CVE-2025-21762: Linux Kernel Use-After-Free Vulnerability

CVE-2025-21762 Overview

CVE-2025-21762 is a use-after-free (UAF) vulnerability [CWE-416] in the Linux kernel's Address Resolution Protocol (ARP) subsystem. The flaw resides in the arp_xmit() function, which can be invoked without RTNL or Read-Copy-Update (RCU) protection. Without proper synchronization, concurrent code paths can free objects while arp_xmit() still references them, leading to memory corruption. The upstream fix introduces RCU protection to prevent the race.

The vulnerability affects multiple Linux kernel branches, including 6.14-rc1 and 6.14-rc2. Downstream distributions such as Debian have shipped LTS advisories, and Siemens has issued product security advisories for affected industrial systems.

Critical Impact
Local, low-privileged attackers can trigger a kernel-space use-after-free, potentially leading to memory corruption, denial of service, or privilege escalation on affected Linux systems.

Affected Products

Linux kernel (multiple stable branches, including 6.14-rc1 and 6.14-rc2)
Debian LTS distributions (per advisories dated March 2025 and May 2025)
Siemens industrial products referenced in advisories SSA-265688 and SSA-503939

Discovery Timeline

2025-02-27 - CVE-2025-21762 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-21762

Vulnerability Analysis

The defect is a classic use-after-free race condition in the kernel networking stack. The arp_xmit() function is responsible for transmitting ARP packets on a network device. The upstream commit message states that arp_xmit() can be called without holding the RTNL (Routing Netlink) lock or without RCU read-side protection.

When a caller executes arp_xmit() without either serialization primitive, a concurrent path may release a networking object (for example, a routing or neighbor structure) while arp_xmit() is still dereferencing it. The result is access to freed kernel memory, a canonical use-after-free condition.

Because the affected code sits in the kernel's packet transmission path, the object lifetime bug can be triggered under normal network activity when a local attacker generates crafted ARP traffic or manipulates network state.

Root Cause

The root cause is missing synchronization around pointer dereferences inside arp_xmit(). Kernel networking objects are typically freed under RCU grace periods, so readers must hold an rcu_read_lock() (or the RTNL mutex) to guarantee that a referenced object remains valid for the duration of use. arp_xmit() lacked that guarantee, opening a UAF window between the read and the concurrent free.

The fix, applied across stable branches in commits including 01d1b5c9abca, 10f555e3f573, 2c331718d338, 307cd1e2d3cb, a42b69f69216, d9366ac2f956, e9f4dee534eb, and f189654459423, wraps the affected accesses in RCU protection so the pointed-to object cannot be freed while arp_xmit() is running.

Attack Vector

Exploitation requires local access with low privileges and no user interaction. The attack surface is the local kernel networking stack. An attacker with the ability to generate ARP traffic or influence network device state can race the transmit path against a concurrent free operation. Successful exploitation of a kernel UAF can escalate to arbitrary kernel memory corruption, kernel information disclosure, denial of service, or local privilege escalation.

The vulnerability manifests in the ARP transmit path. See the upstream kernel commits and referenced advisories for the specific patch diffs and affected call sites.

Detection Methods for CVE-2025-21762

Indicators of Compromise

Unexpected kernel oops or general protection fault messages in dmesg referencing arp_xmit, neigh_, or related ARP/neighbor symbols
KASAN (Kernel Address Sanitizer) reports flagging use-after-free in the ARP transmit path on instrumented builds
Unexpected kernel panics or system freezes correlated with high ARP traffic or network device reconfiguration events

Detection Strategies

Inventory running Linux kernel versions against the fixed commits published on git.kernel.org and vendor advisories from Debian and Siemens
Monitor kernel crash telemetry for signatures involving ARP or neighbor subsystem functions
Correlate local user activity (network namespace manipulation, ARP flooding utilities) with kernel instability events

Monitoring Recommendations

Ship /var/log/kern.log, journald, and crash dumps to a centralized log platform for kernel-fault analysis
Alert on repeated non-fatal kernel warnings originating from networking subsystems on production hosts
Track package versions of linux-image-* across the fleet to identify unpatched nodes

How to Mitigate CVE-2025-21762

Immediate Actions Required

Apply the vendor-supplied kernel updates from your Linux distribution, prioritizing internet-facing and multi-tenant hosts
Restrict local shell and container-breakout paths on shared systems, since exploitation requires local access
Review Siemens advisories SSA-265688 and SSA-503939 for firmware updates on affected industrial products

Patch Information

The fix has been merged into the stable kernel tree across multiple branches. Reference commits include 01d1b5c9abca, 10f555e3f573, 2c331718d338, 307cd1e2d3cb, a42b69f69216, d9366ac2f956, e9f4dee534eb, and f189654459423. Debian users should install kernel updates announced in the Debian LTS Advisory March 2025 and the Debian LTS Advisory May 2025. Industrial operators should follow Siemens Security Advisory SSA-265688 and Siemens Security Advisory SSA-503939.

Workarounds

No configuration-only workaround eliminates the race; patching the kernel is required
Limit accounts with local shell access on multi-tenant systems until updated kernels are deployed
Reboot hosts after installing the updated linux-image package to ensure the patched kernel is running

bash

# Verify running kernel and update on Debian/Ubuntu systems
uname -r
sudo apt-get update
sudo apt-get install --only-upgrade linux-image-$(uname -r | sed 's/[^-]*-[^-]*-//')
sudo reboot