Linux Kernel TLS: zero-length record handling flaw
CVE-2025-39682 Published on September 5, 2025
tls: fix handling of zero-length records on the rx_list
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.
Known Exploited Vulnerability
This Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability is part of CISA's list of Known Exploited Vulnerabilities. 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.
The following remediation steps are recommended / required by September 21, 2026: 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 applicab
Weakness Type
Improper Check for Unusual or Exceptional Conditions
The software 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 software.
Products Associated with CVE-2025-39682
You can be notified by email with stack.watch whenever vulnerabilities like CVE-2025-39682 are published in these products:
Affected Versions
Linux:- Version 84c61fe1a75b4255df1e1e7c054c9e6d048da417 and below 2902c3ebcca52ca845c03182000e8d71d3a5196f is affected.
- Version 84c61fe1a75b4255df1e1e7c054c9e6d048da417 and below c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677 is affected.
- Version 84c61fe1a75b4255df1e1e7c054c9e6d048da417 and below 3439c15ae91a517cf3c650ea15a8987699416ad9 is affected.
- Version 84c61fe1a75b4255df1e1e7c054c9e6d048da417 and below 29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e is affected.
- Version 84c61fe1a75b4255df1e1e7c054c9e6d048da417 and below 62708b9452f8eb77513115b17c4f8d1a22ebf843 is affected.
- Version 6.0 is affected.
- Before 6.0 is unaffected.
- Version 6.1.149, <= 6.1.* is unaffected.
- Version 6.6.103, <= 6.6.* is unaffected.
- Version 6.12.44, <= 6.12.* is unaffected.
- Version 6.16.4, <= 6.16.* is unaffected.
- Version 6.17, <= * is unaffected.
- Before V5.0 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.