CVE-2025-38471 Overview
CVE-2025-38471 is a use-after-free vulnerability [CWE-416] in the Linux kernel's Transport Layer Security (TLS) subsystem. The flaw resides in the tls_strp_check_rcv function inside net/tls/tls_strp.c. Recent changes in net-next TCP began compacting socket buffers (skbs) more aggressively, exposing a latent bug where the TLS receive path may operate on a stale skb when validating decrypt state and geometry across the queue. A local attacker with the ability to use TLS sockets can trigger a slab use-after-free, as confirmed by a KASAN report showing a 4-byte read at a freed address. Successful exploitation impacts kernel confidentiality, integrity, and availability.
Critical Impact
A local, low-privileged user can trigger kernel memory corruption via TLS socket operations, potentially leading to privilege escalation or a denial-of-service condition on affected Linux systems.
Affected Products
- Linux Kernel (multiple versions including the 6.1 and 6.16 release candidate series)
- Debian Linux 11.0
- Siemens products bundling affected kernel versions (see Siemens advisory SSA-082556)
Discovery Timeline
- 2025-07-28 - CVE-2025-38471 published to the National Vulnerability Database (NVD)
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-38471
Vulnerability Analysis
The vulnerability lives in the kernel TLS (kTLS) receive strparser logic. When tls_sw_recvmsg waits for a record via tls_rx_rec_wait, it calls tls_strp_check_rcv to confirm that queued skbs share matching decrypt state and geometry before assembling a TLS record. The check iterates over skbs cached by the TLS strparser, but after recent TCP changes that compact skbs more aggressively, those cached pointers can reference an skb that TCP has already freed or coalesced. Reading fields from the freed skb produces the KASAN slab-use-after-free reported at net/tls/tls_strp.c:436. The fix always reloads the queue when reading from the socket, ensuring the strparser observes the current skb layout rather than stale state.
Root Cause
The root cause is a stale reference in the TLS strparser to skbs owned by the TCP receive queue. The prior implementation only refreshed the cached queue view when strp->stm.full_len was unset. Aggressive skb compaction in the underlying TCP layer invalidated cached skb pointers between refreshes, violating the assumption that queued skbs remain stable across TLS decrypt checks.
Attack Vector
Exploitation requires local access and the ability to open a TLS-enabled socket, then drive TCP receive activity that triggers skb compaction while a concurrent recvmsg() executes. The attack does not require user interaction. Because the freed object is a kernel slab allocation, an attacker who can shape slab reuse may pivot the use-after-free into arbitrary read/write primitives against kernel memory. At minimum, the condition is reliably reachable as a denial-of-service through kernel panic.
No public proof-of-concept exploit code has been released. The KASAN trace in the upstream commit message provides the clearest technical reference for reproducing the crash under a debug kernel; refer to the upstream kernel commit for the exact call path.
Detection Methods for CVE-2025-38471
Indicators of Compromise
- Kernel oops or KASAN reports referencing tls_strp_check_rcv, tls_rx_rec_wait, or tls_sw_recvmsg in dmesg or /var/log/kern.log.
- Unexpected process termination or kernel panics on hosts running TLS-offloaded workloads (for example, NGINX, HAProxy, or Kafka using kTLS).
- Repeated SIGKILL or segmentation events tied to processes performing setsockopt(SOL_TLS, TLS_RX, ...) operations.
Detection Strategies
- Monitor kernel ring buffer output for slab-use-after-free signatures involving the tls module on any host running an affected kernel build.
- Audit loaded kernel modules for tls and correlate against the running kernel release to identify vulnerable systems at scale.
- Use eBPF-based tracing to flag anomalous frequencies of recvmsg() on TLS sockets from unprivileged UIDs.
Monitoring Recommendations
- Forward kernel logs to a centralized SIEM and alert on stack traces containing tls_strp_check_rcv.
- Track kernel version inventory across the fleet and flag hosts running Linux kernels prior to the fix commits (1f3a429c21e0, 4ab26bce3969, 730fed2ff5e2, c76f6f437c46, cdb767915fc9).
- Enable auditd rules for setsockopt syscalls with SOL_TLS to establish baseline TLS socket usage per host.
How to Mitigate CVE-2025-38471
Immediate Actions Required
- Identify Linux hosts running affected kernel versions and prioritize those exposing local shell access to untrusted users or running multi-tenant workloads.
- Apply the vendor-supplied kernel update from your Linux distribution and reboot the affected systems.
- On Debian 11 systems, install the kernel update referenced in the Debian LTS Announcement.
- Siemens operators should follow guidance in the Siemens Security Advisory SSA-082556.
Patch Information
The upstream fix always reloads the TLS receive queue when reading from the socket. The patch is available across stable branches in the following commits: 1f3a429c21e0, 4ab26bce3969, 730fed2ff5e2, c76f6f437c46, and cdb767915fc9. Rebuild custom kernels against these commits or install the vendor-packaged kernel that includes them.
Workarounds
- Where patching is not immediately possible, restrict local shell access on hosts that use kTLS and reduce the number of untrusted local users.
- Disable kernel TLS offload for applications that support user-space TLS as a temporary alternative until the kernel is patched.
- Unload the tls kernel module on systems that do not require kTLS: modprobe -r tls and blacklist the module to prevent auto-load.
# Blacklist the tls module until the kernel is patched
echo 'blacklist tls' | sudo tee /etc/modprobe.d/blacklist-tls.conf
sudo modprobe -r tls
# Verify the module is not loaded
lsmod | grep -w tls
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
