CVE-2026-63920 (GCVE-0-2026-63920)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:55 – Updated: 2026-08-05 12:37
VLAI
Title
ipv6: validate extension header length before copying to cmsg
Summary
In the Linux kernel, the following vulnerability has been resolved: ipv6: validate extension header length before copying to cmsg ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR} cmsgs (and their IPV6_2292* legacy counterparts) by trusting the on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length. The length was validated only at parse time (ipv6_parse_hopopts(), etc.). An nftables payload-write expression can rewrite hdrlen after parsing and before the skb reaches recvmsg; the write itself is in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040 bytes from an 8-byte header. nftables is reachable from an unprivileged user namespace, so this is an unprivileged slab-out-of-bounds read: BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540 put_cmsg+0x3ac/0x540 udpv6_recvmsg+0xca0/0x1250 sock_recvmsg+0xdf/0x190 ____sys_recvmsg+0x1b1/0x620 Add ipv6_get_exthdr_len() which validates that at least two bytes are accessible before reading the hdrlen field, then checks the computed length against skb_tail_pointer(skb), returning 0 on failure. Extension headers are kept in the linear skb area by pskb_may_pull() during input, so skb_tail_pointer() is the correct bound. Use ipv6_get_exthdr_len() at all non-AH call sites: the five standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR) and the three standard cases in the extension-header walk loop (DSTOPTS, ROUTING, default). AH retains an inline bounds check because its length formula differs ((ptr[1]+2)<<2). The walk loop also gets a pre-read bounds check at the top to validate ptr before any case accesses ptr[0] or ptr[1]. When the walk loop detects a corrupted header, return from the function instead of continuing to process later socket options.
Severity
7.1 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < 81394827dfb72772c50d0ae3bdfa094428a5d76d (git)
Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < 931b4a1f13408c2507719890f78f7227c34a0282 (git)
Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < 08464413e628803bd10cb1df68d0138665f2f885 (git)
Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < a29768d56eb3798c052ad3281b05596e695a17af (git)
Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < 0d330eff318c0f44d4fb0ad2c2aef38f87f24c90 (git)
Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < a35daeabb433686234b010ebf7b53778dbd6c9b8 (git)
Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < eb18a1b1644e4cad978df2131e2bb9a2e6886992 (git)
Affected: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 , < dd433671fef381fdaf7b530c631e6b782d66e224 (git)
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Linux Linux Affected: 2.6.12
Unaffected: 0 , < 2.6.12 (semver)
Unaffected: 5.10.259 , ≤ 5.10.* (semver)
Unaffected: 5.15.210 , ≤ 5.15.* (semver)
Unaffected: 6.1.176 , ≤ 6.1.* (semver)
Unaffected: 6.6.143 , ≤ 6.6.* (semver)
Unaffected: 6.12.93 , ≤ 6.12.* (semver)
Unaffected: 6.18.35 , ≤ 6.18.* (semver)
Unaffected: 7.0.12 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
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{
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: validate extension header length before copying to cmsg\n\nip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}\ncmsgs (and their IPV6_2292* legacy counterparts) by trusting the\non-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.\nThe length was validated only at parse time (ipv6_parse_hopopts(),\netc.).  An nftables payload-write expression can rewrite hdrlen after\nparsing and before the skb reaches recvmsg; the write itself is\nin-bounds but put_cmsg() then reads up to ((hdrlen+1) \u003c\u003c 3) = 2040\nbytes from an 8-byte header.  nftables is reachable from an\nunprivileged user namespace, so this is an unprivileged\nslab-out-of-bounds read:\n\n  BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540\n   put_cmsg+0x3ac/0x540\n   udpv6_recvmsg+0xca0/0x1250\n   sock_recvmsg+0xdf/0x190\n   ____sys_recvmsg+0x1b1/0x620\n\nAdd ipv6_get_exthdr_len() which validates that at least two bytes\nare accessible before reading the hdrlen field, then checks the\ncomputed length against skb_tail_pointer(skb), returning 0 on\nfailure.  Extension headers are kept in the linear skb area by\npskb_may_pull() during input, so skb_tail_pointer() is the correct\nbound.\n\nUse ipv6_get_exthdr_len() at all non-AH call sites: the five\nstandalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)\nand the three standard cases in the extension-header walk loop\n(DSTOPTS, ROUTING, default).  AH retains an inline bounds check\nbecause its length formula differs ((ptr[1]+2)\u003c\u003c2).\n\nThe walk loop also gets a pre-read bounds check at the top to\nvalidate ptr before any case accesses ptr[0] or ptr[1].\n\nWhen the walk loop detects a corrupted header, return from the\nfunction instead of continuing to process later socket options."
