CVE-2020-8037: Tcpdump PPP Decapsulator DOS Vulnerability

CVE-2020-8037 Overview

CVE-2020-8037 is a resource exhaustion vulnerability affecting the PPP (Point-to-Point Protocol) decapsulator component in tcpdump version 4.9.3. The vulnerability allows remote attackers to cause a denial of service condition by convincing the application to allocate a large amount of memory when processing specially crafted network packets. This flaw is classified as CWE-770 (Allocation of Resources Without Limits or Throttling).

tcpdump is one of the most widely used command-line network packet analyzers, commonly employed by network administrators, security professionals, and developers for network traffic analysis and debugging. The widespread deployment of tcpdump across Linux distributions, macOS, and various Unix-like systems makes this vulnerability particularly impactful.

Critical Impact

Remote attackers can cause denial of service by triggering excessive memory allocation in tcpdump's PPP decapsulator, potentially crashing systems running network monitoring operations or exhausting system resources.

Affected Products

• tcpdump 4.9.3
• Debian Linux 9.0
• Fedora 32 and 33
• Apple macOS (multiple versions including 10.14.6, 10.15.7, and later)
• Apple Mac OS X (multiple versions with various security updates)

Discovery Timeline

• November 4, 2020 - CVE-2020-8037 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-8037

Vulnerability Analysis

The vulnerability resides in the PPP decapsulator functionality within tcpdump 4.9.3. When parsing PPP-encapsulated network traffic, the decapsulator fails to properly validate or limit the size of memory allocations requested during packet processing. An attacker can craft malicious network packets that, when captured and processed by tcpdump, trigger the allocation of excessive amounts of memory.

This type of resource exhaustion attack exploits the lack of proper bounds checking on allocation sizes. The PPP decapsulator accepts length values from untrusted network input without adequate validation, allowing attackers to specify arbitrarily large allocation sizes. When tcpdump attempts to allocate the requested memory, it can lead to memory exhaustion, system slowdown, or application crash.

The vulnerability can be exploited remotely without authentication since tcpdump operates by passively capturing network traffic. An attacker positioned on the same network segment or capable of injecting packets that reach the monitored interface can trigger this condition.

Root Cause

The root cause of CVE-2020-8037 is insufficient input validation in the PPP decapsulator code path. Specifically, the vulnerability stems from the application's failure to implement proper resource allocation limits when processing PPP packet headers. The decapsulator trusts length fields from network packets without verifying that the requested allocation size is within reasonable bounds, violating the principle of defensive programming.

This represents a classic CWE-770 scenario where resources are allocated based on untrusted input without enforcing limits. The fix implemented in the tcpdump codebase adds proper validation to prevent excessive memory allocations during PPP decapsulation.

Attack Vector

The attack vector is network-based and requires no user interaction or privileges. An attacker can exploit this vulnerability through the following scenario:

1. The target system runs tcpdump to monitor network traffic on an interface
2. The attacker sends specially crafted PPP-encapsulated packets to the monitored network
3. tcpdump captures and attempts to parse the malicious packets
4. The PPP decapsulator attempts to allocate memory based on attacker-controlled values
5. Excessive memory allocation causes resource exhaustion or application crash

The vulnerability is particularly dangerous in scenarios where tcpdump runs as a long-running monitoring service, on resource-constrained systems, or in environments where network traffic cannot be fully trusted.

Detection Methods for CVE-2020-8037

Indicators of Compromise

• Unusual memory consumption spikes in tcpdump processes
• tcpdump process crashes or unexpected termination during network monitoring
• System logs indicating out-of-memory conditions coinciding with tcpdump activity
• Abnormal PPP-encapsulated traffic patterns on monitored network interfaces

Detection Strategies

• Monitor system resource utilization for tcpdump processes using tools like top, htop, or process monitoring agents
• Implement memory usage thresholds and alerts for network monitoring applications
• Review system logs for OOM (Out of Memory) killer events targeting tcpdump
• Deploy network intrusion detection rules to identify malformed PPP packets

Monitoring Recommendations

• Configure resource limits (ulimit) for tcpdump processes to prevent complete system exhaustion
• Enable process monitoring with automatic restart capabilities for critical monitoring services
• Implement centralized logging to correlate tcpdump failures across multiple systems
• Consider deploying SentinelOne endpoint protection to detect abnormal process behavior

How to Mitigate CVE-2020-8037

Immediate Actions Required

• Update tcpdump to a patched version that addresses CVE-2020-8037
• Apply vendor-specific security updates for affected operating systems (Debian, Fedora, macOS)
• Implement process resource limits using ulimit or cgroups to contain potential impact
• Review network monitoring configurations and consider isolating tcpdump processes

Patch Information

The tcpdump project has released a fix for this vulnerability. The patch is available in the official tcpdump GitHub repository commit 32027e199368dad9508965aae8cd8de5b6ab5231. Operating system vendors have also released patches through their respective security update channels:

• Debian: Security updates available via Debian LTS Security Notice
• Fedora: Updates available through Fedora package announcements for Fedora 32 and 33
• Apple: Patches included in macOS security updates as documented in Apple Support HT212325, HT212326, and HT212327

Workarounds

• Apply resource limits using ulimit -v to restrict maximum virtual memory for tcpdump processes
• Run tcpdump in containerized environments with memory constraints enforced
• Use BPF filters to exclude PPP traffic from capture if not needed: tcpdump -i eth0 not ppp
• Deploy alternative packet capture tools temporarily while patching is scheduled
• Implement network segmentation to limit exposure of monitoring systems to untrusted traffic

# Configuration example - Apply memory limits for tcpdump processes
# Set maximum virtual memory to 512MB before running tcpdump
ulimit -v 524288

# Run tcpdump with BPF filter to exclude PPP traffic
tcpdump -i eth0 not ppp

# Alternative: Use systemd resource controls for tcpdump service
# Add to tcpdump.service unit file:
# [Service]
# MemoryMax=512M
# MemoryHigh=400M