Analysis Summary

A newly disclosed Linux kernel vulnerability, CVE-2025-38236, discovered by researcher, exposes a flaw in the MSG_OOB (out-of-band) feature of UNIX domain sockets. Present in Linux kernel versions 6.9 and above but originating from code introduced in version 5.15, the bug allows attackers to escalate privileges from within Chrome’s renderer sandbox. Though rarely used outside specific Oracle products, MSG_OOB was enabled by default and accessible in the sandbox due to unfiltered syscall flags. The flaw creates a use-after-free (UAF) condition that can be triggered through specific socket operations, enabling attackers to manipulate kernel memory and gain elevated privileges. Both the Linux kernel and Chrome have since been patched to disable MSG_OOB in the renderer sandbox.

Horn’s proof-of-concept exploit demonstrates how a UAF can provide a read primitive to leak arbitrary kernel memory, bypassing usercopy hardening. By reallocating freed memory as pipe pages or kernel stacks, and using page table manipulation along with mprotect() for delay injection, attackers can precisely corrupt memory. Interestingly, the exploit also leverages Debian’s CONFIG_RANDOMIZE_KSTACK_OFFSET—a mitigation—into an advantage for aligning memory targets. The attack was shown to work on a Debian Trixie x86-64 system, showcasing the potential for kernel-level compromise starting from native code execution in Chrome’s renderer sandbox.

The bug was initially discovered during Horn’s review of new kernel code in June, with a related issue later identified by Google’s syzkaller fuzzing tool in August 2024. While one flaw required six syscalls to trigger, another more complex variant needed eight, underscoring the challenges fuzzers face in navigating intricate kernel data structures like socket buffers (SKBs). Horn notes that targeted fuzzing on specific kernel subsystems could improve detection of such vulnerabilities, as conventional fuzzers struggle with deep, sequence-specific bugs in obscure features.

This discovery also highlights significant risks in Chrome’s Linux renderer sandbox design, which unnecessarily exposes extensive kernel interfaces such as UNIX sockets, pipes, anonymous VMAs, and syscalls like sendmsg() and mprotect(). Past Chrome exploits involving futex(), memfd_create(), and pipe2() have similarly abused little-used kernel features. Horn’s findings question the effectiveness of probabilistic mitigations such as stack randomization when arbitrary read capabilities exist. The case reinforces the need for stricter sandbox restrictions, reduced kernel feature exposure, and closer scrutiny of esoteric functionalities to limit attack surfaces in high-risk environments. Linux users are urged to apply patches immediately.

Impact

• Privilege Escalation
• Security Bypass
• Gain Access

Indicators of Compromise

CVE

• CVE-2025-38236

Affected Vendors

Remediation

• Apply the latest patched Linux kernel version from your distribution to remove the MSG_OOB vulnerability.
• Update Chrome to the latest version, which blocks MSG_OOB messages in the renderer sandbox.
• Limit unnecessary kernel interfaces (e.g., UNIX sockets, pipes, certain syscalls) accessible within renderer sandboxes to reduce the attack surface.
• Review kernel configurations and disable rarely used or high-risk features such as MSG_OOB unless explicitly required.
• Implement targeted fuzzing for specific kernel subsystems like socket buffers (SKBs) to detect complex sequence-based bugs earlier.
• Reassess reliance on stack randomization and similar mitigations, adding additional hardening layers against arbitrary read/write exploits.
• Apply strict syscall filtering (e.g., using seccomp) for sandboxed processes to allow only necessary operations.
• Implement kernel-level logging and security monitoring to detect suspicious syscall patterns and socket behavior.