Vulnerability-Lookup

FKIE_CVE-2024-49940

Vulnerability from fkie_nvd - Published: 2024-10-21 18:15 - Updated: 2026-10-30 13:17

Severity

7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
5.5 (Medium) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Summary

In the Linux kernel, the following vulnerability has been resolved: l2tp: prevent possible tunnel refcount underflow When a session is created, it sets a backpointer to its tunnel. When the session refcount drops to 0, l2tp_session_free drops the tunnel refcount if session->tunnel is non-NULL. However, session->tunnel is set in l2tp_session_create, before the tunnel refcount is incremented by l2tp_session_register, which leaves a small window where session->tunnel is non-NULL when the tunnel refcount hasn't been bumped. Moving the assignment to l2tp_session_register is trivial but l2tp_session_create calls l2tp_session_set_header_len which uses session->tunnel to get the tunnel's encap. Add an encap arg to l2tp_session_set_header_len to avoid using session->tunnel. If l2tpv3 sessions have colliding IDs, it is possible for l2tp_v3_session_get to race with l2tp_session_register and fetch a session which doesn't yet have session->tunnel set. Add a check for this case.

References

URL	Tags
416baaa9-dc9f-4396-8d5f-8c081fb06d67	https://git.kernel.org/stable/c/24256415d18695b46da06c93135f5b51c548b950	Patch
416baaa9-dc9f-4396-8d5f-8c081fb06d67	https://git.kernel.org/stable/c/98691b2e28a6c3dac0680914493d54c77f215755
416baaa9-dc9f-4396-8d5f-8c081fb06d67	https://git.kernel.org/stable/c/aa7132362e79abc4f2add4d8657dd1679341edc5
416baaa9-dc9f-4396-8d5f-8c081fb06d67	https://git.kernel.org/stable/c/f7415e60c25a6108cd7955a20b2e66b6251ffe02	Patch

Impacted products

	Vendor	Product	Version
	linux	linux_kernel	*

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "net/l2tp/l2tp_core.c",
            "net/l2tp/l2tp_core.h",
            "net/l2tp/l2tp_netlink.c",
            "net/l2tp/l2tp_ppp.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "version": "3953ae7b218df4d1e544b98a393666f9ae58a78c",
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            {
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              "version": "b102bfc2a90d14f342580285782a9a51c74f7369",
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            },
            {
              "status": "affected",
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              "versionType": "git"
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            {
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              "version": "8d1c650d452c53fcb3f02a7b1d772741639f89a4",
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            },
            {
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              "version": "4.14.182",
              "versionType": "semver"
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        },
        {
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          "product": "Linux",
          "programFiles": [
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            "net/l2tp/l2tp_core.h",
            "net/l2tp/l2tp_netlink.c",
            "net/l2tp/l2tp_ppp.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
              "version": "4.15"
            },
            {
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              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
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              "status": "unaffected",
              "version": "6.1.191",
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            },
            {
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              "status": "unaffected",
              "version": "6.6.160",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.11.*",
              "status": "unaffected",
              "version": "6.11.3",
              "versionType": "semver"
            },
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              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "6.12",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
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  "configurations": [
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      "nodes": [
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          "cpeMatch": [
            {
              "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
              "matchCriteriaId": "6D5FF9C2-A011-4A64-B614-F9244ED2EA0D",
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          "operator": "OR"
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  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nl2tp: prevent possible tunnel refcount underflow\n\nWhen a session is created, it sets a backpointer to its tunnel. When\nthe session refcount drops to 0, l2tp_session_free drops the tunnel\nrefcount if session-\u003etunnel is non-NULL. However, session-\u003etunnel is\nset in l2tp_session_create, before the tunnel refcount is incremented\nby l2tp_session_register, which leaves a small window where\nsession-\u003etunnel is non-NULL when the tunnel refcount hasn\u0027t been\nbumped.\n\nMoving the assignment to l2tp_session_register is trivial but\nl2tp_session_create calls l2tp_session_set_header_len which uses\nsession-\u003etunnel to get the tunnel\u0027s encap. Add an encap arg to\nl2tp_session_set_header_len to avoid using session-\u003etunnel.\n\nIf l2tpv3 sessions have colliding IDs, it is possible for\nl2tp_v3_session_get to race with l2tp_session_register and fetch a\nsession which doesn\u0027t yet have session-\u003etunnel set. Add a check for\nthis case."
    },
    {
      "lang": "es",
      "value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: l2tp: evitar un posible desbordamiento del recuento de referencias del t\u00fanel Cuando se crea una sesi\u00f3n, establece un puntero hacia atr\u00e1s a su t\u00fanel. Cuando el recuento de referencias de la sesi\u00f3n cae a 0, l2tp_session_free descarta el recuento de referencias del t\u00fanel si session-\u0026gt;tunnel no es NULL. Sin embargo, session-\u0026gt;tunnel se establece en l2tp_session_create, antes de que el recuento de referencias del t\u00fanel se incremente mediante l2tp_session_register, lo que deja una peque\u00f1a ventana donde session-\u0026gt;tunnel no es NULL cuando el recuento de referencias del t\u00fanel no se ha incrementado. Mover la asignaci\u00f3n a l2tp_session_register es trivial, pero l2tp_session_create llama a l2tp_session_set_header_len, que usa session-\u0026gt;tunnel para obtener el encap del t\u00fanel. Agregue un argumento de encap a l2tp_session_set_header_len para evitar usar session-\u0026gt;tunnel. Si las sesiones l2tpv3 tienen identificadores en conflicto, es posible que l2tp_v3_session_get compita con l2tp_session_register y obtenga una sesi\u00f3n que a\u00fan no tenga configurado session-\u0026gt;tunnel. Agregue una verificaci\u00f3n para este caso."
    }
  ],
  "id": "CVE-2024-49940",
  "lastModified": "2026-10-30T13:17:39.873",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "LOCAL",
          "availabilityImpact": "HIGH",
          "baseScore": 7.8,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
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        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "type": "Secondary"
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        "source": "nvd@nist.gov",
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          "role": "CISA Coordinator",
          "timestamp": "2024-10-22T13:37:52.827630Z",
          "version": "2.0.3"
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  "published": "2024-10-21T18:15:15.703",
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "tags": [
        "Patch"
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    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Modified",
  "weaknesses": [
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        {
          "lang": "en",
          "value": "NVD-CWE-noinfo"
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      "source": "nvd@nist.gov",
      "type": "Primary"
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}

