SETLINK; unprivileged local users obtain this in a user+network namespace (unshare -Urn) and can then race MTU changes against PMTU exception updates on the same host.\nUI:N - Exploitation requires only the attacker\u0027s own MTU ioctl/netlink operations and concurrent network or socket traffic to populate/evict fib_nh_exception entries; no victim mount, click, or other voluntary action is needed beyond normal host networking.\nS:U - The slab use-after-free corrupts host kernel IPv4 routing state within the same kernel security authority; it is not a VM escape, IOMMU bypass, or other cross-boundary scope change.\nC:H - KASAN reported a slab use-after-free read of 8 bytes in fib_nhc_update_mtu() on a fib_nh_exception freed by kfree_rcu() from fnhe_remove_oldest(); UAF on this kmalloc object enables heap grooming for arbitrary kernel memory disclosure.\nI:H - The buggy walk both reads and writes fnhe_pmtu and fnhe_mtu_locked on potentially freed fib_nh_exception objects without fnhe_lock, providing standard slab reuse primitives for attacker-controlled kernel writes and potential control-flow hijack.\nA:H - The confirmed KASAN slab-use-after-free in fib_nhc_update_mtu() can immediately BUG/oops the kernel; the attacker can retrigger the race by repeating MTU toggles concurrent with PMTU exception flooding for persistent denial of service."
            }
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        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-25T05:41:13.058Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/fd39e711866498ae94fcf9acf6f422a4f045b681"
        },
        {
          "url": "https://git.kernel.org/stable/c/e1e602d6b22d5cb1641c4459c487eb18bf569e0a"
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        {
          "url": "https://git.kernel.org/stable/c/e00f7d2b5f2540a3415a229c982af7a25ff6362e"
        },
        {
          "url": "https://git.kernel.org/stable/c/dfe388da13aa784851e5ebbea90afbb099075761"
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        {
          "url": "https://git.kernel.org/stable/c/ed503eaad62f20cdd5122d7c3078a648a99c8f16"
        },
        {
          "url": "https://git.kernel.org/stable/c/bc5bde9ce3cc36502839dfe98e068f7303a50982"
        }
      ],
      "title": "ipv4: fix use-after-free in fib_nhc_update_mtu()",
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  "cveMetadata": {
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-74656",
    "datePublished": "2026-08-22T15:32:30.729Z",
    "dateReserved": "2026-08-15T05:44:03.923Z",
    "dateUpdated": "2026-08-25T05:41:13.058Z",
    "state": "PUBLISHED"
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      "cve": "CVE-2026-74656",
      "date": "2026-08-29",
      "epss": "0.00126",
      "percentile": "0.0253"
    },
    "microsoft_vex": {
      "current_release_date": "2026-08-24T14:50:22.000Z",
      "cve": "CVE-2026-74656",
      "id": "msrc_CVE-2026-74656",
      "initial_release_date": "2026-08-24T01:07:27.000Z",
      "product_status:known_affected": "1",
      "source": "Microsoft CSAF VEX",
      "status": "final",
      "title": "ipv4: fix use-after-free in fib_nhc_update_mtu()",
      "url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-74656.json",
      "version": "2"
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    "nvd": "{\"cve\":{\"id\":\"CVE-2026-74656\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2026-08-22T16:16:39.177\",\"lastModified\":\"2026-08-25T06:18:46.560\",\"vulnStatus\":\"Received\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"In the Linux kernel, the following vulnerability has been resolved:\\n\\nipv4: fix use-after-free in fib_nhc_update_mtu()\\n\\nfib_nhc_update_mtu() walks the nexthop exception table under RTNL, but\\nRTNL does not serialize this walk with PMTU exception updates. The walk\\nuses rcu_dereference_protected() with a constant true condition without\\nholding fnhe_lock.\\n\\nThe following interleaving can therefore occur:\\n\\n  CPU 0                              CPU 1\\n  fib_nhc_update_mtu()               update_or_create_fnhe()\\n    load fnhe                          spin_lock_bh(\u0026fnhe_lock)\\n                                       fnhe_remove_oldest()\\n                                         unlink fnhe\\n                                         kfree_rcu(fnhe, rcu)\\n    \u003cquiescent state\u003e\\n    access fnhe after grace period\\n\\nKASAN reported:\\n\\n  BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410\\n  Read of size 8 at addr ffff888107d49000 by task poc/90\\n  Call Trace:\\n   fib_nhc_update_mtu+0x3df/0x410\\n   fib_sync_mtu+0x7a/0xd0\\n   fib_netdev_event+0x229/0x3f0\\n   netif_set_mtu_ext+0x33a/0x570\\n   dev_set_mtu+0x88/0x120\\n\\nThe same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a\\npair and other writers serialize them with fnhe_lock. RCU alone prevents\\nreclamation, but would still allow concurrent writers to leave a mixed\\npair.