        }
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            "baseSeverity": "HIGH",
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            "version": "3.1"
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              "value": "AV:L - Exploitation requires local syscalls (nftables netlink to install a payload-set rule, setsockopt for IPV6_RECV* options, and recvmsg). Although the trigger packet is received over IPv6, the mandatory nftables corruption step is classified as Local per kernel guidance.\nAC:L - The attacker fully controls the nftables payload-set rule, the on-wire extension header, socket receive options, and the recvmsg call, making the slab out-of-bounds read reliably reproducible without races or external conditions.\nPR:L - An unprivileged local user can create a user+network namespace, obtain CAP_NET_ADMIN there, configure nftables, open a UDPv6/raw socket with extension-header cmsg options, and trigger the bug without real root in the initial namespace.\nUI:N - No victim interaction is required; the attacker can send the crafted IPv6 datagram and call recvmsg on their own socket to trigger the out-of-bounds read.\nS:U - Impact is confined to kernel memory disclosure and potential kernel instability within the same kernel security domain; it does not cross a VM/host or IOMMU boundary.\nC:H - put_cmsg() can copy up to 2040 bytes past the actual extension header into userspace ancillary data, leaking adjacent slab/kernel memory (confirmed by KASAN slab-out-of-bounds in put_cmsg).\nI:N - The bug is an out-of-bounds read that copies kernel data to userspace; it does not modify kernel memory, corrupt control structures, or provide a write primitive.\nA:H - The out-of-bounds slab read can provoke a KASAN fault/kernel oops during put_cmsg(), and repeated triggering can deny availability of the affected system."
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        "dateUpdated": "2026-08-05T12:37:14.814Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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      "title": "ipv6: validate extension header length before copying to cmsg",
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    "cveId": "CVE-2026-63920",
    "datePublished": "2026-07-19T14:55:22.813Z",
    "dateReserved": "2026-07-19T07:54:57.020Z",
    "dateUpdated": "2026-08-05T12:37:14.814Z",
    "state": "PUBLISHED"
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      "date": "2026-09-21",
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        "descriptions": [
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            "lang": "en",
            "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: validate extension header length before copying to cmsg\n\nip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}\ncmsgs (and their IPV6_2292* legacy counterparts) by trusting the\non-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.\nThe length was validated only at parse time (ipv6_parse_hopopts(),\netc.).  An nftables payload-write expression can rewrite hdrlen after\nparsing and before the skb reaches recvmsg; the write itself is\nin-bounds but put_cmsg() then reads up to ((hdrlen+1) \u003c\u003c 3) = 2040\nbytes from an 8-byte header.  nftables is reachable from an\nunprivileged user namespace, so this is an unprivileged\nslab-out-of-bounds read:\n\n  BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540\n   put_cmsg+0x3ac/0x540\n   udpv6_recvmsg+0xca0/0x1250\n   sock_recvmsg+0xdf/0x190\n   ____sys_recvmsg+0x1b1/0x620\n\nAdd ipv6_get_exthdr_len() which validates that at least two bytes\nare accessible before reading the hdrlen field, then checks the\ncomputed length against skb_tail_pointer(skb), returning 0 on\nfailure.  Extension headers are kept in the linear skb area by\npskb_may_pull() during input, so skb_tail_pointer() is the correct\nbound.\n\nUse ipv6_get_exthdr_len() at all non-AH call sites: the five\nstandalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)\nand the three standard cases in the extension-header walk loop\n(DSTOPTS, ROUTING, default).  AH retains an inline bounds check\nbecause its length formula differs ((ptr[1]+2)\u003c\u003c2).\n\nThe walk loop also gets a pre-read bounds check at the top to\nvalidate ptr before any case accesses ptr[0] or ptr[1].\n\nWhen the walk loop detects a corrupted header, return from the\nfunction instead of continuing to process later socket options."
          }
        ],
        "id": "CVE-2026-63920",
        "lastModified": "2026-07-27T17:44:23.777",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "LOCAL",
                "availabilityImpact": "HIGH",
                "baseScore": 7.1,
                "baseSeverity": "HIGH",
                "confidentialityImpact": "HIGH",
                "integrityImpact": "NONE",
                "privilegesRequired": "LOW",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H",
                "version": "3.1"
              },
              "exploitabilityScore": 1.8,
              "impactScore": 5.2,
              "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
              "type": "Secondary"
            }
          ]
        },
        "published": "2026-07-19T16:17:10.153",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/08464413e628803bd10cb1df68d0138665f2f885"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/0d330eff318c0f44d4fb0ad2c2aef38f87f24c90"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/81394827dfb72772c50d0ae3bdfa094428a5d76d"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/931b4a1f13408c2507719890f78f7227c34a0282"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/a29768d56eb3798c052ad3281b05596e695a17af"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/a35daeabb433686234b010ebf7b53778dbd6c9b8"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/dd433671fef381fdaf7b530c631e6b782d66e224"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/eb18a1b1644e4cad978df2131e2bb9a2e6886992"
          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Awaiting Analysis"
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-07-21T21:37:50+00:00",
      "cve": "CVE-2026-63920",
      "id": "CVE-2026-63920",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_affected": "274",
      "product_status:known_not_affected": "1",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: ipv6: validate extension header length before copying to cmsg",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63920.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "important",
      "current_release_date": "2026-09-18T00:46:28Z",
      "cve": "CVE-2026-63920",
      "id": "CVE-2026-63920",
      "initial_release_date": "2026-07-21T13:03:33Z",
      "product_status:first_fixed": "127",
      "product_status:known_affected": "733",
      "product_status:recommended": "107",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-63920",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-63920.json",
      "version": "10"
    }
  }
}



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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.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.