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

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run Linux Kernel. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-09-18, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-09-21; treat that as the outer limit, not the target.
• EPSS 2.9%, percentile 87: FIRST’s model estimates a 2.9% chance of exploitation activity in the next 30 days, higher than 87% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Editorial verdict: Patch this week. Linux Kernel: Five Exploited Root Bugs Added to CISA KEV — Five local root bugs, all with fixes and most with public exploits, are now confirmed exploited; get every Linux host onto a current kernel and reboot this week, container hosts, CI runners and shared servers first.

What it is

In the Linux kernel, the following vulnerability has been resolved: tls: fix handling of zero-length records on the rxlist 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 rxlist. Next recvmsg() will pick it up from there. Queuing the skb to rxlist 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 rxlist, and it's zero length.

CISA’s summary

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.

Weakness type: CWE-754.

Product: Linux Kernel · Assigned by: Linux · Published: 2025-09-05

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2026-09-18; federal due date 2026-09-21. Known ransomware use: Unknown.
• 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 SSVC assessment: exploitation active, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 2.88% probability of exploitation activity in the next 30 days; percentile 87.

Timeline

• 2025-09-05 — CVE record published by Linux
• 2026-09-18 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2026-09-18 — erroneo.us brief: Linux Kernel: Five Exploited Root Bugs Added to CISA KEV
• 2026-09-21 — CISA remediation deadline for US federal agencies
• 2026-10-07 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. Listed in CISA KEV notes (git.kernel.org) git.kernel.org
2. Listed in CISA KEV notes (git.kernel.org) git.kernel.org
3. Listed in CISA KEV notes (git.kernel.org) git.kernel.org
4. Listed in CISA KEV notes (git.kernel.org) git.kernel.org
5. Listed in CISA KEV notes (git.kernel.org) git.kernel.org

Vulnerability records

1. CVE.org record CVE-2025-39682 cve.org
2. NVD entry CVE-2025-39682 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov
2. Listed in CISA KEV notes (cisa.gov) cisa.gov
3. Listed in CISA KEV notes (cisa.gov) cisa.gov

Other references from the CVE record and advisory

1. lists.debian.org lists.debian.org
2. cert-portal.siemens.com cert-portal.siemens.com

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