CVE-2024-49940 (GCVE-0-2024-49940)

Vulnerability from cvelistv5 – Published: 2024-10-21 18:01 – Updated: 2026-10-30 12:19

Title

l2tp: prevent possible tunnel refcount underflow

Summary

Severity

7.8 (High)


                        
                          CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2024-10-22 13:37 UTC

Assigner

Linux CNA under the mitre root

CNA scorecard D 53/100 over 7033 records in the last 180 days details

References

4 references

URL	Tags
https://git.kernel.org/stable/c/aa7132362e79abc4f…
https://git.kernel.org/stable/c/98691b2e28a6c3dac…
https://git.kernel.org/stable/c/f7415e60c25a6108c…
https://git.kernel.org/stable/c/24256415d18695b46…

Impacted products

2 products

Vendor	Product	Version
Linux	Linux	Affected: 3953ae7b218df4d1e544b98a393666f9ae58a78c , < aa7132362e79abc4f2add4d8657dd1679341edc5 (git) Affected: 3953ae7b218df4d1e544b98a393666f9ae58a78c , < 98691b2e28a6c3dac0680914493d54c77f215755 (git) Affected: 3953ae7b218df4d1e544b98a393666f9ae58a78c , < f7415e60c25a6108cd7955a20b2e66b6251ffe02 (git) Affected: 3953ae7b218df4d1e544b98a393666f9ae58a78c , < 24256415d18695b46da06c93135f5b51c548b950 (git) Affected: b102bfc2a90d14f342580285782a9a51c74f7369 (git) Affected: 10c15ddabbcf888922adbdd44ca3fecf6eab19d9 (git) Affected: 8d1c650d452c53fcb3f02a7b1d772741639f89a4 (git) Affected: 12b5fb58ac993c24210cf8cbc72d407d3a4e6490 (git) Affected: aef37401b467a0b1a9517c69924a1d66937e0789 (git) Affected: 3.2.99 , < 3.3 (semver) Affected: 3.16.54 , < 3.17 (semver) Affected: 4.4.225 , < 4.5 (semver) Affected: 4.9.225 , < 4.10 (semver) Affected: 4.14.182 , < 4.15 (semver)
Linux	Linux	Affected: 4.15 Unaffected: 0 , < 4.15 (semver) Unaffected: 6.1.191 , ≤ 6.1.* (semver) Unaffected: 6.6.160 , ≤ 6.6.* (semver) Unaffected: 6.11.3 , ≤ 6.11.* (semver) Unaffected: 6.12 , ≤ * (original_commit_for_fix)

