CVE-2025-21764: Linux Kernel Use-After-Free Vulnerability

CVE-2025-21764 Overview

CVE-2025-21764 is a Use After Free (UAF) vulnerability discovered in the Linux kernel's Neighbor Discovery (ndisc) subsystem. The vulnerability exists in the ndisc_alloc_skb() function, which can be called without proper RTNL (Routing Netlink) or RCU (Read-Copy-Update) protection being held. This missing synchronization mechanism creates a race condition that can lead to memory corruption through use-after-free scenarios.

Critical Impact

Local attackers with low privileges can potentially exploit this vulnerability to achieve high confidentiality, integrity, and availability impact on affected Linux systems. The lack of proper RCU protection in network stack allocation functions represents a serious kernel-level security risk.

Affected Products

• Linux Kernel (multiple versions)
• Linux Kernel 6.14-rc1
• Linux Kernel 6.14-rc2

Discovery Timeline

• 2025-02-27 - CVE-2025-21764 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-21764

Vulnerability Analysis

The vulnerability resides in the ndisc_alloc_skb() function within the Linux kernel's IPv6 Neighbor Discovery protocol implementation. This function is responsible for allocating socket buffers (skb) used in neighbor discovery operations, which are essential for IPv6 address resolution and router discovery.

The core issue stems from insufficient synchronization when accessing shared data structures. In the Linux kernel, RCU is a critical synchronization mechanism that allows readers to access shared data without blocking, while ensuring that memory is not freed until all readers have finished. When ndisc_alloc_skb() is called without RTNL or RCU protection, there exists a window where memory referenced by the function could be freed by another CPU or thread while still being accessed.

This type of vulnerability can allow a local attacker to manipulate kernel memory, potentially leading to privilege escalation, information disclosure, or system crashes.

Root Cause

The root cause is a missing RCU read-side critical section in the ndisc_alloc_skb() function. The function accesses data structures that can be modified or freed by other kernel paths, but it does not acquire the necessary RCU read lock before doing so. Without this protection, a concurrent operation (such as interface deletion or configuration change) could free the memory being accessed, resulting in a use-after-free condition.

The fix adds proper RCU protection by wrapping the critical code path with rcu_read_lock() and rcu_read_unlock() calls, ensuring that referenced memory cannot be reclaimed while the function is executing.

Attack Vector

Exploitation requires local access to the system with low privileges. An attacker would need to trigger specific network operations that invoke ndisc_alloc_skb() while simultaneously causing concurrent modifications to the network device or neighbor discovery data structures.

The attack scenario involves creating a race condition between:

1. A thread invoking neighbor discovery operations that call ndisc_alloc_skb()
2. A concurrent thread modifying network interface configuration or triggering memory deallocation

Successful exploitation could allow the attacker to corrupt kernel memory, potentially leading to privilege escalation or arbitrary code execution in kernel context.

Detection Methods for CVE-2025-21764

Indicators of Compromise

• Unexpected kernel panics or oops messages referencing ndisc_alloc_skb or related ndisc functions
• System instability when handling IPv6 neighbor discovery operations
• Kernel memory corruption indicators in dmesg logs related to the networking subsystem
• KASAN (Kernel Address Sanitizer) reports indicating use-after-free in network allocation paths

Detection Strategies

• Enable KASAN in development and testing environments to detect memory safety violations
• Monitor system logs for kernel warnings or panics involving the ndisc or IPv6 networking code paths
• Deploy kernel live patching solutions to track and alert on vulnerable kernel versions
• Use SentinelOne Singularity platform to monitor for suspicious kernel-level activity and exploitation attempts

Monitoring Recommendations

• Configure auditd to monitor for unusual network namespace operations
• Enable kernel tracing on ndisc-related functions in high-security environments
• Implement automated kernel version tracking across your infrastructure to identify unpatched systems
• Monitor for abnormal IPv6 neighbor discovery traffic patterns that could indicate exploitation attempts

How to Mitigate CVE-2025-21764

Immediate Actions Required

• Update the Linux kernel to a patched version that includes RCU protection in ndisc_alloc_skb()
• Prioritize patching systems that are exposed to local user access or container escape risks
• Review and restrict local system access where possible until patches can be applied
• Consider enabling kernel security modules (SELinux/AppArmor) in enforcing mode to limit exploitation potential

Patch Information

The Linux kernel maintainers have released patches across multiple stable kernel branches. The fix adds proper RCU protection to the ndisc_alloc_skb() function to prevent the use-after-free condition. Patches are available through the following kernel git commits:

• Kernel Patch 3c2d705f5adf
• Kernel Patch 628e6d18930b
• Kernel Patch 96fc896d0e5b
• Kernel Patch 9e0ec817eb41

Debian users should consult the Debian LTS security announcements for distribution-specific package updates.

Workarounds

• Limit local user access to systems running vulnerable kernel versions where possible
• Implement strict access controls and user privilege restrictions to reduce attack surface
• Consider disabling IPv6 on systems where it is not required as a temporary mitigation
• Deploy network segmentation to limit exposure of vulnerable systems

# Check current kernel version
uname -r

# Verify if IPv6 can be temporarily disabled (if not required)
sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1

# Make persistent (add to /etc/sysctl.conf if IPv6 is not needed)
echo "net.ipv6.conf.all.disable_ipv6 = 1" >> /etc/sysctl.conf
echo "net.ipv6.conf.default.disable_ipv6 = 1" >> /etc/sysctl.conf