CVE-2020-3810: Debian Apt DOS Vulnerability

CVE-2020-3810 Overview

CVE-2020-3810 is an input validation vulnerability affecting the ar/tar implementations in APT (Advanced Package Tool) before version 2.1.2. This vulnerability allows attackers to cause a denial of service condition by processing specially crafted deb files. The flaw stems from missing input validation when APT parses archive files during package installation or inspection operations.

Critical Impact
Attackers can disrupt system operations by providing malicious deb packages that crash APT, potentially blocking system updates and security patch installations on affected Linux distributions.

Affected Products

Debian APT (versions prior to 2.1.2)
Debian Linux 9.0 and 10.0
Fedora 32
Ubuntu Linux 12.04, 14.04, 16.04, 18.04, 19.10, and 20.04

Discovery Timeline

May 15, 2020 - CVE-2020-3810 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-3810

Vulnerability Analysis

This vulnerability is classified as CWE-20 (Improper Input Validation), indicating that the APT package manager fails to properly validate input when processing ar and tar archive formats within deb packages. The attack requires local access and user interaction, meaning an attacker must convince a user or system process to install or process a malicious deb file.

The denial of service condition occurs because APT's archive handling code does not adequately verify the integrity and format of archive headers and data structures. When encountering specially crafted malformed content within a deb file's ar or tar components, the application enters an error state that prevents normal operation.

Root Cause

The root cause lies in insufficient bounds checking and format validation within APT's ar/tar parsing routines. Deb packages use a layered archive format—an outer ar archive containing control and data tar archives. The vulnerability exists because the code paths handling these nested archives did not validate all input parameters before processing, allowing malformed data to trigger unexpected behavior.

The fix, implemented in commit dceb1e49e4b8e4dadaf056be34088b415939cda6, adds proper input validation checks to ensure archive data conforms to expected formats before processing continues.

Attack Vector

The attack requires local access to the target system and user interaction. An attacker would need to:

Craft a malicious deb package with specially constructed ar/tar archive components
Distribute this package through social engineering, compromised repositories, or man-in-the-middle attacks
Wait for a user or automated system to attempt installation of the malicious package

When APT processes the malicious deb file, the improper input validation causes the denial of service condition, potentially crashing APT operations and blocking legitimate package management activities.

The vulnerability mechanism involves malformed archive headers that bypass initial format checks but cause processing failures during extraction. Due to the lack of verified code examples, readers should consult the Debian APT Commit for detailed technical implementation information.

Detection Methods for CVE-2020-3810

Indicators of Compromise

Unexpected APT crashes or termination during package installation
Error messages related to malformed archive headers in /var/log/apt/ logs
Failed automated update processes without clear network-related causes
Presence of unusual or untrusted deb files in download directories

Detection Strategies

Monitor APT process stability and exit codes during package operations
Implement file integrity monitoring on package cache directories (/var/cache/apt/archives/)
Review APT logs for repeated parsing failures or unexpected terminations
Deploy endpoint detection to identify malicious deb file characteristics

Monitoring Recommendations

Configure alerts for APT service crashes or abnormal terminations
Monitor system logs (/var/log/syslog, /var/log/apt/) for ar/tar parsing errors
Track package installation attempts from non-official repository sources
Implement file scanning for downloaded deb packages before installation

How to Mitigate CVE-2020-3810

Immediate Actions Required

Update APT to version 2.1.2 or later immediately
Verify the source and integrity of all deb packages before installation
Restrict untrusted package sources in APT configuration
Monitor systems for signs of exploitation attempts

Patch Information

Debian has released APT version 2.1.2 which addresses this vulnerability. The fix adds proper input validation to the ar/tar implementation. Patches are available for all affected distributions:

Debian: See the Debian Security Announcement and Debian Tracker News
Ubuntu: Apply updates per Ubuntu Security Notice USN-4359-1 and USN-4359-2
Fedora: Follow the Fedora Package Announcement

For additional technical details, refer to the Launchpad Bug Report and GitHub Issue #111.

Workarounds

Only install packages from trusted, official repositories
Verify package checksums using GPG signatures before installation
Implement network-level controls to prevent downloading packages from untrusted sources
Consider temporary manual verification of all new packages until patching is complete

bash

# Update APT to patched version
sudo apt update
sudo apt install --only-upgrade apt

# Verify APT version after update
apt --version
# Should show version 2.1.2 or higher

# Configure APT to require valid signatures
echo 'APT::Get::AllowUnauthenticated "false";' | sudo tee /etc/apt/apt.conf.d/99security