CVE-2025-30193: DNSdist TCP Stack Exhaustion DoS Vulnerability

CVE-2025-30193 Overview

CVE-2025-30193 is a denial of service vulnerability in DNSdist, a highly DNS-, DoS-, and abuse-aware load balancer. When DNSdist is configured to allow an unlimited number of queries on a single incoming TCP connection from a client, an attacker can craft a malicious TCP exchange that triggers stack exhaustion, resulting in a crash of the DNSdist service. This vulnerability allows remote attackers to disrupt DNS services without requiring authentication or user interaction.

Critical Impact

Remote attackers can crash DNSdist servers by exploiting the unbounded TCP query handling, causing complete denial of DNS load balancing services.

Affected Products

• DNSdist versions prior to 1.9.10
• DNSdist installations configured with unlimited TCP queries per connection
• DNSdist deployments without setMaxTCPQueriesPerConnection restrictions

Discovery Timeline

• 2025-05-20 - CVE CVE-2025-30193 published to NVD
• 2025-05-21 - Last updated in NVD database

Technical Details for CVE-2025-30193

Vulnerability Analysis

This vulnerability is classified under CWE-674 (Uncontrolled Recursion), indicating that the root cause involves recursive function calls or iterative processes that can be exploited to exhaust system resources. When DNSdist processes incoming TCP connections with no query limit configured, the application enters a state where continued processing leads to stack exhaustion.

The vulnerability specifically targets the TCP connection handling mechanism in DNSdist. Under default or misconfigured conditions where unlimited queries are permitted on a single TCP connection, an attacker can maintain a persistent connection and issue a carefully crafted sequence of DNS queries that causes the internal processing stack to grow unbounded until system memory is exhausted, resulting in a crash.

Root Cause

The vulnerability stems from insufficient bounds checking in the TCP query handling logic. When setMaxTCPQueriesPerConnection is not configured or set to allow unlimited queries, DNSdist fails to properly limit the depth of recursive operations or iterative processing associated with handling queries on a persistent TCP connection. This creates an uncontrolled recursion scenario (CWE-674) where stack frames accumulate without proper release, eventually exhausting available stack space.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker establishes a TCP connection to the vulnerable DNSdist instance and sends specially crafted DNS queries designed to trigger the stack exhaustion condition. Since the attack leverages legitimate TCP connections to the DNS service port, it may be difficult to distinguish from normal traffic without specific detection mechanisms.

The exploitation mechanism involves:

1. Establishing a TCP connection to the DNSdist service
2. Sending a sequence of malformed or specifically crafted DNS queries
3. Maintaining the connection while the server processes queries recursively
4. The accumulated stack frames eventually exhaust available stack memory
5. DNSdist crashes, causing denial of service for all clients

Detection Methods for CVE-2025-30193

Indicators of Compromise

• Unexpected DNSdist service crashes or restarts
• Abnormally long-lived TCP connections to the DNS service port
• Memory utilization spikes immediately preceding service crashes
• Log entries indicating stack overflow or segmentation faults

Detection Strategies

• Monitor DNSdist process stability and implement alerting on unexpected restarts
• Track TCP connection duration and flag connections exceeding normal operational thresholds
• Implement network flow analysis to detect unusual patterns of TCP-based DNS queries
• Review DNSdist logs for crash indicators or error messages related to memory exhaustion

Monitoring Recommendations

• Deploy application-level monitoring for DNSdist service health and availability
• Configure alerts for TCP connection count anomalies per source IP
• Implement stack size monitoring on systems running DNSdist
• Use SentinelOne Singularity platform to detect and alert on process crashes indicative of exploitation attempts

How to Mitigate CVE-2025-30193

Immediate Actions Required

• Upgrade DNSdist to version 1.9.10 or later immediately
• If immediate patching is not possible, implement the setMaxTCPQueriesPerConnection workaround
• Review current DNSdist configuration for unlimited TCP query settings
• Monitor DNSdist service stability for signs of active exploitation

Patch Information

PowerDNS has released version 1.9.10 of DNSdist to address this vulnerability. The patch implements proper bounds checking on TCP query processing to prevent stack exhaustion. Organizations should upgrade to this version as the primary remediation. For detailed information, refer to the PowerDNS Security Advisory.

Workarounds

• Configure setMaxTCPQueriesPerConnection to a safe value such as 50 to limit queries per TCP connection
• Implement rate limiting on incoming DNS TCP connections at the network perimeter
• Consider temporary deployment of additional DNS load balancers to maintain service availability during patching

# Configuration example - Add to DNSdist configuration file
# Limit maximum TCP queries per connection to prevent stack exhaustion
setMaxTCPQueriesPerConnection(50)