CVE-2021-29763: IBM Db2 Denial of Service Vulnerability

CVE-2021-29763 Overview

IBM Db2 for Linux, UNIX and Windows (includes Db2 Connect Server) versions 11.1 and 11.5 contains a resource exhaustion vulnerability that, under very specific conditions, could allow a local user to keep running a procedure that causes the system to run out of memory, resulting in a denial of service condition. This vulnerability is tracked by IBM X-Force ID: 202267.

Critical Impact

A local attacker can exploit specific procedures in IBM Db2 to exhaust system memory, causing database service disruption and potential system-wide denial of service.

Affected Products

• IBM Db2 11.1 for Linux, UNIX and Windows
• IBM Db2 11.5 for Linux, UNIX and Windows
• IBM Db2 Connect Server 11.1 and 11.5
• Supported operating systems: IBM AIX, Linux, Microsoft Windows, Oracle Solaris, and UNIX

Discovery Timeline

• 2021-09-16 - CVE-2021-29763 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-29763

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling), which indicates that the IBM Db2 database engine fails to properly limit memory allocation when executing certain procedures. The issue requires local access and specific conditions to trigger, making successful exploitation more complex but still presenting a significant availability risk to database operations.

The vulnerability allows repeated procedure execution that progressively consumes system memory without proper cleanup or resource limits. Once memory is exhausted, the Db2 instance becomes unresponsive, affecting all connected applications and potentially impacting other services running on the same system.

Root Cause

The root cause of CVE-2021-29763 lies in improper resource management within IBM Db2's procedure execution engine. Specifically, the database fails to implement adequate memory allocation limits or proper cleanup mechanisms when certain procedures are executed repeatedly. This absence of resource throttling (CWE-770) allows memory consumption to grow unbounded until system resources are depleted.

Attack Vector

The attack requires local access to the system running IBM Db2. An attacker with the ability to execute database procedures can trigger the memory exhaustion condition by repeatedly invoking specific procedures under particular conditions. While the attack complexity is high due to the requirement of "very specific conditions," a motivated local attacker with knowledge of the vulnerable procedure patterns could systematically exhaust available memory.

The exploitation mechanism involves:

1. Local user gains access to execute procedures on the Db2 instance
2. Attacker identifies or triggers the specific conditions required for exploitation
3. Repeated execution of the targeted procedure causes incremental memory consumption
4. System memory becomes exhausted, leading to denial of service
5. The Db2 instance and potentially other system services become unresponsive

Detection Methods for CVE-2021-29763

Indicators of Compromise

• Abnormal memory consumption patterns on systems running IBM Db2 11.1 or 11.5
• Repeated execution of the same database procedures by local users
• Gradual degradation of Db2 instance performance over time
• System-wide memory exhaustion alerts or out-of-memory errors
• Unexplained Db2 service restarts or crashes

Detection Strategies

• Monitor system memory utilization on Db2 hosts for unusual consumption patterns
• Implement database activity monitoring to track procedure execution frequency
• Configure alerting for memory thresholds approaching system limits
• Review Db2 diagnostic logs for memory allocation warnings or errors
• Audit local user activity for suspicious repeated procedure calls

Monitoring Recommendations

• Establish baseline memory usage patterns for normal Db2 operations
• Implement real-time monitoring dashboards for Db2 host system resources
• Configure automated alerts when memory usage exceeds 80% of available resources
• Enable Db2 diagnostic monitoring to capture procedure execution statistics
• Review and correlate system logs with Db2 activity logs for anomaly detection

How to Mitigate CVE-2021-29763

Immediate Actions Required

• Apply the IBM security patch as referenced in the official IBM Support documentation
• Restrict local access to Db2 instances to only authorized administrators
• Implement system resource limits using OS-level controls where possible
• Monitor affected systems for signs of exploitation until patching is complete
• Review and restrict procedure execution privileges for non-essential users

Patch Information

IBM has released a security update to address this vulnerability. Administrators should apply the patch as documented in the IBM Support Page for Vulnerability. Additional details about this vulnerability can be found in the IBM X-Force Vulnerability #202267. NetApp has also published guidance in the NetApp Security Advisory NTAP-20211029-0005.

Workarounds

• Limit local user access to the Db2 server to trusted administrators only
• Implement OS-level resource limits (ulimit, cgroups) to constrain Db2 memory usage
• Configure Db2 instance memory parameters to set hard limits on allocation
• Monitor and alert on memory usage to enable rapid response before exhaustion occurs
• Consider implementing network isolation for Db2 servers to reduce local access vectors

# Example: Set memory limits for Db2 processes using ulimit
# Add to Db2 instance owner's profile
ulimit -v 8388608  # Limit virtual memory to 8GB
ulimit -m 4194304  # Limit resident set size to 4GB

# Example: Configure Db2 instance memory parameters
db2 update dbm cfg using INSTANCE_MEMORY 75
db2 update db cfg for <database> using DATABASE_MEMORY 50