CVE-2026-23077: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23077 Overview

A Use-After-Free (UAF) vulnerability has been identified in the Linux kernel's memory management subsystem, specifically in the mm/vma component. The flaw occurs during mremap() operations when merging faulted and unfaulted Virtual Memory Areas (VMAs). This vulnerability was introduced by commit 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges") which enabled previously unavailable VMA merge scenarios but failed to properly handle certain edge cases involving anonymous VMA (anon_vma) state management.

Critical Impact

Successful exploitation could lead to memory corruption, denial of service, or potentially privilege escalation in affected Linux systems through improper handling of VMA merges during memory remapping operations.

Affected Products

• Linux kernel versions with commit 879bca0a2c4f applied
• Linux kernel builds utilizing the affected mm/vma merge functionality
• Systems performing mremap() operations on faulted VMAs adjacent to unfaulted VMAs

Discovery Timeline

• 2026-02-04 - CVE-2026-23077 published to NVD
• 2026-02-05 - Last updated in NVD database

Technical Details for CVE-2026-23077

Vulnerability Analysis

This vulnerability resides in the Linux kernel's virtual memory area management code, specifically within the vma_expand() function's handling of anonymous VMA duplication during merge operations. The core issue stems from the fact that dup_anon_vma() is only invoked when the target VMA (the VMA that will ultimately become the merged VMA) is not the next VMA appearing after the range where the new VMA is being established.

The mremap() system call presents a unique scenario not adequately handled by the original merge logic. When a faulted VMA is copied and needs to be merged with adjacent unfaulted VMAs, the code fails to properly duplicate the anon_vma state in three specific cases:

1. Previous VMA unfaulted: The copied faulted VMA attempts to merge with an unfaulted VMA preceding it
2. Next VMA unfaulted: The copied faulted VMA attempts to merge with an unfaulted VMA following it
3. Both adjacent VMAs unfaulted: The copied faulted VMA is sandwiched between two unfaulted VMAs

In each scenario, the improper anon_vma handling can result in a use-after-free condition when the kernel attempts to access memory structures that have already been freed.

Root Cause

The root cause is the incomplete logic in vma_expand() which handles dup_anon_vma() calls. The function was designed to work correctly for standard VMA expansion cases (such as __mmap_region(), do_brk_flags(), vma_merge_extend(), and relocate_vma_down()), but mremap() introduces a unique situation requiring duplication of anon_vma state from a VMA that is neither the previous nor next VMA being merged.

Additionally, the code failed to verify whether the faulted VMA was forked (using the now-renamed vma_is_fork_child() function, previously vma_had_uncowed_parents()) when merging a copied VMA in the problematic scenarios.

Attack Vector

The attack vector involves local access to the system with the ability to perform memory mapping operations. An attacker would need to craft a specific sequence of memory operations using mremap() to trigger the vulnerable code path where a faulted VMA is merged with an adjacent unfaulted VMA without proper anon_vma duplication.

The exploitation scenario would involve:

1. Allocating memory regions that result in faulted and unfaulted VMAs in adjacent memory ranges
2. Using mremap() to copy the faulted VMA in a way that triggers a merge operation
3. Exploiting the use-after-free condition when the kernel accesses the improperly managed anon_vma structure

The vulnerability was discovered via a syzbot report, indicating it can be triggered through automated fuzzing of the kernel's memory management interfaces.

Detection Methods for CVE-2026-23077

Indicators of Compromise

• Kernel panic or oops messages referencing mm/vma, anon_vma, or mremap functions
• Memory corruption errors in kernel logs related to VMA operations
• Unexpected system crashes during memory-intensive operations
• KASAN (Kernel Address Sanitizer) reports indicating use-after-free in VMA handling code

Detection Strategies

• Enable KASAN in kernel builds to detect use-after-free conditions at runtime
• Monitor kernel logs for anomalous messages related to VMA merge operations or anon_vma handling
• Deploy kernel crash dump analysis to identify patterns consistent with this vulnerability
• Utilize Linux audit framework to monitor mremap() system calls for unusual patterns

Monitoring Recommendations

• Configure syslog monitoring for kernel memory management errors and panics
• Implement runtime memory debugging tools in non-production environments to identify exploitation attempts
• Monitor for process crashes that coincide with memory remapping operations
• Review kernel oops reports for call traces involving vma_expand(), dup_anon_vma(), or related functions

How to Mitigate CVE-2026-23077

Immediate Actions Required

• Apply the official kernel patches as soon as available for your distribution
• Evaluate workloads for heavy mremap() usage that could trigger the vulnerable code path
• Consider temporarily limiting access to memory mapping system calls for untrusted users if patch deployment is delayed
• Monitor systems for signs of exploitation while remediation is in progress

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fixes ensure proper dup_anon_vma() handling for all merge scenarios involving faulted and unfaulted VMAs during mremap() operations.

Relevant kernel commits:

• Kernel Git Commit 61f67c23
• Kernel Git Commit a4d9dbfc

The patch series includes:

• Fixes for all three problematic VMA merge scenarios
• Additional validation to check if faulted VMAs were forked before merging
• Cleanup of vma_expand() function
• Renaming of vma_had_uncowed_parents() to vma_is_fork_child() for clarity
• Self-tests to verify the fixes

Workarounds

• Apply kernel patches from your distribution's security repository as the primary remediation
• If patches are not immediately available, consider restricting access to the mremap() system call using seccomp filters for high-risk applications
• Deploy runtime protection solutions that can detect memory corruption exploitation attempts
• Maintain updated kernel versions and apply security patches promptly as part of ongoing security hygiene

# Check current kernel version for affected status
uname -r

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

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

# Verify patch application by checking kernel version after update
uname -r