CVE-2024-1085: Linux Kernel Privilege Escalation Flaw

CVE-2024-1085 Overview

A use-after-free vulnerability exists in the Linux kernel's netfilter nf_tables component that can be exploited to achieve local privilege escalation. The flaw resides in the nft_setelem_catchall_deactivate() function, which improperly checks whether the catch-all set element is active in the current generation instead of the next generation before freeing it. This function only flags the element inactive in the next generation, creating a window where the element can be freed multiple times, resulting in a double-free condition.

Critical Impact

Local attackers with low privileges can exploit this use-after-free vulnerability to escalate privileges to root, potentially gaining complete control over the affected Linux system.

Affected Products

• Linux Kernel (multiple versions affected)
• Linux distributions using vulnerable kernel versions
• Systems with netfilter/nf_tables components enabled

Discovery Timeline

• 2024-01-31 - CVE-2024-1085 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-1085

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a critical memory corruption flaw that occurs when a program continues to use a pointer after it has been freed. In the context of the Linux kernel's netfilter subsystem, this creates a particularly dangerous scenario where kernel memory can be corrupted by an attacker with local access.

The netfilter framework, specifically the nf_tables component, provides packet filtering and classification capabilities for the Linux kernel. The vulnerability exists in the handling of catch-all set elements during deactivation. The nft_setelem_catchall_deactivate() function performs a generation check that is logically flawed—it verifies element activity against the current generation rather than the next generation before proceeding with memory deallocation.

Since the element is only marked inactive in the next generation (not the current one), subsequent operations can trigger additional free operations on the same memory region. This double-free condition can be leveraged by attackers to corrupt kernel heap structures, potentially leading to arbitrary code execution with kernel privileges.

Root Cause

The root cause lies in the improper synchronization between generation tracking and memory deallocation in the nft_setelem_catchall_deactivate() function. The function checks the element's active state against the wrong generation context, creating a race condition between the visibility of the deactivation and the actual memory free operation. This temporal mismatch allows the same memory to be freed twice when properly timed operations trigger redundant deactivation sequences.

Attack Vector

The attack requires local access with low privileges on the target system. An attacker must be able to interact with the netfilter subsystem, typically through the nft userspace tool or direct netlink socket communications. By carefully crafting netfilter rule operations that manipulate catch-all set elements, an attacker can trigger the double-free condition. Once kernel heap corruption is achieved, standard exploitation techniques such as heap spraying can be employed to gain code execution in kernel context, effectively escalating to root privileges.

The vulnerability manifests in the catch-all set element handling within nft_setelem_catchall_deactivate(). The function's improper generation check logic allows memory to be freed multiple times. For detailed technical analysis of the fix, see the Linux Kernel Commit.

Detection Methods for CVE-2024-1085

Indicators of Compromise

• Unexpected kernel panics or system crashes related to netfilter or nf_tables subsystem
• Suspicious local user activity involving netfilter rule manipulation via nft commands
• Kernel log entries showing memory corruption or double-free errors in the nft_tables module
• Unusual privilege escalation events from non-root users

Detection Strategies

• Monitor for suspicious nft command executions by unprivileged users
• Implement kernel auditing to track netlink socket communications to the netfilter subsystem
• Deploy runtime kernel integrity monitoring to detect heap corruption attempts
• Use system call monitoring to identify patterns consistent with exploit development activity

Monitoring Recommendations

• Enable kernel SLUB debug options (slub_debug=FZP) in non-production environments to detect double-free attempts
• Configure auditd rules to log all netfilter configuration changes
• Implement SentinelOne's kernel-level monitoring for real-time detection of privilege escalation attempts
• Review system logs regularly for signs of exploitation attempts against the netfilter subsystem

How to Mitigate CVE-2024-1085

Immediate Actions Required

• Update the Linux kernel to a version containing commit b1db244ffd041a49ecc9618e8feb6b5c1afcdaa7
• Apply vendor-provided kernel security patches from your Linux distribution
• Restrict access to netfilter administration to only trusted administrative users
• Monitor systems for signs of exploitation attempts until patches can be applied

Patch Information

The vulnerability has been addressed in the Linux kernel with commit b1db244ffd041a49ecc9618e8feb6b5c1afcdaa7. This patch corrects the generation check logic in nft_setelem_catchall_deactivate() to properly verify element state against the next generation before freeing, preventing the double-free condition. Organizations should upgrade their kernels past this commit or apply equivalent patches from their distribution vendors.

For the official fix, refer to the Linux Kernel Commit and Kernel Dance Reference.

Workarounds

• Disable or unload the nf_tables kernel module if not required for operations
• Use iptables-legacy instead of nftables where possible as a temporary measure
• Implement strict access controls to limit netfilter administration capabilities
• Consider deploying kernel hardening options such as KASLR and SMEP/SMAP to increase exploitation difficulty

# Check current kernel version
uname -r

# Verify nf_tables module status
lsmod | grep nf_tables

# Temporarily unload nf_tables if not needed (may disrupt firewall operations)
# modprobe -r nf_tables

# Restrict nft command to root only
chmod 700 /usr/sbin/nft