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CVE Record

CVE-2024-49940: l2tp: prevent possible tunnel refcount underflow

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.

HighCVSS 7.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A race in the Linux kernel’s L2TP session lifecycle can mismanage a tunnel reference count. A locally authenticated, low-privilege user may be able to trigger serious kernel-level consequences, potentially affecting confidentiality, integrity, and availability. Prioritize systems using affected kernels where untrusted local users can reach L2TP functionality.

Executive priority

Treat this as a high-priority kernel maintenance issue, especially on shared Linux systems or hosts running untrusted workloads. It is not documented as remotely exploitable or actively exploited in the supplied evidence, so urgency should be driven by local-user exposure, L2TP reachability, and whether supported security updates are available.

Technical view

The flaw occurs because an L2TP session can reference its tunnel before registration increments the tunnel reference count. Concurrent session lookup, registration, and teardown can therefore cause a tunnel refcount underflow or expose a session lacking its tunnel pointer. The kernel fix changes when the backpointer is assigned and adds a missing-tunnel check for colliding L2TPv3 session IDs.

Likely exposure

Exposure is limited to Linux systems running an affected kernel where a low-privilege local user can reach the vulnerable L2TP session paths. The supplied version data is irregular and includes commit identifiers, so vulnerability managers should map distribution kernel packages against vendor advisories or the referenced stable fixes rather than rely only on upstream version labels.

Exploitation context

The CVSS vector describes a local, low-complexity attack requiring low privileges and no user interaction, with potentially high confidentiality, integrity, and availability impact. The bundle marks this CVE as absent from KEV and provides no evidence of active exploitation or a public exploit. Exact triggering prerequisites beyond the documented L2TP race are not established here.

Researcher notes

The vulnerable window spans session creation, registration, lookup, and destruction. The correction delays tunnel backpointer assignment, passes encapsulation explicitly while calculating header length, and rejects L2TPv3 lookup results whose tunnel pointer is not yet set. The supplied affected-version list is insufficiently clear for definitive branch-level conclusions; confirm inclusion or backport status through kernel or distribution records.

Mitigation direction

  • Apply a vendor-supported kernel update containing the referenced upstream fixes.
  • Confirm distribution backports rather than judging exposure solely by the displayed kernel version.
  • If patching is delayed, review vendor guidance for safely restricting unused L2TP functionality.
  • Prioritize multi-user systems where untrusted local accounts or workloads can access L2TP paths.

Validation and detection

  • Inventory running kernel package versions across Linux hosts.
  • Compare package changelogs or vendor advisories with CVE-2024-49940 and the referenced commits.
  • Determine whether L2TP functionality is present and reachable by untrusted local users or workloads.
  • After updating, verify the patched kernel is running following any required reboot.
  • Record exceptions where vendor evidence confirms a backport or unaffected build.
Prepared
Confidence
medium
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2024-49940 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.8 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
3Timeline events
1ADP providers
3Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.8High
CVSS 3.1 vector shape for CVE-2024-49940Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux3953ae7b218df4d1e544b98a393666f9ae58a78c, 3953ae7b218df4d1e544b98a393666f9ae58a78c, b102bfc2a90d14f342580285782a9a51c74f7369, 10c15ddabbcf888922adbdd44ca3fecf6eab19d9, 8d1c650d452c53fcb3f02a7b1d772741639f89a4, 12b5fb58ac993c24210cf8cbc72d407d3a4e6490, aef37401b467a0b1a9517c69924a1d66937e0789, 3.2.99, 3.16.54, 4.4.225, 4.9.225, 4.14.182unaffected
LinuxLinux4.15, 0, 6.11.3, 6.12affected
Weakness

CWE details

No CWE listed

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