CVE-2025-68816: Linux Kernel Format String Vulnerability

CVE-2025-68816 Overview

A format string vulnerability has been identified in the Linux kernel's Mellanox mlx5 network driver, specifically in the firmware tracer component. The net/mlx5 firmware tracer receives format strings from device firmware and uses them to format trace messages. Without proper validation, malicious or malfunctioning firmware could provide format strings with dangerous format specifiers (such as %s, %p, or %n) that could lead to system crashes, information disclosure, or other undefined behavior.

This vulnerability has been addressed by adding the mlx5_tracer_validate_params() function to validate that all format specifiers in trace strings are limited to safe integer/hex formats (%x, %d, %i, %u, %llx, %lx, etc.). Invalid format strings are now logged with a "BAD_FORMAT:" prefix for visibility.

Critical Impact

Malformed format strings from firmware could cause kernel crashes, memory corruption, or potentially allow arbitrary memory access through unsafe format specifiers like %n or %s.

Affected Products

• Linux Kernel (systems using Mellanox mlx5 network drivers)
• Systems with Mellanox ConnectX network adapters
• Linux-based network appliances and servers utilizing mlx5 firmware tracing

Discovery Timeline

• 2026-01-13 - CVE CVE-2025-68816 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2025-68816

Vulnerability Analysis

The vulnerability exists in the Linux kernel's mlx5 network driver firmware tracer subsystem. The firmware tracer is a diagnostic component that receives format strings from the Mellanox network adapter's firmware and uses them to generate human-readable trace output. The core issue is that format strings received from firmware were being passed directly to formatting functions without validation of the format specifiers contained within them.

Format string vulnerabilities occur when user-controlled (or in this case, firmware-controlled) input is used as a format string argument in functions like printf() or similar kernel equivalents. When an attacker controls the format string, they can inject format specifiers that cause the function to read from or write to arbitrary memory locations.

In this case, the attack surface is through firmware-provided format strings. While legitimate firmware would only use safe integer format specifiers, compromised or malicious firmware could provide dangerous specifiers like %n (which writes to memory), %s (which reads from an arbitrary pointer), or %p (which can leak kernel addresses).

Root Cause

The root cause is the lack of input validation for format string parameters received from firmware in the mlx5 tracer component. The firmware tracer was implicitly trusting that all format strings received from the device firmware would only contain safe format specifiers. This trust assumption was incorrect from a defense-in-depth perspective, as firmware could potentially be compromised, buggy, or malicious.

The fix implements proper input validation through a new mlx5_tracer_validate_params() function that examines each format specifier in the received string and rejects any that are not in the allowlist of safe integer/hex formats.

Attack Vector

The attack vector for this vulnerability requires an attacker to control or influence the firmware on a Mellanox mlx5 network adapter. This could potentially occur through:

1. Compromised firmware updates - An attacker who can supply malicious firmware to the network adapter
2. Supply chain attacks - Hardware devices shipped with pre-compromised firmware
3. Firmware bugs - Unintentional malformed format strings from buggy firmware

When the vulnerable firmware tracer processes a malicious format string containing dangerous specifiers like %n, %s, or multiple %p specifiers, it could lead to kernel memory corruption, information disclosure of kernel memory addresses, or denial of service through kernel crashes.

The fix ensures that only safe format specifiers (%x, %d, %i, %u, %llx, %lx, and similar integer formats) are permitted, while rejecting potentially dangerous ones and marking them with a "BAD_FORMAT:" prefix in the trace output.

Detection Methods for CVE-2025-68816

Indicators of Compromise

• Kernel crash dumps or oops messages originating from the mlx5_core or mlx5_fw_tracer modules
• Unusual trace messages containing "BAD_FORMAT:" prefix after patching, indicating potentially malicious firmware
• System instability or unexpected behavior on systems with Mellanox network adapters
• Memory corruption or KASAN (Kernel Address Sanitizer) warnings related to mlx5 driver components

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for mlx5-related crashes or warnings
• Enable kernel crash dump collection (kdump) to capture and analyze any kernel panics
• Use KASAN-enabled kernels in testing environments to detect memory corruption issues
• Review firmware versions on Mellanox adapters to ensure they are from trusted sources

Monitoring Recommendations

• Implement continuous monitoring of kernel message buffers for mlx5 driver anomalies
• Configure alerting for system crashes or unexpected reboots on systems with affected hardware
• After applying patches, monitor for "BAD_FORMAT:" messages which indicate blocked malicious or malformed format strings
• Maintain an inventory of firmware versions deployed across your Mellanox network adapter fleet

How to Mitigate CVE-2025-68816

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the mlx5_tracer_validate_params() fix
• Review and verify the integrity of firmware installed on Mellanox network adapters
• Monitor systems for signs of exploitation or kernel instability related to mlx5 drivers
• Consider disabling firmware tracing on production systems if not required

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. Multiple kernel versions have been updated with the fix. The patch introduces the mlx5_tracer_validate_params() function that validates all format specifiers before they are processed.

The following kernel git commits contain the fix:

• Kernel Git Commit 38ac688b52ef
• Kernel Git Commit 45bd283b1d69
• Kernel Git Commit 8ac688c0e430
• Kernel Git Commit 8c35c2448086
• Kernel Git Commit b35966042d20

Apply the appropriate patch for your kernel version or upgrade to a kernel release that includes the fix.

Workarounds

• Disable mlx5 firmware tracing if it is not required for your operational needs
• Restrict access to firmware update mechanisms to prevent unauthorized firmware modifications
• Use kernel module parameters to limit mlx5 tracer functionality if available
• Implement network segmentation to limit exposure of systems with potentially vulnerable configurations

# Configuration example
# Check current mlx5 module status and firmware tracing
lsmod | grep mlx5

# View mlx5 module parameters
modinfo mlx5_core

# Monitor kernel logs for mlx5-related issues
dmesg | grep -i mlx5

# After patching, check for BAD_FORMAT messages indicating blocked malicious strings
dmesg | grep "BAD_FORMAT"