---
id: CVE-2026-15924
title: >-
  Zephyr's TLS socket layer in subsys/net/lib/sockets/sockets_tls.c keeps a
  single process-global array, client_cache, of cached client sessions that is
  shared by every TLS socket context
summary: >-
  Zephyr's TLS socket layer in subsys/net/lib/sockets/sockets_tls.c keeps a
  single process-global array, client_cache, of cached client sessions that is
  shared by every TLS socket context. The functions that mutate and read it —
  tls_sessio…
severity: medium
cvss: 5.9
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-416
vendor: zephyrproject
product: zephyr
affected:
  - zephyr >= 3.1.0 < 4.4.2
published: '2026-09-14'
updated: '2026-09-14'
sourceUpdated: '2026-09-14T21:10:41.650'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-15924'
references:
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/commit/7f9d8ee32ba9a93fc1dbb192ca2a591ac0853bdc
    label: vulnerabilities@zephyrproject.org
  - url: >-
      https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-wcgm-pq6x-v2gf
    label: vulnerabilities@zephyrproject.org
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-09-14T20:04:00.148079Z'
ingestedAt: '2026-09-14T20:14:21.146Z'
epss: 0.00308
epssPercentile: 0.21062
---

## Overview

Zephyr's TLS socket layer in subsys/net/lib/sockets/sockets_tls.c keeps a single process-global array, client_cache, of cached client sessions that is shared by every TLS socket context. The functions that mutate and read it — tls_session_save(), tls_session_get(), tls_session_cache_reset(), and the settings restore handler — allocate, free, and dereference each entry's heap buffer (entry->session). Before the fix these accesses were serialized only by the per-socket context mutex ctx->lock (assigned per socket in ctx_set_lock()), which provides no mutual exclusion between different sockets touching the shared cache.

Because CONFIG_NET_SOCKETS_TLS_MAX_CLIENT_SESSION_COUNT defaults to 1, any two concurrent client sockets contend for the same slot. A thread in tls_session_get() reading entry->session inside mbedtls_ssl_session_load() can run concurrently with another thread in tls_session_save() that selects the same entry for reuse and executes mbedtls_free(entry->session) before reallocating — a use-after-free read, and a double-free when two saves evict the same entry. Both corrupt the mbedTLS heap. The cache is reached on ordinary client paths: at connect time via tls_session_store()/tls_session_restore(), and (on main) whenever a TLS 1.3 session ticket arrives during recv()/poll() via tls_session_store_current().

Exploitation requires an application that opts into per-socket client session caching (the TLS_SESSION_CACHE socket option, off by default) and runs concurrent TLS client connections on multiple threads; the timing that opens the window is influenced by the remote peer(s), so a malicious or compromised server can raise session-ticket frequency to widen it. The reliably-demonstrable impact is memory corruption leading to a crash or heap corruption (denial of service). The fix adds a dedicated session_cache_lock mutex taken across every accessor of client_cache, serializing all reads and frees and closing the race.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.