CVE-2025-39682
CVE-2025-39682 is a security flaw in Linux Kernel, the core of Linux, which runs most servers, Android phones and many home routers. Attackers are using a flaw in Linux Kernel. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 18 Sep 2026.[1]
Linux Kernel is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 2902c3ebcca52ca845c03182000e8d71d3a5196f, c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677, 3439c15ae91a517cf3c650ea15a8987699416ad9 and 29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e and others.
Description
CISA calls it an improper check for unusual or exceptional conditions in Linux Kernel. In its words: “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.”[1]
The CVE record adds: “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…”[2]
The underlying weakness is CWE-754, in MITRE’s list of common software weaknesses.
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Linux | commit 2902c3ebcca5 |
| Linux | commit c09dd3773b59 |
| Linux | commit 3439c15ae91a |
| Linux | commit 29c0ce3c8cdb |
| Linux | commit 62708b9452f8 |
| SIMATIC CN 4100 | V5.0 |
Most families won’t have Linux Kernel; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 18 Sep 2026, 378 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 21 Sep 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 3%. That’s in the top 12% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
Linux has published a fix or advice.[3] CISA’s 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).”[1]
- Check whether you use it.Linux Kernel is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 2902c3ebcca52ca845c03182000e8d71d3a5196f, c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677, 3439c15ae91a517cf3c650ea15a8987699416ad9 and 29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e and others), or not in use. CISA gave US government agencies until 21 Sep 2026 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 3%.[4] |
See also
Other Linux Kernel flaws
- CVE-2026-46300: missing initialization of a variable ·
- CVE-2026-53266: out-of-bounds write ·
- CVE-2025-39964: race condition ·
- CVE-2026-53362: unspecified ·
- CVE-2022-0995: out-of-bounds write ·
- CVE-2022-0492: improper authentication ·
Similar flaws
- CVE-2024-3393, Palo Alto Networks PAN-OS: malicious DNS packet ·
- CVE-2023-41993, Apple devices: webKit code execution ·
- CVE-2023-41992, Apple devices: kernel privilege escalation ·
Added the same day
- CVE-2026-53266, Linux Kernel: out-of-bounds write ·
- CVE-2025-39964, Linux Kernel: race condition ·
Every Linux flaw on CISA’s list · Everything added in September 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability”. Known Exploited Vulnerabilities Catalog, added 18 Sep 2026.
- ^ National Institute of Standards and Technology. “CVE-2025-39682 Detail”. National Vulnerability Database, published 5 Sep 2025.
- ^ Linux. Advisories and fixes: git.kernel.org, git.kernel.org, git.kernel.org, git.kernel.org, git.kernel.org.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.033, percentile 88.3, 31 Oct 2026.
- CVE Program. “CVE-2025-39682”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.