CVE-2021-44730: Snapd Privilege Escalation Vulnerability

CVE-2021-44730 Overview

CVE-2021-44730 is a privilege escalation vulnerability in Canonical snapd version 2.54.2 that arises from improper validation of the snap-confine binary location. A local attacker with standard user privileges can exploit this flaw by creating a hardlink to the snap-confine binary in an attacker-controlled location, causing the binary to execute arbitrary commands with elevated privileges.

The vulnerability is classified as CWE-59 (Improper Link Resolution Before File Access), commonly known as a hardlink attack or symlink attack. This type of vulnerability occurs when a privileged program follows filesystem links without proper validation, allowing attackers to redirect execution flow to malicious code.

Critical Impact

Local attackers can achieve full root privilege escalation on affected Linux systems by exploiting improper hardlink validation in the snap-confine binary, potentially compromising the entire system.

Affected Products

• Canonical snapd (versions prior to 2.54.3)
• Canonical Ubuntu Linux 18.04 LTS, 20.04 LTS, and 21.10
• Fedora 34 and 35
• Debian Linux 10.0 and 11.0

Discovery Timeline

• 2022-02-17 - CVE-2021-44730 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-44730

Vulnerability Analysis

The snap-confine binary is a core component of the snapd package management system that runs with elevated privileges to set up the execution environment for snap applications. This binary is responsible for creating confined sandboxes and managing security namespaces for snap packages.

The vulnerability exists because snapd version 2.54.2 failed to properly validate the filesystem location of the snap-confine binary before execution. In Unix-like operating systems, hardlinks create additional directory entries pointing to the same inode as the original file, inheriting all of its properties including setuid bits.

When an attacker creates a hardlink to snap-confine in a directory they control, they can manipulate the execution environment (such as library paths or configuration files relative to the binary's location) to achieve arbitrary code execution with the privileges of the snap-confine binary—typically root.

Root Cause

The root cause of CVE-2021-44730 is the absence of proper path canonicalization and validation in the snapd service. The snap-confine binary did not verify that it was being executed from its legitimate installation path (/usr/lib/snapd/snap-confine or similar system locations). This oversight allowed local users to create hardlinks to the setuid binary in user-writable directories, bypassing intended security boundaries.

The vulnerability falls under CWE-59 (Improper Link Resolution Before File Access), which describes scenarios where software does not properly handle hardlinks or symbolic links, leading to security bypasses.

Attack Vector

The attack requires local access to a vulnerable system. An attacker with a standard user account can exploit this vulnerability by performing the following actions:

1. Create a hardlink to the snap-confine binary in a directory under their control
2. Manipulate the execution environment (e.g., library paths, configuration files) relative to the hardlinked binary location
3. Execute the hardlinked binary, which runs with elevated privileges
4. The privileged execution occurs within the attacker-controlled environment, allowing execution of arbitrary code with root privileges

This attack does not require user interaction and can be executed with low complexity once local access is obtained. The scope is changed because the vulnerability allows breaking out of the user's privilege boundary to affect the entire system.

Detection Methods for CVE-2021-44730

Indicators of Compromise

• Presence of hardlinks to snap-confine binary in non-standard directories (outside /usr/lib/snapd/ or /usr/libexec/snapd/)
• Unexpected files in user-writable directories with setuid permissions
• Evidence of privilege escalation attempts in system logs related to snap-confine execution
• Suspicious snap-confine processes spawned from unusual working directories

Detection Strategies

• Monitor filesystem events for hardlink creation targeting /usr/lib/snapd/snap-confine or similar system binaries
• Implement file integrity monitoring (FIM) to detect unauthorized hardlinks to setuid binaries
• Deploy EDR solutions capable of detecting privilege escalation patterns and anomalous process ancestry
• Use auditd rules to log link creation events: auditctl -w /usr/lib/snapd/snap-confine -p l -k snap_confine_hardlink

Monitoring Recommendations

• Enable and monitor snap daemon logs for unusual execution patterns
• Configure alerts for any setuid binary execution from user home directories or temporary locations
• Regularly audit hardlink counts for sensitive system binaries using find or similar tools
• Implement real-time monitoring for privilege escalation attempts using SentinelOne's behavioral AI detection

How to Mitigate CVE-2021-44730

Immediate Actions Required

• Update snapd to patched versions: 2.54.3+18.04 for Ubuntu 18.04 LTS, 2.54.3+20.04 for Ubuntu 20.04 LTS, or 2.54.3+21.10.1 for Ubuntu 21.10
• Apply security updates from your distribution's package manager immediately
• Audit systems for existing hardlinks to snap-confine in non-standard locations
• Consider temporarily restricting hardlink creation using fs.protected_hardlinks sysctl if patching is delayed

Patch Information

Canonical has released security patches addressing this vulnerability. The fixed versions properly validate the execution path of the snap-confine binary before allowing privileged operations.

Workarounds

• Enable hardlink protection system-wide by ensuring fs.protected_hardlinks=1 is set in sysctl configuration
• Remove the setuid bit from snap-confine temporarily if snap functionality is not critical (note: this will break snap functionality)
• Restrict user access to systems where patching cannot be immediately applied
• Implement filesystem restrictions to prevent hardlink creation in user-writable directories pointing to system binaries

# Enable hardlink protection (recommended security hardening)
echo "fs.protected_hardlinks = 1" | sudo tee -a /etc/sysctl.d/99-hardlink-protection.conf
sudo sysctl -p /etc/sysctl.d/99-hardlink-protection.conf

# Verify snapd version after patching
snap version

# Search for suspicious hardlinks to snap-confine
find / -type f -samefile /usr/lib/snapd/snap-confine 2>/dev/null | grep -v "^/usr/lib/snapd/"