CVE-2021-32036: MongoDB Server DOS Vulnerability

CVE-2021-32036 Overview

CVE-2021-32036 is a resource exhaustion vulnerability in MongoDB Server that allows an authenticated user without any specific authorizations to repeatedly invoke the features command. When executed at high volume, this can lead to resource depletion or generate high lock contention, resulting in denial of service conditions. In rare cases, this vulnerability could also result in id field collisions, potentially impacting data integrity.

Critical Impact

Authenticated attackers can cause denial of service through resource exhaustion by repeatedly invoking the features command, with potential for id field collisions affecting data integrity.

Affected Products

• MongoDB Server v5.0 versions prior to and including 5.0.3
• MongoDB Server v4.4 versions prior to and including 4.4.9
• MongoDB Server v4.2 versions prior to and including 4.2.16
• MongoDB Server v4.0 versions prior to and including 4.0.28

Discovery Timeline

• 2022-02-04 - CVE CVE-2021-32036 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-32036

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue stems from MongoDB Server's failure to implement proper rate limiting or resource controls on the features command endpoint. An authenticated user, even one without elevated privileges or specific authorizations, can exploit this weakness by sending a high volume of requests to the features command.

The attack exploits the network-accessible interface requiring only low-privilege authentication, making it accessible to any authenticated user on the system. The primary impact is on system availability through resource exhaustion, with a secondary concern being potential integrity impacts through id field collisions in rare edge cases.

Root Cause

The root cause is the absence of proper resource allocation controls and rate limiting mechanisms for the features command in MongoDB Server. The server does not enforce throttling or quotas on how frequently this command can be invoked by authenticated users, allowing for abuse through repeated high-volume requests. This design oversight enables attackers to consume server resources disproportionately, leading to lock contention and potential system instability.

Attack Vector

The attack can be executed remotely over the network by any authenticated user. The attacker needs only basic authentication to the MongoDB instance—no special authorizations or privileges are required. The exploitation involves repeatedly calling the features command at high frequency, which:

1. Consumes server CPU and memory resources
2. Generates high lock contention as the server processes concurrent requests
3. Potentially causes id field collisions due to race conditions under extreme load
4. Results in degraded performance or complete denial of service for legitimate users

The vulnerability requires no user interaction and can be scripted for automated exploitation.

Detection Methods for CVE-2021-32036

Indicators of Compromise

• Unusually high frequency of features command invocations from single authenticated sessions
• Abnormal lock contention patterns in MongoDB server logs
• Resource exhaustion alerts showing elevated CPU or memory utilization
• Unexpected id field collision errors in application logs

Detection Strategies

• Monitor MongoDB slow query logs for repeated features command executions
• Implement alerting on command frequency thresholds per authenticated user
• Track system resource utilization metrics (CPU, memory, lock wait times) for anomalies
• Review authentication logs for sessions exhibiting suspicious command patterns

Monitoring Recommendations

• Enable MongoDB profiling to capture detailed command execution statistics
• Configure SIEM alerts for rapid successive features command invocations
• Establish baseline metrics for normal features command usage patterns
• Monitor for signs of resource contention such as increased query latency across all operations

How to Mitigate CVE-2021-32036

Immediate Actions Required

• Upgrade MongoDB Server to patched versions: 5.0.4 or later, 4.4.10 or later, 4.2.17 or later, or 4.0.28 or later
• Review authentication logs to identify any potential exploitation attempts
• Implement network-level rate limiting for MongoDB connections as an interim measure
• Restrict network access to MongoDB instances to trusted hosts only

Patch Information

MongoDB has addressed this vulnerability in subsequent releases. Organizations should upgrade to the following minimum versions:

• MongoDB Server v5.0.4 or later
• MongoDB Server v4.4.10 or later
• MongoDB Server v4.2.17 or later
• MongoDB Server v4.0.28 or later (note: v4.0 branch has reached end of life)

For detailed patch information, refer to the MongoDB Jira Issue SERVER-59294.

Workarounds

• Implement application-level rate limiting for database commands where possible
• Use network firewalls to restrict MongoDB access to only authorized application servers
• Deploy a reverse proxy or connection pooler with rate limiting capabilities
• Review and tighten MongoDB user permissions to minimize authenticated user exposure

# Example: Restrict MongoDB access using iptables
# Allow connections only from trusted application servers
iptables -A INPUT -p tcp --dport 27017 -s 10.0.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 27017 -j DROP