CVE-2025-21759: Linux Kernel Use-After-Free Vulnerability

CVE-2025-21759 Overview

CVE-2025-21759 is a Use-After-Free vulnerability in the Linux kernel's IPv6 multicast implementation, specifically within the igmp6_send() function. The vulnerability arises because igmp6_send() can be called without proper RTNL (Route Netlink) or RCU (Read-Copy-Update) protection being held, potentially leading to unsafe memory access when fetching the network namespace pointer.

The vulnerability was addressed by extending RCU protection to safely fetch the net pointer and prevent the potential Use-After-Free condition. As part of the fix, the code was modified to use alloc_skb() instead of sock_alloc_send_skb() because ipv6.igmp_sk uses GFP_KERNEL allocations which can sleep, making it incompatible with RCU critical sections.

Critical Impact

Local attackers with low privileges can potentially exploit this Use-After-Free vulnerability to achieve high impact on confidentiality, integrity, and availability of affected Linux systems.

Affected Products

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

Discovery Timeline

• 2025-02-27 - CVE CVE-2025-21759 published to NVD
• 2025-03-24 - Last updated in NVD database

Technical Details for CVE-2025-21759

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption flaw where a program continues to use a pointer after the memory it references has been freed. In the context of the Linux kernel's IPv6 multicast subsystem, the igmp6_send() function is responsible for sending ICMPv6 Multicast Listener Discovery (MLD) messages.

The core issue stems from insufficient synchronization mechanisms when accessing shared kernel data structures. The function could be invoked in contexts where neither RTNL (the primary locking mechanism for network configuration in Linux) nor RCU (a scalable reader-writer synchronization mechanism) protection was active. This creates a race condition where the network namespace pointer (net) could be dereferenced after the underlying memory has been freed by a concurrent operation.

The local attack vector requires an attacker with low-privilege access to the system. Successful exploitation could allow manipulation of kernel memory, potentially leading to privilege escalation, information disclosure, or denial of service.

Root Cause

The root cause of this vulnerability is the lack of proper RCU synchronization in the igmp6_send() function when accessing the network namespace pointer. The function retrieves the net pointer to access net->ipv6.igmp_sk (the IGMP socket for IPv6) without ensuring the underlying data structure remains valid throughout the operation.

In the Linux kernel, RCU is a critical synchronization mechanism that allows readers to access data structures without blocking, while ensuring that memory isn't freed until all readers have completed. Without this protection, a concurrent operation could free the network namespace while igmp6_send() is still accessing it.

Attack Vector

The vulnerability requires local access to the system with low privileges. An attacker could potentially trigger the race condition by:

1. Creating conditions where igmp6_send() is called without proper locking
2. Simultaneously initiating network namespace destruction or modification operations
3. Exploiting the timing window between the net pointer access and its actual use to corrupt memory

The exploitation mechanism involves triggering the Use-After-Free condition in kernel space, which could allow an attacker to manipulate freed memory that gets reallocated for other purposes, potentially achieving arbitrary code execution with kernel privileges.

Detection Methods for CVE-2025-21759

Indicators of Compromise

• Kernel panic or system crash related to IPv6 multicast operations
• Unexpected kernel oops messages referencing igmp6_send or related MLD functions
• Anomalous network behavior during IPv6 multicast group management operations
• KASAN (Kernel Address Sanitizer) reports indicating Use-After-Free in networking subsystem

Detection Strategies

• Enable KASAN in development/testing kernels to detect Use-After-Free conditions
• Monitor kernel logs for signs of memory corruption in the IPv6 networking stack
• Deploy kernel runtime protection tools that can detect UAF exploitation attempts
• Use SentinelOne Singularity Platform for real-time kernel-level threat detection

Monitoring Recommendations

• Monitor system logs for kernel warnings or errors related to IPv6 multicast handling
• Track process behavior for unusual network namespace operations
• Implement auditing for privileged operations that interact with network configuration
• Deploy endpoint detection solutions capable of monitoring kernel-level activities

How to Mitigate CVE-2025-21759

Immediate Actions Required

• Update to a patched Linux kernel version that includes the security fix
• Monitor systems for any signs of exploitation attempts or unusual kernel behavior
• Consider temporarily limiting local user access on critical systems until patching is complete
• Enable additional kernel hardening options such as KASLR and SMEP/SMAP if not already active

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix extends RCU protection throughout the igmp6_send() function to ensure safe access to the network namespace pointer. The patch also modifies the socket buffer allocation mechanism to use alloc_skb() with explicit socket charging under RCU protection, replacing the previous sock_alloc_send_skb() call.

Official patches are available through the following kernel git commits:

• Kernel Commit 087c1faa594f
• Kernel Commit 0bf8e2f37686
• Kernel Commit 81b25a07ebf5
• Kernel Commit 8e92d6a413fe

Workarounds

• Restrict local user access to reduce the attack surface for this local privilege escalation vector
• Deploy kernel runtime protection solutions that can detect and prevent UAF exploitation
• Enable kernel hardening features such as KASLR, SMEP, and SMAP to complicate exploitation
• Consider using SELinux or AppArmor policies to limit access to network namespace operations

# Check current kernel version
uname -r

# Update kernel packages (Debian/Ubuntu)
sudo apt update && sudo apt upgrade linux-image-generic

# Update kernel packages (RHEL/CentOS)
sudo yum update kernel

# Reboot to apply new kernel
sudo reboot