CVE-2025-21673Network attack vector

Kernel crash via SMB hostname double free

Published Jan 31, 2025 · Updated Aug 5, 2026

A double free in affected Linux kernel SMB client releases allows remote attackers to crash the kernel through crafted DFS reconnects. cifs_put_tcp_session() frees TCP_Server_Info::hostname while the cifsd thread can still reuse and free that pointer as it reconnects across multiple DFS targets. Exploitation requires an active SMB DFS connection and a shutdown race overlapping attacker-controlled disconnects and referrals; successful triggering corrupts kernel memory and can terminate service.

CVSS severity9.8
Critical
EPSS probability0.41%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A double free in affected Linux kernel SMB client releases allows remote attackers to crash the kernel through crafted DFS reconnects. cifs_put_tcp_session() frees TCP_Server_Info::hostname while the cifsd thread can still reuse and free that pointer as it reconnects across multiple DFS targets. Exploitation requires an active SMB DFS connection and a shutdown race overlapping attacker-controlled disconnects and referrals; successful triggering corrupts kernel memory and can terminate service.

The record

CVE
CVE-2025-21673
Published
Jan 31, 2025
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-415, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 31, 2025CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=1c35a216ef77db708178ca225d796271f2f60a7a
  2. Affected versionversion=49f933bb3016269dc50074eac5f6033d127644f1
  3. Affected versionversion=5.14.19 <5.15
  4. Affected versionversion=5.15.3 <5.16
  5. Affected versionversion=5.16
  6. Affected versionversion=7be3248f313930ff3d3436d4e9ddbe9fccc1f541 <1ea68070338518a1d31ce71e6abfe1b30001b27a
  7. Affected versionversion=7be3248f313930ff3d3436d4e9ddbe9fccc1f541 <a2be5f2ba34d0c6d5ef2624b24e3d852561fcd6a
  8. Affected versionversion=7be3248f313930ff3d3436d4e9ddbe9fccc1f541 <fa2f9906a7b333ba757a7dbae0713d8a5396186e

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=1c35a216ef77db708178ca225d796271f2f60a7a; version=49f933bb3016269dc50074eac5f6033d127644f1; version=5.14.19 <5.15; version=5.15.3 <5.16; version=5.16; version=7be3248f313930ff3d3436d4e9ddbe9fccc1f541 <1ea68070338518a1d31ce71e6abfe1b30001b27a; version=7be3248f313930ff3d3436d4e9ddbe9fccc1f541 <a2be5f2ba34d0c6d5ef2624b24e3d852561fcd6a; version=7be3248f313930ff3d3436d4e9ddbe9fccc1f541 <fa2f9906a7b333ba757a7dbae0713d8a5396186e

Published CVSS scores

9.8Linux kernel CVE teamCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
5.5NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-415Double Free
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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