\\n\\nWalk the table under RCU and acquire fnhe_lock only while updating each\\nexception. RCU keeps the current entry alive while the short critical\\nsection serializes its paired PMTU fields. This avoids holding the global\\nlock while scanning all 2048 buckets for every nexthop.\"}],\"affected\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"affectedData\":[{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"unaffected\",\"programFiles\":[\"include/net/route.h\",\"net/ipv4/fib_semantics.c\",\"net/ipv4/route.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"fd39e711866498ae94fcf9acf6f422a4f045b681\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"e1e602d6b22d5cb1641c4459c487eb18bf569e0a\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"e00f7d2b5f2540a3415a229c982af7a25ff6362e\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"dfe388da13aa784851e5ebbea90afbb099075761\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"5a28a4b22dde92f9d293b94236314b8d6181dc4a\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"63996ffc594d128ccec8fc0983f91effd2d3adc4\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"ed503eaad62f20cdd5122d7c3078a648a99c8f16\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"af7d6cce53694a88d6a1bb60c9a239a6a5144459\",\"lessThan\":\"bc5bde9ce3cc36502839dfe98e068f7303a50982\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"b427832009b97a3ee412ef643a80b15372a7754d\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"2b7e4c735933be79882aba2bed9afa789e03c62f\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"8d59c3a6376bbc6dd3f7303968a719ffba75a4f1\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"9b4869cf385aa16f89c0f019eed4ec4e36aa441c\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"ff34695ced21e2dfa04d0fa1c5f6c35011fa8117\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"3.16.62\",\"lessThan\":\"3.17\",\"versionType\":\"semver\",\"status\":\"affected\"},{\"version\":\"4.4.162\",\"lessThan\":\"4.5\",\"versionType\":\"semver\",\"status\":\"affected\"},{\"version\":\"4.9.134\",\"lessThan\":\"4.10\",\"versionType\":\"semver\",\"status\":\"affected\"},{\"version\":\"4.14.77\",\"lessThan\":\"4.15\",\"versionType\":\"semver\",\"status\":\"affected\"},{\"version\":\"4.18.15\",\"lessThan\":\"4.19\",\"versionType\":\"semver\",\"status\":\"affected\"}]},{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"affected\",\"programFiles\":[\"include/net/route.h\",\"net/ipv4/fib_semantics.c\",\"net/ipv4/route.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"4.19\",\"status\":\"affected\"},{\"version\":\"0\",\"lessThan\":\"4.19\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"5.10.265\",\"lessThanOrEqual\":\"5.10.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"5.15.216\",\"lessThanOrEqual\":\"5.15.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.1.183\",\"lessThanOrEqual\":\"6.1.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.6.152\",\"lessThanOrEqual\":\"6.6.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.12.104\",\"lessThanOrEqual\":\"6.12.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"6.18.45\",\"lessThanOrEqual\":\"6.18.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.1.9\",\"lessThanOrEqual\":\"7.1.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.2\",\"lessThanOrEqual\":\"*\",\"versionType\":\"original_commit_for_fix\",\"status\":\"unaffected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H\",\"baseScore\":7.8,\"baseSeverity\":\"HIGH\",\"attackVector\":\"LOCAL\",\"attackComplexity\":\"LOW\",\"privilegesRequired\":\"LOW\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"HIGH\",\"integrityImpact\":\"HIGH\",\"availabilityImpact\":\"HIGH\"},\"exploitabilityScore\":1.8,\"impactScore\":5.9}]},\"references\":[{\"url\":\"https://git.kernel.org/stable/c/5a28a4b22dde92f9d293b94236314b8d6181dc4a\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/63996ffc594d128ccec8fc0983f91effd2d3adc4\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/bc5bde9ce3cc36502839dfe98e068f7303a50982\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/dfe388da13aa784851e5ebbea90afbb099075761\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/e00f7d2b5f2540a3415a229c982af7a25ff6362e\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/e1e602d6b22d5cb1641c4459c487eb18bf569e0a\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/ed503eaad62f20cdd5122d7c3078a648a99c8f16\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/fd39e711866498ae94fcf9acf6f422a4f045b681\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"}]}}",
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-08-25T08:53:25+00:00",
      "cve": "CVE-2026-74656",
      "id": "CVE-2026-74656",
      "initial_release_date": "2026-08-22T00:00:00+00:00",
      "product_status:known_affected": "274",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: ipv4: fix use-after-free in fib_nhc_update_mtu()",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-74656.json",
      "version": "3"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.