CVE-2020-25643: Linux Kernel HDLC_PPP Module DOS Flaw

CVE-2020-25643 Overview

A memory corruption vulnerability was discovered in the HDLC_PPP module of the Linux kernel affecting versions before 5.9-rc7. The flaw exists in the ppp_cp_parse_cr function where improper input validation leads to memory corruption and a read overflow condition. This vulnerability can be exploited to crash the system or cause a denial of service, with potential impacts to data confidentiality and integrity.

Critical Impact

Memory corruption in the Linux kernel HDLC_PPP module can lead to system crashes, denial of service, and potential compromise of data confidentiality and integrity through improper input validation in network protocol handling.

Affected Products

• Linux Kernel (versions before 5.9-rc7)
• Red Hat Enterprise Linux 7.0 and 8.0
• openSUSE Leap 15.1 and 15.2
• Debian Linux 9.0 and 10.0
• NetApp H410C Firmware
• StarWind Virtual SAN v8 (multiple builds)

Discovery Timeline

• October 6, 2020 - CVE-2020-25643 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25643

Vulnerability Analysis

This vulnerability resides in the HDLC_PPP (High-Level Data Link Control Point-to-Point Protocol) module of the Linux kernel. The ppp_cp_parse_cr function fails to properly validate input data when processing PPP control protocol configuration requests. When malformed or specially crafted input is passed to this function, it results in memory corruption and an out-of-bounds read condition.

The vulnerability is classified under CWE-20 (Improper Input Validation), indicating that the root issue lies in insufficient checks on data being processed by the kernel module. An attacker who can send network traffic to an affected system running the vulnerable HDLC_PPP module could potentially trigger this condition.

The impact of successful exploitation extends beyond simple denial of service. The memory corruption could potentially allow an attacker to read sensitive kernel memory (information disclosure) or corrupt critical data structures (integrity impact), in addition to causing system instability or crashes (availability impact).

Root Cause

The root cause of CVE-2020-25643 is improper input validation in the ppp_cp_parse_cr function within the HDLC_PPP kernel module. The function fails to adequately verify the bounds and format of incoming configuration request data before processing it, allowing maliciously crafted packets to corrupt memory and trigger buffer over-read conditions.

Attack Vector

The attack vector for this vulnerability is network-based. An attacker with high privileges who can send specially crafted PPP control protocol packets to a system running a vulnerable Linux kernel with the HDLC_PPP module loaded could exploit this flaw. The attack does not require user interaction, making it potentially exploitable in automated attack scenarios against exposed systems.

The exploitation involves sending malformed PPP configuration request packets that trigger the improper input validation flaw in ppp_cp_parse_cr. When the kernel processes these packets, memory corruption occurs, which can lead to kernel panics, system crashes, or potentially more severe consequences including information leakage from kernel memory.

Detection Methods for CVE-2020-25643

Indicators of Compromise

• Unexpected kernel panics or system crashes related to the HDLC_PPP or PPP subsystem
• Kernel oops messages referencing ppp_cp_parse_cr or related PPP functions in system logs
• Abnormal PPP-related network traffic patterns targeting affected systems
• Memory corruption indicators in kernel crash dumps

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for PPP module errors or memory corruption messages
• Implement network intrusion detection rules to identify malformed PPP control protocol packets
• Use kernel debugging tools to detect out-of-bounds memory access in the PPP subsystem
• Deploy endpoint detection solutions capable of identifying kernel-level exploitation attempts

Monitoring Recommendations

• Enable enhanced kernel logging for PPP and networking subsystems
• Configure alerts for kernel oops or panic events related to network modules
• Monitor system stability metrics and correlate with network activity patterns
• Implement network traffic analysis for PPP protocol anomalies on affected systems

How to Mitigate CVE-2020-25643

Immediate Actions Required

• Update the Linux kernel to version 5.9-rc7 or later where the vulnerability is patched
• Apply vendor-specific security patches from Red Hat, Debian, openSUSE, or other affected distributions
• If immediate patching is not possible, consider disabling the HDLC_PPP module if not required
• Restrict network access to affected systems to minimize exposure to potential attacks

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix is available in the Linux Kernel Git Repository with commit hash 66d42ed8b25b64eb63111a2b8582c5afc8bf1105. This patch corrects the input validation in the ppp_cp_parse_cr function to prevent memory corruption and read overflow conditions.

Multiple Linux distributions have released security updates addressing this vulnerability:

• Red Hat Bug Report
• Debian Security Advisory DSA-4774
• openSUSE Security Announcements
• NetApp Security Advisory
• StarWind Security Advisory

Workarounds

• Disable or blacklist the HDLC_PPP kernel module if PPP functionality is not required (modprobe -r hdlc_ppp or add to module blacklist)
• Implement network segmentation to limit exposure of vulnerable systems to untrusted networks
• Use firewall rules to restrict PPP-related traffic from untrusted sources
• Monitor for and alert on any attempts to load or interact with the HDLC_PPP module

# Disable HDLC_PPP module temporarily
sudo modprobe -r hdlc_ppp

# Blacklist the module permanently (add to /etc/modprobe.d/blacklist.conf)
echo "blacklist hdlc_ppp" | sudo tee -a /etc/modprobe.d/blacklist-hdlc-ppp.conf

# Verify the module is not loaded
lsmod | grep hdlc_ppp