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2024-49940",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2024-10-22T13:37:52.827630Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2024-10-22T13:38:50.656Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
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        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
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          ],
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              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nl2tp: prevent possible tunnel refcount underflow\n\nWhen a session is created, it sets a backpointer to its tunnel. When\nthe session refcount drops to 0, l2tp_session_free drops the tunnel\nrefcount if session-\u003etunnel is non-NULL. However, session-\u003etunnel is\nset in l2tp_session_create, before the tunnel refcount is incremented\nby l2tp_session_register, which leaves a small window where\nsession-\u003etunnel is non-NULL when the tunnel refcount hasn\u0027t been\nbumped.\n\nMoving the assignment to l2tp_session_register is trivial but\nl2tp_session_create calls l2tp_session_set_header_len which uses\nsession-\u003etunnel to get the tunnel\u0027s encap. Add an encap arg to\nl2tp_session_set_header_len to avoid using session-\u003etunnel.\n\nIf l2tpv3 sessions have colliding IDs, it is possible for\nl2tp_v3_session_get to race with l2tp_session_register and fetch a\nsession which doesn\u0027t yet have session-\u003etunnel set. Add a check for\nthis case."
        }
      ],
      "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"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The refcount underflow is triggered by local session creation \u2014 either the L2TP generic-netlink `L2TP_CMD_SESSION_CREATE` command or a `connect()` on an `AF_PPPOX`/`PX_PROTO_OL2TP` socket. A remote peer cannot itself drive `l2tp_session_create()`/`l2tp_session_register()`, though remote packets in `l2tp_udp_encap_recv()` are what subsequently dereference the prematurely freed tunnel.\nAC:L - The attacker fully controls both sides: they choose the session ID, so they can deterministically force `l2tp_session_register()` to fail (`-EEXIST` on a colliding L2TPv3 ID, or `-ENODEV`), and they can run concurrent create/teardown threads to widen the window between `l2tp_session_create()` and the tunnel refcount bump. No condition outside attacker influence is required, and L2TP is a stock distro module.\nPR:L - `pppol2tp_create()`/`pppox_create()` perform no capability check, so a plain unprivileged local user can reach `l2tp_session_create()` and `l2tp_session_register()` through `connect()`. The netlink path is `GENL_UNS_ADMIN_PERM` (CAP_NET_ADMIN against the netns user_ns), which any unprivileged user obtains via `unshare -Urn`.\nUI:N - The attacker performs the entire sequence \u2014 tunnel setup, session creation with a colliding ID, and session teardown \u2014 from their own process. No victim action or cooperating user is involved.\nS:U - The underflow and the resulting use-after-free are confined to the kernel\u0027s own l2tp objects within the same security authority; no VM, IOMMU, or sandbox boundary is crossed.\nC:H - The dropped-too-early tunnel reference frees `struct l2tp_tunnel` while `session-\u003etunnel` and the datapath still reference it, giving a use-after-free on a kmalloc\u0027d object that can be reclaimed with attacker-sprayed data and then read back (e.g. `tunnel-\u003esock`, `tunnel-\u003eencap`, `tunnel-\u003eversion` used in `l2tp_recv_common()`/`l2tp_xmit_skb()`), enabling kernel memory disclosure.\nI:H - A use-after-free on the tunnel object lets an attacker reoccupy the freed slab and control fields such as `tunnel-\u003esock`, which the transmit and receive paths dereference and pass to socket operations, yielding a write/control-flow-hijack primitive. Additionally the extra `l2tp_tunnel_dec_refcount()` corrupts kernel reference-counting state itself.\nA:H - Premature tunnel free reliably produces an oops/panic when the still-live session or a received L2TP packet dereferences the dangling `session-\u003etunnel`, and the `refcount_t` underflow itself raises a WARN that panics on `panic_on_warn` systems."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-30T12:19:44.806Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/aa7132362e79abc4f2add4d8657dd1679341edc5"
        },
        {
          "url": "https://git.kernel.org/stable/c/98691b2e28a6c3dac0680914493d54c77f215755"
        },
        {
          "url": "https://git.kernel.org/stable/c/f7415e60c25a6108cd7955a20b2e66b6251ffe02"
        },
        {
          "url": "https://git.kernel.org/stable/c/24256415d18695b46da06c93135f5b51c548b950"
        }
      ],
      "title": "l2tp: prevent possible tunnel refcount underflow",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2024-49940",
    "datePublished": "2024-10-21T18:01:59.668Z",
    "dateReserved": "2024-10-21T12:17:06.043Z",
    "dateUpdated": "2026-10-30T12:19:44.806Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

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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.

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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