Executive Summary

CVE-2025-39682 exposes a flaw in the Linux kernel’s TLS stack where zero‑length records from the rx_list can be mishandled during recvmsg() processing. The bug allows an attacker to manipulate record type detection, potentially bypassing security checks or causing denial‑of‑service. The vulnerability is actively exploited in the wild (CISA KEV).

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Authoritative CVE Metadata - CVSS Base Score: 9.8 (CRITICAL) - Published: 2025-09-05T18:15:44.670 - Last Modified: 2026-09-19T04:17:35.263

[!CAUTION] Known Exploited Vulnerability: YES (CISA KEV Added: 2026-09-18)

Original 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.

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