CVE-2025-39682: Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability

In short

Linux Kernel has a weakness classified as improper check for unusual or exceptional conditions (CWE-754). CISA added it to its Known Exploited Vulnerabilities catalog on 2026-09-18. NVD lists CVSS 3.1 at 9.8 CRITICAL.

What it is

“Linux Kernel contains an improper check for unusual or exceptional conditions vulnerability in the TLS receive path which allows a zero-length record retrieved from the rx_list to bypass the intended recvmsg() record-type handling, potentially causing subsequent TLS records to be processed using incorrect zero-copy and queuing assumptions. The impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.” — CISA KEV, retrieved 2026-11-01

“In the Linux kernel, the following vulnerability has been resolved: tls: fix handling of zero-length records on the rx_list Each recvmsg() call must process either - only contiguous DATA records (any number of them) - one non-DATA record If the next record has different type than what has already been processed we break out of the main processing loop. If the record has already been decrypted (which may be the case for TLS 1.3 where we don't know type until decryption) we queue the pending record to the rx_list. Next recvmsg() will pick it up from there. Queuing the skb to rx_list after zero-copy decrypt is not possible, since in that case we decrypted directly to the user space buffer, and we don't have an skb to queue (darg.skb points to the ciphertext skb for access to metadata like length). Only data records are allowed zero-copy, and we break the processing loop after each non-data record. So we should never zero-copy and then find out that the record type has changed. The corner case we missed is when the initial record comes from rx_list, and it's zero length.” — NVD, retrieved 2026-11-01

Who is affected

Linux Kernel. NVD lists the following versions as affected:

• Linux Linux Kernel: from 6.0 inclusive, before 6.1.149
• Linux Linux Kernel: from 6.2 inclusive, before 6.6.103
• Linux Linux Kernel: from 6.7 inclusive, before 6.12.44
• Linux Linux Kernel: from 6.13 inclusive, before 6.16.4
• Linux Linux Kernel: version 6.17
• Debian Debian Linux: version 11.0
• Siemens Simatic Cn 4100 Firmware: before 5.0

Exploitation status

CISA KEV added: 2026-09-18. Known ransomware campaign use: Not reported by CISA. Due date CISA set for US federal civilian agencies: 2026-09-21.

Scores

NVD CVSS 3.1: 9.8 CRITICAL; vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H; source [email protected].

EPSS by FIRST: 3.25%; percentile 88.34; score date 2026-10-31. EPSS estimates the probability of exploitation in the next 30 days.

CWE: CWE-754.

Timeline

CISA required action

“Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.” — CISA KEV

What to do

• Confirm whether you run Linux Kernel and which version.
• Apply the vendor update or mitigation in the advisory.
• Follow CISA’s required action.
• Review the vendor guidance on signs of compromise.
• Track the fix through completion.

References

• Patch: https://git.kernel.org/stable/c/2902c3ebcca52ca845c03182000e8d71d3a5196f
• Patch: https://git.kernel.org/stable/c/29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e
• Patch: https://git.kernel.org/stable/c/3439c15ae91a517cf3c650ea15a8987699416ad9
• Patch: https://git.kernel.org/stable/c/62708b9452f8eb77513115b17c4f8d1a22ebf843
• Patch: https://git.kernel.org/stable/c/c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677
• Mailing List, Third Party Advisory: https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html
• Third Party Advisory: https://cert-portal.siemens.com/productcert/html/ssa-032379.html
• NVD detail
• CVE.org record
• CISA KEV catalog

Sources and dates

CISA Known Exploited Vulnerabilities Catalog, version 2026.10.30; retrieved 2026-11-01. CISA KEV data: CC0 1.0. No endorsement by CISA.

NVD retrieved: 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.

EPSS scores by FIRST; score date 2026-10-31.