CVE-2025-37749: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-37749 Overview

CVE-2025-37749 is an out-of-bounds read vulnerability in the Linux kernel's PPP (Point-to-Point Protocol) synchronous networking implementation. The vulnerability exists in the ppp_sync_txmung function, which fails to properly validate the length of socket buffer (skb) data before accessing initial bytes. When processing packets with empty payloads, the function may attempt to access memory beyond the bounds of the linear buffer, potentially leading to information disclosure or system instability.

Critical Impact

Local attackers with low privileges can exploit this vulnerability to read sensitive kernel memory or cause denial of service conditions through crafted network packets with empty payloads.

Affected Products

• Linux Kernel versions 2.6.12 through affected stable branches
• Linux Kernel 6.15-rc1
• Debian Linux 11.0

Discovery Timeline

• May 1, 2025 - CVE-2025-37749 published to NVD
• November 4, 2025 - Last updated in NVD database

Technical Details for CVE-2025-37749

Vulnerability Analysis

The ppp_sync_txmung function in the Linux kernel's PPP networking subsystem processes outgoing synchronous PPP packets. The function accesses specific byte offsets within the socket buffer's data region to perform protocol-specific transformations. However, the code lacks adequate bounds checking before accessing data[2] and other initial bytes of the buffer.

When an incoming PPPoE (PPP over Ethernet) packet arrives with a payload length of zero (as indicated by the length = 0x0 field in the PPPoE header), the skb structure reflects len = 0x0 and data_len = 0x0. In this state, the linear buffer contains no valid payload data, yet the vulnerable code path proceeds to dereference memory at specific offsets within the data region.

Root Cause

The root cause is missing boundary validation in the ppp_sync_txmung function. The code assumes that the socket buffer always contains sufficient data in the linear buffer region before accessing indexed positions. When processing malformed or specially crafted packets with empty payloads, the function fails to verify that skb->len is greater than the offset being accessed, resulting in an out-of-bounds read condition classified as CWE-125 (Out-of-bounds Read).

Attack Vector

This vulnerability requires local access to exploit. An attacker with low-level privileges on the system can craft or manipulate PPP network packets with empty payloads to trigger the out-of-bounds read condition. The attack does not require user interaction.

The vulnerability can be triggered when:

1. The PPP synchronous driver receives a packet with a PPPoE header indicating zero payload length
2. The ppp_sync_txmung function processes this packet without validating the data length
3. The function attempts to access data[2] or other offsets when the actual data length is zero

This can result in disclosure of sensitive information from adjacent kernel memory or cause kernel crashes leading to denial of service.

Detection Methods for CVE-2025-37749

Indicators of Compromise

• Kernel panic or oops messages referencing ppp_sync_txmung or PPP synchronous driver components
• Unexpected system crashes or reboots when PPP connections are in use
• Memory corruption warnings in kernel logs related to the network subsystem
• Abnormal PPPoE packets with zero-length payloads observed in network traffic

Detection Strategies

• Monitor kernel logs (dmesg) for stack traces involving ppp_sync_txmung, ppp_sync.c, or related PPP driver functions
• Deploy network intrusion detection rules to identify malformed PPPoE packets with length = 0x0 in the header
• Use kernel address sanitizer (KASAN) builds in testing environments to detect out-of-bounds memory accesses
• Implement endpoint detection solutions that monitor for abnormal PPP subsystem behavior

Monitoring Recommendations

• Enable enhanced kernel logging for network subsystem events on systems utilizing PPP connections
• Configure SentinelOne agents to monitor for kernel-level anomalies and memory access violations
• Review system stability metrics on infrastructure using PPP/PPPoE connections for signs of exploitation attempts

How to Mitigate CVE-2025-37749

Immediate Actions Required

• Apply the appropriate kernel patch for your distribution immediately
• Review and update Linux kernel packages on all affected systems, particularly those using PPP networking
• Prioritize patching on systems with PPPoE or synchronous PPP connections exposed to untrusted networks
• Consider disabling unused PPP functionality until patches can be applied

Patch Information

The Linux kernel maintainers have released patches across multiple stable branches to address this vulnerability. The fix adds proper bounds checking to ensure sufficient data exists in the linear buffer before accessing initial bytes. The patches are available through the following kernel commits:

• Kernel Commit 1f6eb9f
• Kernel Commit 529401c
• Kernel Commit 6e8a6bf
• Kernel Commit 99aa698
• Kernel Commit aabc659
• Kernel Commit b4c836d
• Kernel Commit b78f2b4
• Kernel Commit de5a4f0
• Kernel Commit fbaffe8

Debian users should refer to the Debian LTS Security Announcements for distribution-specific packages.

Workarounds

• Disable PPP synchronous networking functionality if not actively required by unloading the ppp_synctty module
• Implement network filtering to drop malformed PPPoE packets before they reach the kernel
• Restrict access to PPP device interfaces to trusted users only
• Use network segmentation to isolate systems utilizing PPP connections from untrusted network segments

# Disable PPP synchronous module if not required
modprobe -r ppp_synctty

# Blacklist the module to prevent automatic loading
echo "blacklist ppp_synctty" >> /etc/modprobe.d/blacklist-ppp.conf

# Verify current kernel version
uname -r

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