CVE-2026-3038: Kernel Buffer Overflow Vulnerability

CVE-2026-3038 Overview

CVE-2026-3038 is a stack buffer overflow vulnerability in the FreeBSD kernel's rtsock_msg_buffer() function. This function serializes routing information into a buffer, copying sockaddr structures into a sockaddr_storage structure on the stack. The vulnerability arises because the function assumes the source sockaddr length field has already been validated, but this is not necessarily the case. A malicious userspace program can craft a request that triggers a 127-byte stack overflow.

In practice, this overflow immediately overwrites the stack canary for the rtsock_msg_buffer() stack frame, resulting in a kernel panic once the function returns. While the stack canary mitigation limits the immediate impact to a denial of service condition, other kernel vulnerabilities could potentially allow an attacker to bypass this protection and achieve local privilege escalation.

Critical Impact

An unprivileged user can crash the FreeBSD kernel through a crafted routing socket request, causing system-wide denial of service. Potential for privilege escalation exists if stack canary values can be leaked through other vulnerabilities.

Affected Products

• FreeBSD (routing socket subsystem)
• Systems using FreeBSD-based network routing functionality

Discovery Timeline

• 2026-03-09 - CVE-2026-3038 published to NVD
• 2026-03-09 - Last updated in NVD database

Technical Details for CVE-2026-3038

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), specifically manifesting as a stack buffer overflow in kernel space. The rtsock_msg_buffer() function is responsible for serializing routing information for inter-process communication via routing sockets. During this serialization process, the function copies sockaddr structures to a stack-allocated sockaddr_storage buffer without properly validating the length field of the source sockaddr structure.

The lack of bounds checking on the sockaddr length field means a malicious userspace program can supply a length value that exceeds the destination buffer's capacity. This results in a 127-byte overflow that corrupts adjacent stack memory, including the stack canary value placed by the compiler's stack protection mechanism.

Root Cause

The root cause is improper input validation (CWE-787) in the kernel's routing socket message buffer handling. The rtsock_msg_buffer() function trusts that the sockaddr length field has been validated by callers, but this assumption is violated in certain code paths. This trust boundary violation allows unprivileged userspace processes to supply arbitrary length values that are not sanitized before being used in memory copy operations.

The affected code path does not enforce the maximum size constraints for sockaddr structures before copying them to the fixed-size sockaddr_storage structure on the stack.

Attack Vector

The vulnerability is exploitable from userspace without requiring elevated privileges. An attacker can exploit this vulnerability by:

1. Opening a routing socket (AF_ROUTE)
2. Crafting a malicious routing message with an oversized sockaddr length field
3. Sending the crafted message through the routing socket interface
4. Triggering the rtsock_msg_buffer() function to process the malformed request
5. Causing a 127-byte stack buffer overflow that overwrites the stack canary

The overflow mechanism involves the kernel copying more data than the stack buffer can accommodate. While the stack canary protection currently converts this to a denial of service (kernel panic), the underlying vulnerability could potentially be weaponized for privilege escalation if an attacker can first leak or predict the canary value through a separate information disclosure vulnerability.

For detailed technical information about the vulnerability and exploitation mechanics, refer to the FreeBSD Security Advisory.

Detection Methods for CVE-2026-3038

Indicators of Compromise

• Unexpected kernel panics with stack smashing detection errors in rtsock_msg_buffer()
• Crash dumps indicating stack canary corruption in routing socket handling code
• Unusual routing socket activity from unprivileged processes
• System logs showing repeated kernel crashes originating from routing subsystem

Detection Strategies

• Monitor for kernel panic events with stack protector violations in routing socket functions
• Implement system call auditing to detect suspicious routing socket operations from unprivileged users
• Deploy kernel crash analysis tools to identify exploitation attempts
• Configure alerting on AF_ROUTE socket creation by non-root processes in high-security environments

Monitoring Recommendations

• Enable kernel crash dump collection and analysis for forensic investigation
• Monitor system uptime metrics for unexpected reboots that may indicate exploitation attempts
• Implement process behavior analysis to detect unusual routing socket usage patterns
• Review audit logs for routing socket syscalls from processes that typically don't require network configuration capabilities

How to Mitigate CVE-2026-3038

Immediate Actions Required

• Apply the security patch from FreeBSD as soon as available
• Consider restricting routing socket access to privileged processes where operationally feasible
• Implement network segmentation to limit potential attack surface from compromised local users
• Enable enhanced kernel logging to detect exploitation attempts

Patch Information

FreeBSD has released a security advisory addressing this vulnerability. System administrators should consult the FreeBSD Security Advisory for specific patch information and upgrade instructions. The patch adds proper validation of the sockaddr length field before copying data to the stack buffer, preventing the overflow condition.

Workarounds

• Restrict access to routing sockets by limiting which users and processes can create AF_ROUTE sockets
• Implement mandatory access control policies (MAC) to restrict routing socket access to trusted system services
• Consider enabling additional kernel hardening options if available in your FreeBSD version
• Monitor for and investigate any suspicious process activity involving routing socket operations

# Example: Restrict routing socket access using MAC framework (if available)
# Consult FreeBSD documentation for your specific version
# Enable security.mac.enabled in /boot/loader.conf
# Configure MAC policies to limit AF_ROUTE socket creation