CVE-2020-8597: PPP Protocol Buffer Overflow Vulnerability

CVE-2020-8597 Overview

CVE-2020-8597 is a critical buffer overflow vulnerability affecting the Point-to-Point Protocol Daemon (pppd) versions 2.4.2 through 2.4.8. The vulnerability exists in the eap.c file, specifically within the eap_request and eap_response functions, where improper bounds checking allows for a stack-based buffer overflow in the rhostname buffer. This flaw enables unauthenticated remote attackers to execute arbitrary code or cause a denial of service on vulnerable systems.

The vulnerability is particularly dangerous because it can be exploited remotely over the network without any user interaction or authentication, affecting a wide range of systems including Linux distributions, embedded devices, and industrial control systems.

Critical Impact
Unauthenticated remote attackers can exploit this buffer overflow to execute arbitrary code with the privileges of the pppd daemon, potentially gaining root-level access to affected systems including network equipment, embedded devices, and industrial control systems.

Affected Products

Point-to-Point Protocol (ppp) versions 2.4.2 through 2.4.8
WAGO PFC100 and PFC200 controllers with PFC firmware
Debian Linux 9.0 and 10.0
Canonical Ubuntu Linux 12.04, 14.04 (ESM), 16.04 (ESM), 18.04 LTS, and 19.04

Discovery Timeline

2020-02-03 - CVE-2020-8597 published to NVD
2025-12-03 - Last updated in NVD database

Technical Details for CVE-2020-8597

Vulnerability Analysis

The vulnerability stems from a logic error in the bounds checking within the EAP (Extensible Authentication Protocol) handling code of pppd. The eap_request and eap_response functions in eap.c fail to properly validate the length of data being copied into the fixed-size rhostname buffer.

During EAP packet processing, when the daemon receives a malformed EAP packet with a crafted hostname length, the incorrect bounds check allows an attacker to overflow the rhostname buffer. Since pppd typically runs with elevated privileges (often as root) to manage network interfaces, successful exploitation grants attackers significant system access.

The vulnerability affects not only traditional Linux desktop and server systems but also extends to embedded devices, routers, and industrial control systems that utilize PPP for network connectivity. CISA has issued an ICS Advisory (ICSA-20-224-04) highlighting the impact on industrial systems.

Root Cause

The root cause is a flawed bounds check in the EAP processing code. The original code used an incorrect comparison: if (vallen >= len + sizeof (rhostname)) which fails to properly prevent buffer overflow when processing peer hostnames from EAP packets. This logic error allows data larger than the allocated buffer to be copied, leading to stack memory corruption.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can send specially crafted EAP packets to a vulnerable pppd process to trigger the overflow. This can occur through:

Direct PPP connections where the attacker controls one endpoint
PPPoE (PPP over Ethernet) sessions common in DSL and fiber connections
VPN implementations using PPP tunneling
Any network service that processes EAP packets through pppd

The following patch demonstrates the fix for the bounds check vulnerability:

 		}
 
 		/* Not so likely to happen. */
-		if (vallen >= len + sizeof (rhostname)) {
+		if (len - vallen >= sizeof (rhostname)) {
 			dbglog("EAP: trimming really long peer name down");
 			BCOPY(inp + vallen, rhostname, sizeof (rhostname) - 1);
 			rhostname[sizeof (rhostname) - 1] = '\0';

Source: GitHub Commit on PPP

The patch corrects the bounds check by changing the comparison from vallen >= len + sizeof (rhostname) to len - vallen >= sizeof (rhostname), which properly validates that the data length does not exceed the buffer size.

Detection Methods for CVE-2020-8597

Indicators of Compromise

Unexpected crashes or restarts of the pppd daemon process
Anomalous EAP packets with unusually large hostname fields in network traffic
Suspicious memory access patterns or segmentation faults in system logs related to pppd
Unauthorized processes spawned by or running with pppd privileges

Detection Strategies

Deploy network intrusion detection signatures to identify malformed EAP packets with oversized hostname fields
Monitor system logs for pppd crash events or unexpected termination signals (SIGSEGV, SIGBUS)
Implement file integrity monitoring on pppd binaries and related configuration files
Use endpoint detection tools to identify exploitation attempts targeting the EAP handling functions

Monitoring Recommendations

Enable verbose logging for pppd to capture EAP authentication events and potential anomalies
Configure SIEM rules to alert on repeated pppd restarts or authentication failures
Monitor network traffic for PPP and EAP protocol anomalies, particularly packets with unusual length fields
Track system call patterns from pppd processes for signs of code execution attempts

How to Mitigate CVE-2020-8597

Immediate Actions Required

Update pppd to version 2.4.9 or later, which contains the security fix
Apply vendor-specific security patches for affected operating systems and devices
If patching is not immediately possible, consider disabling EAP authentication methods in pppd configuration
Restrict network access to PPP services where possible using firewall rules

Patch Information

The vulnerability has been addressed in the upstream PPP project through commit 8d7970b8f3db727fe798b65f3377fe6787575426. Major Linux distributions have released security updates:

Debian: DSA-4632 and LTS announcement
Ubuntu: USN-4288-1 and USN-4288-2
Red Hat: RHSA-2020:0630, RHSA-2020:0631, RHSA-2020:0633, RHSA-2020:0634
Gentoo: GLSA 202003-19
openSUSE: Security Announcement

For embedded and industrial devices, consult vendor-specific advisories from Siemens, WAGO, Netgear, Synology, and NetApp.

Workarounds

Disable EAP authentication in pppd by removing require-eap or similar options from configuration files
Use alternative authentication methods such as PAP or CHAP if EAP is not required
Implement network segmentation to limit exposure of PPP services to trusted networks only
Deploy intrusion prevention systems with signatures for CVE-2020-8597 exploitation attempts

bash

# Example: Disable EAP authentication in pppd configuration
# Edit /etc/ppp/options or connection-specific options file
# Remove or comment out EAP-related options:
# require-eap
# refuse-eap

# Verify pppd version after patching
pppd --version