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CVE-2026-17177 is a denial-of-service vulnerability affecting IBM Db2 Mirror for i versions 7.4, 7.5, and 7.6. The flaw stems from uncontrolled recursion within the database engine’s processing logic, allowing a remote, unauthenticated attacker to exhaust system resources and render the database service unresponsive.
At its core, the vulnerability exploits how Db2 Mirror for i handles recursive operations—specifically, nested function calls or iterative data processing that lack proper termination conditions. Under normal circumstances, recursive algorithms in database systems are bounded by depth limits or resource thresholds to prevent stack overflows or memory exhaustion. However, in the affected versions, certain input sequences can trigger deep recursion paths that never hit a safe termination guard.
The attack vector is remote and does not require authentication, making it particularly dangerous for exposed database instances. An attacker can craft a malicious request—likely a specially formed SQL query or a malformed protocol message—that forces the Db2 engine to enter an infinite or excessively deep recursive loop. Each recursive call consumes stack memory and CPU cycles. Without a cap on recursion depth, the process continues until the system’s memory is depleted or the stack overflows, causing the database service to crash or enter a hung state.
The uncontrolled recursion is classified under CWE-674 (Uncontrolled Recursion). This weakness is common in software that processes hierarchical data structures (e.g., XML, JSON, or nested queries) without enforcing recursion limits. In Db2 Mirror for i, the vulnerable component likely involves the parser or query optimizer when handling deeply nested expressions or self-referential data constructs.
Once triggered, the DoS condition persists until the Db2 process is manually restarted or the system is rebooted. In clustered or mirrored environments, the attack may also affect failover nodes if they process the same malicious input, amplifying the impact. While the vulnerability does not allow data theft or code execution, its ease of exploitation and remote accessibility make it a credible threat to availability.
IBM has acknowledged the issue and is expected to release a cumulative fix. Administrators are advised to restrict network access to Db2 Mirror for i services and monitor for unusual CPU or memory spikes that may indicate exploitation attempts.
DailyCVE Form:
Platform: IBM i
Version: 7.4, 7.5, 7.6
Vulnerability: Uncontrolled Recursion DoS
Severity: Medium (CVSS 6.5)
Date: 2026-08-14
Prediction: 2026-09-15
What Undercode Say:
Monitor Db2 Mirror for i processes for excessive CPU/memory usage
ps -ef | grep db2
top -p $(pgrep -d',' -f db2)
Check system logs for recursion-related errors
grep -i "recursion|stack overflow|out of memory" /var/log/messages
Test for vulnerability (educational use only) - simulate deep recursion payload
echo "SELECT FROM xmltable('/a/b/c/d/e/f/g/h/i/j/k/l/m/n/o/p/q/r/s/t/u/v/w/x/y/z' passing ...)" | db2 -x
Apply PTF fix when available (example command)
SNDPTFCMD PTFID=SI82345
Restrict network access to Db2 port (default 50000/tcp) temporarily
iptables -A INPUT -p tcp --dport 50000 -j DROP
Exploit: (Educational Purposes!)
A remote attacker can exploit this vulnerability by sending a crafted SQL statement containing a deeply nested XML or JSON structure to the Db2 Mirror for i listener. The database parser, lacking a recursion depth limit, will attempt to process the nested hierarchy, consuming stack frames until memory exhaustion occurs. For example:
-- Crafted payload with excessive nesting (conceptual) SELECT xmlparse(document '<a><b><c>...<z>trigger</z>...</c></b></a>') FROM sysibm.sysdummy1;
By increasing the nesting depth beyond the engine’s implicit capacity, the recursion never terminates, leading to a denial of service.
Protection:
- Apply the official IBM PTF (Program Temporary Fix) as soon as it is released.
- Restrict inbound access to Db2 Mirror for i ports (typically 50000/tcp) using firewalls or network ACLs.
- Implement rate limiting and request size restrictions at the application or proxy layer.
- Enable database audit logging to detect and block anomalous query patterns.
Impact:
- Availability: Complete loss of database service until manual intervention.
- Operational Disruption: Mirrored or clustered environments may experience failover storms if multiple nodes are targeted.
- Recovery Time: Service restoration requires process termination and restart, leading to extended downtime.
- No Data Compromise: The vulnerability does not expose or alter data, limiting its scope to denial of service.
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Sources:
Reported By: nvd.nist.gov
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