CISA alerts of active exploitation of three Linux kernel flaws
CISA says three Linux kernel flaws, one critical, are actively exploited and due for federal fixes today.
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CVE-2025-39682
Improper Condition Check in Linux Kernel kTLS Receive Path Enables Local Info Leak/DoS
CISA: Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability
- CVSS 3.1
- 9.8 critical
- EPSS
- 3%p88
- Published
- ·
- Added to KEV
AI analysisSummaries by Vani AI
The Linux kernel's in-kernel TLS (kTLS) implementation mishandles zero-length records on the rx_list: each recvmsg() call must process either contiguous DATA records or a single non-DATA record, and the missed corner case occurs when the initial record is pulled from the rx_list and is zero length, breaking the check on record-type changes after zero-copy decryption. A local, low-privileged attacker able to use kTLS sockets can trigger this flaw, and the CVSS vector indicates high confidentiality impact (kernel information disclosure) and high availability impact (crash/DoS), with no integrity impact. Any Linux deployment running a kernel with the TLS (kTLS) feature enabled is affected, including Debian systems, which are listed as affected vendors in CISA's data. CISA added this vulnerability to the Known Exploited Vulnerabilities catalog on 2026-09-18, indicating known exploitation in the wild, though no public proof-of-concept is known and ransomware use is unconfirmed.
What to do: Update the Linux kernel to a release containing the upstream fix for this kTLS rx_list issue, applying Debian security updates where applicable, consistent with CISA BOD 26-04 patching guidance. As an interim mitigation, check whether kernel TLS is active (e.g., 'lsmod | grep tls' or CONFIG_TLS in the running kernel) and unload/disable the tls module on hosts that do not require kTLS. Limit local unprivileged access on multi-user systems, since exploitation requires a local low-privileged user.
| Product | Affected |
|---|---|
| Linux Kernel | — |
| Debian Linux | — |
Estimated exposure
mass≈100M+ systems potentially exposed (Linux kernel ubiquitous; only hosts with the kTLS module enabled and in use are vulnerable) — The Linux kernel runs on the majority of servers, cloud instances, and hundreds of millions of embedded devices, but the flaw is locally exploitable (AV:L/PR:L) and only reachable where kernel TLS (CONFIG_TLS/tls module) is loaded and used.
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
Description · NVD
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.
KEVCISA Known Exploited Vulnerability
- Affected
- Linux Kernel
- 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.
- Due date
- Ransomware use
- Unknown
- Products
- linux kernel, debian linux, simatic cn 4100 firmware
- Weakness
- CWE-754
- Vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
In the news
7 storiesSeptember 2026 CVE Landscape
Recorded Future says 80 vulnerabilities were actively exploited in September 2026, including NetScaler zero-days.
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CISA Warns of Linux Kernel Vulnerabilities Actively Exploited in Attacks
CISA added three actively exploited Linux kernel flaws (CVE-2025-39682, CVE-2026-53266, CVE-2025-39964) to KEV with a September 21 federal patching deadline.
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CISA Flags Three Linux Kernel Vulnerabilities Exploited in the Wild
CISA added three actively exploited Linux kernel flaws (CVE-2025-39682, CVE-2026-53266, CVE-2025-39964) to KEV; federal patch deadline September 21, 2026.
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U.S. CISA adds Linux Kernel flaws to its Known Exploited Vulnerabilities catalog
CISA added three actively exploited Linux kernel flaws (CVE-2025-39682, CVE-2025-39964, CVE-2026-53266) to its KEV catalog, federal patch deadline September 21, 2026.
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Organizations Warned of 3 Exploited Linux Kernel Vulnerabilities
CISA added three exploited Linux kernel flaws to its KEV catalog, ordering federal agencies to patch within three days.
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CISA Adds One Known Exploited Vulnerability to Catalog
CISA added actively exploited Linux kernel flaw CVE-2025-39682 to its KEV catalog, requiring federal agencies to prioritize remediation under BOD 26-04.
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