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CVE Record

CVE-2025-21792: ax25: Fix refcount leak caused by setting SO_BINDTODEVICE sockopt

In the Linux kernel, the following vulnerability has been resolved: ax25: Fix refcount leak caused by setting SO_BINDTODEVICE sockopt If an AX25 device is bound to a socket by setting the SO_BINDTODEVICE socket option, a refcount leak will occur in ax25_release(). Commit 9fd75b66b8f6 ("ax25: Fix refcount leaks caused by ax25_cb_del()") added decrement of device refcounts in ax25_release(). In order for that to work correctly the refcounts must already be incremented when the device is bound to the socket. An AX25 device can be bound to a socket by either calling ax25_bind() or setting SO_BINDTODEVICE socket option. In both cases the refcounts should be incremented, but in fact it is done only in ax25_bind(). This bug leads to the following issue reported by Syzkaller: ================================================================ refcount_t: decrement hit 0; leaking memory. WARNING: CPU: 1 PID: 5932 at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31 Modules linked in: CPU: 1 UID: 0 PID: 5932 Comm: syz-executor424 Not tainted 6.13.0-rc4-syzkaller-00110-g4099a71718b0 #0 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 RIP: 0010:refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31 Call Trace: <TASK> __refcount_dec include/linux/refcount.h:336 [inline] refcount_dec include/linux/refcount.h:351 [inline] ref_tracker_free+0x710/0x820 lib/ref_tracker.c:236 netdev_tracker_free include/linux/netdevice.h:4156 [inline] netdev_put include/linux/netdevice.h:4173 [inline] netdev_put include/linux/netdevice.h:4169 [inline] ax25_release+0x33f/0xa10 net/ax25/af_ax25.c:1069 __sock_release+0xb0/0x270 net/socket.c:640 sock_close+0x1c/0x30 net/socket.c:1408 ... do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f ... </TASK> ================================================================ Fix the implementation of ax25_setsockopt() by adding increment of refcounts for the new device bound, and decrement of refcounts for the old unbound device.

MediumCVSS 5.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2025-21792 is a Linux kernel availability flaw in AX.25 networking. A local user can trigger incorrect device reference counting when a socket is bound to an AX.25 device using SO_BINDTODEVICE. The reported impact is service disruption, not data theft or privilege escalation.

Executive priority

Treat as a routine but real Linux kernel availability risk. Prioritize patching on multi-user or externally managed Linux systems where local users can run code. It does not currently justify emergency response based on the provided evidence.

Technical view

The AX.25 socket cleanup path decremented network device references in ax25_release(), but ax25_setsockopt() did not increment references when binding a device through SO_BINDTODEVICE. This mismatch can produce a refcount leak or saturation warning during socket release. The kernel fix adds reference increments for newly bound devices and decrements for old unbound devices.

Likely exposure

Exposure is most likely on Linux systems running affected kernel builds with AX.25 networking enabled or available. This is a local attack surface, so internet-facing exposure is not indicated by the sources.

Exploitation context

The issue was reported by Syzkaller. The source bundle does not show CISA KEV listing or other evidence of active exploitation. CVSS marks local access, low complexity, low privileges, no user interaction, and high availability impact.

Researcher notes

Focus validation on AX.25 socket/device binding behavior and kernel versions carrying the stable fixes. The public description provides root cause and crash context, but not complete distro-by-distro affected package mapping beyond the Debian LTS reference.

Mitigation direction

  • Update to a vendor kernel containing the referenced stable AX.25 fixes.
  • Review Debian LTS or other distribution advisories for corrected package versions.
  • Disable or restrict AX.25 support where it is not operationally required.
  • Prioritize shared multi-user Linux systems over isolated single-user hosts.

Validation and detection

  • Inventory Linux kernel versions against vendor advisories and fixed stable commits.
  • Check whether AX.25 support is enabled, loadable, or required on affected hosts.
  • Confirm updated kernels include the AX.25 SO_BINDTODEVICE refcount fix.
  • Verify no production dependency requires delaying AX.25-related kernel updates.
Prepared
Confidence
high
Sources
8

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-2025-21792 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
5.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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
2ADP providers
7Source 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
5.5CVSS 3.1MediumCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H1.83.6CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

5.5Medium
CVSS 3.1 vector shape for CVE-2025-21792Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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
cvssV3_1other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux9fd75b66b8f68498454d685dc4ba13192ae069b0, 9fd75b66b8f68498454d685dc4ba13192ae069b0, 9fd75b66b8f68498454d685dc4ba13192ae069b0, 9fd75b66b8f68498454d685dc4ba13192ae069b0, 9fd75b66b8f68498454d685dc4ba13192ae069b0, c44a453ffe16eb08acdc6129ac4fa0192dbc0456, de55a1338e6a48ff1e41ea8db1432496fbe2a62b, 9e1e088a57c23251f1cfe9601bbd90ade2ea73b9, b20a5ab0f5fb175750c6bafd4cf12daccf00c738, 452ae92b99062d2f6a34324eaf705a3b7eac9f8b, 534156dd4ed768e30a43de0036f45dca7c54818f, 4.14.277, 4.19.240, 5.4.190, 5.10.112, 5.15.35, 5.17.2unaffected
LinuxLinux5.18, 0, 6.1.129, 6.6.79, 6.12.16, 6.13.4, 6.14affected
Weakness

CWE details

No CWE listed

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