CVE-2021-20333: MongoDB Server Log Injection Vulnerability

CVE-2021-20333 Overview

CVE-2021-20333 is a log injection vulnerability affecting MongoDB Server that allows attackers to send specially crafted commands resulting in artificial log entries being generated or existing log entries being split. This vulnerability stems from improper output encoding/neutralization of log entries (CWE-117, CWE-116), enabling attackers to manipulate log files which can be leveraged to obscure malicious activity, forge audit trails, or potentially exploit downstream log processing systems.

Critical Impact

Attackers can inject malicious content into MongoDB Server logs, potentially hiding evidence of compromise, creating false audit trails, or exploiting log analysis tools that process these entries.

Affected Products

• MongoDB Server v3.6 versions prior to 3.6.20
• MongoDB Server v4.0 versions prior to 4.0.21
• MongoDB Server v4.2 versions prior to 4.2.10

Discovery Timeline

• 2021-07-23 - CVE-2021-20333 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-20333

Vulnerability Analysis

This vulnerability exists in MongoDB Server's log handling mechanism where user-controlled input is written to log files without proper sanitization or encoding. The weakness falls under CWE-117 (Improper Output Neutralization for Logs) and CWE-116 (Improper Encoding or Escaping of Output), indicating that the server fails to properly neutralize special characters or sequences before writing them to log files.

When MongoDB Server processes certain commands, it logs relevant information for debugging and auditing purposes. However, the vulnerable versions do not adequately validate or encode user-supplied data before including it in log entries. This allows an attacker to inject newline characters, escape sequences, or other special characters that can manipulate the log format.

The attack can be executed remotely over the network without requiring any authentication or user interaction. While the integrity impact is limited in scope, the ability to manipulate logs poses significant risks to forensic investigations and compliance auditing.

Root Cause

The root cause is improper output neutralization in MongoDB Server's logging subsystem. When processing commands from clients, the server includes portions of the command data in log messages without proper encoding of special characters. This allows attackers to inject control characters (such as carriage returns and newlines) that can split log entries or insert entirely artificial log lines.

Attack Vector

The attack is network-based and does not require authentication. An attacker can craft malicious MongoDB commands containing special characters designed to manipulate log output. When these commands are processed by the server, the injected content becomes part of the log entries, enabling the attacker to:

1. Split legitimate log entries across multiple lines
2. Inject completely fabricated log entries that appear authentic
3. Potentially interfere with log parsing tools and SIEM systems
4. Obscure evidence of malicious activity by corrupting log integrity

The vulnerability requires no privileges and no user interaction, making it straightforward to exploit from any network-accessible position relative to the MongoDB server.

Detection Methods for CVE-2021-20333

Indicators of Compromise

• Unusual patterns in MongoDB log files including malformed entries or unexpected line breaks
• Log entries containing suspicious control characters or escape sequences
• Discrepancies between expected log entry format and actual logged content
• Evidence of log tampering or gaps in chronological log sequences

Detection Strategies

• Implement log integrity monitoring to detect unexpected modifications or format anomalies
• Deploy network monitoring to identify unusually crafted MongoDB commands
• Configure SIEM rules to alert on malformed or suspicious log entry patterns
• Review MongoDB logs for entries with unexpected newline characters or split formatting

Monitoring Recommendations

• Enable detailed auditing on MongoDB servers to maintain secondary audit trails
• Implement centralized logging with tamper-evident storage separate from the MongoDB host
• Monitor for connections from unexpected sources to MongoDB server ports
• Establish baseline log patterns to detect anomalies indicative of injection attempts

How to Mitigate CVE-2021-20333

Immediate Actions Required

• Upgrade MongoDB Server v3.6 to version 3.6.20 or later
• Upgrade MongoDB Server v4.0 to version 4.0.21 or later
• Upgrade MongoDB Server v4.2 to version 4.2.10 or later
• Restrict network access to MongoDB servers using firewall rules and access controls

Patch Information

MongoDB has released patched versions addressing this vulnerability. The fix involves proper sanitization and encoding of user-controlled input before it is written to log files. For detailed information about the fix, refer to the MongoDB Bug Report SERVER-50605.

Organizations should upgrade to the following minimum versions:

• MongoDB v3.6: Upgrade to 3.6.20 or later
• MongoDB v4.0: Upgrade to 4.0.21 or later
• MongoDB v4.2: Upgrade to 4.2.10 or later

Workarounds

• Implement network-level access controls to limit MongoDB server exposure to trusted sources only
• Deploy a web application firewall or network filter capable of inspecting MongoDB protocol traffic
• Configure log forwarding to an immutable, centralized logging system to preserve evidence integrity
• Enable MongoDB authentication and authorization to reduce the attack surface from unauthenticated sources

# Configuration example - Restrict MongoDB network binding and enable authentication
# In mongod.conf:
net:
  bindIp: 127.0.0.1,192.168.1.100  # Limit to specific interfaces
  port: 27017
security:
  authorization: enabled  # Require authentication