CVE-2025-39682

Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability

Actively exploited (CISA KEV) Critical 9.8 CVSS 3.1

Description

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.

Description as published by the source, in English.

Key facts

Published: 2025-09-05
Last modified: 2026-09-21
Assigned by: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Scored by: nvd@nist.gov (NVD Primary)
Probability of exploitation: 0.5 % higher than 38 % of all known vulnerabilities
EPSS, next 30 days, model v2026.06.15
Fix available: Yes, see references

Actively exploited

CISA added this vulnerability to the Known Exploited Vulnerabilities catalogue on 2026-09-18, with a remediation deadline of 2026-09-21 for US federal agencies.

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.

Affected products

Vendor	Product	Versions	Status
Debian	Debian Linux	11.0	Affected
Linux	Linux Kernel	6.17	Affected
Linux	Linux Kernel	≥ 6.0, < 6.1.149	Affected
Linux	Linux Kernel	≥ 6.13, < 6.16.4	Affected
Linux	Linux Kernel	≥ 6.2, < 6.6.103	Affected
Linux	Linux Kernel	≥ 6.7, < 6.12.44	Affected
Siemens	Simatic Cn 4100	all versions listed	Not affected
Siemens	Simatic Cn 4100 Firmware	< 5.0	Affected

Weakness type

CWE-754: Improper Check for Unusual or Exceptional Conditions
The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.

References

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 Patch
https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html Mailing List, Third Party Advisory
https://cert-portal.siemens.com/productcert/html/ssa-032379.html Third Party Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-39682 US Government Resource

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Sources and freshness

CVE

What it is: A reference number naming one specific vulnerability, for example CVE-2024-9474.
What it tells you: Which vulnerability we are talking about, and in which product.
What it does NOT tell you: Nothing about how serious it is, whether anyone is exploiting it, or whether it concerns you.
Why it matters: Without a shared number, vendor, authority and tooling cannot discuss the same flaw.
Used with the others: The number is the key. NVD, KEV and EPSS all describe the same CVE.

NVD CWE CISA KEV EPSS

How to read this together

No single value should be used on its own. A high CVSS score does not mean a vulnerability is being exploited, and a low one does not mean it is harmless. The four sources answer four different questions.

CVE names the specific vulnerability.
NVD adds severity and technical detail about it.
CWE describes the type of mistake behind it.
CISA KEV shows that it is already being exploited.
EPSS estimates how likely exploitation is soon.

The order that decides what is urgent

Do you actually have the product?
Is the version you run affected?
Is it listed in CISA KEV?
Does it have a high EPSS score?
Is the system reachable from the internet or from others?
How important is the system to the business?
Are there compensating controls already in place?

The first two questions are not about the vulnerability at all. They are about you, and they remove most of the list before you start.

A short example

Fix this week

CVSS 6,5 (medium)

In CISA KEV: yes
EPSS: 94 per cent
A firewall facing the internet

Can be scheduled

CVSS 9,8 (critical)

In CISA KEV: no
EPSS: 0,04 per cent
A test server with no internet access

Sorted by CVSS alone, the one on the right comes first. It is the more serious flaw in theory. But the one on the left is being used against real organisations right now, and it sits where an attacker can reach it. That is the one to fix first.

The critical one still has to be fixed. It just does not have to be fixed today.

This product uses the NVD API but is not endorsed or certified by the NVD. CVE and the CVE logo are registered trademarks of The MITRE Corporation; CVE records are used under the CVE Terms of Use. CWE is a trademark of The MITRE Corporation, used per the CWE Terms of Use. KEV data from CISA (CC0). Exploitation probability from EPSS, a FIRST.org project. Open-source package data from OSV and the GitHub Advisory Database (CC-BY 4.0).

Scores and vectors are shown as published by the sources. The ordering of results is Berigo's own editorial assessment, described on the search page, and is not NVD data. An empty result never means a system is safe.