CVE-2025-24294: DNS Packet Parsing DoS Vulnerability

CVE-2025-24294 Overview

CVE-2025-24294 is a Denial of Service (DoS) vulnerability in the Ruby resolv library caused by an insufficient check on the length of a decompressed domain name within a DNS packet. An attacker can craft a malicious DNS packet containing a highly compressed domain name. When the resolv library parses such a packet, the name decompression process consumes a large amount of CPU resources, as the library does not limit the resulting length of the name. This resource consumption can cause the application thread to become unresponsive, resulting in a Denial of Service condition.

Critical Impact

Applications using Ruby's resolv library are vulnerable to CPU exhaustion attacks through specially crafted DNS packets with highly compressed domain names, potentially causing complete service unavailability.

Affected Products

• Ruby resolv library (all versions prior to patch)
• Applications utilizing Ruby DNS resolution functionality

Discovery Timeline

• 2025-07-12 - CVE CVE-2025-24294 published to NVD
• 2025-07-16 - Last updated in NVD database

Technical Details for CVE-2025-24294

Vulnerability Analysis

This vulnerability stems from CWE-400 (Uncontrolled Resource Consumption), a class of weaknesses where an application fails to properly control resource usage. The Ruby resolv library, which provides DNS resolution capabilities for Ruby applications, processes DNS response packets that may contain compressed domain names using DNS message compression (RFC 1035). The vulnerability exists because the library does not impose adequate limits on the resulting length of decompressed domain names.

DNS compression allows domain names to be represented using pointers to previously occurring domain name segments within the same message, reducing packet size. However, maliciously crafted compression schemes can exploit this mechanism to create domain names that expand to disproportionately large sizes when decompressed, causing the parsing routine to consume excessive CPU cycles.

Root Cause

The root cause is an insufficient validation check on the length of domain names after the decompression process completes. The resolv library fails to enforce reasonable bounds on the expanded domain name length, allowing attackers to craft DNS packets where the decompression algorithm runs for an extended period. This unbounded processing creates an algorithmic complexity attack vector where a relatively small input packet can trigger substantial computational overhead.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Crafting a malicious DNS response packet with carefully constructed domain name compression pointers
2. Sending this packet to an application that uses Ruby's resolv library to parse DNS responses
3. The library's decompression routine processes the compressed name without length limits
4. CPU resources are exhausted as the decompression continues processing the malicious payload

The attack can be launched remotely against any Ruby application that processes DNS packets using the vulnerable library. This could occur in scenarios where applications handle DNS responses from untrusted sources or when an attacker can perform DNS response injection attacks.

Detection Methods for CVE-2025-24294

Indicators of Compromise

• Unusual CPU utilization spikes in Ruby application processes
• Application threads becoming unresponsive during DNS resolution operations
• Presence of abnormally large or malformed DNS response packets in network traffic
• DNS packets with excessive compression pointer chains

Detection Strategies

• Monitor for anomalous CPU consumption patterns in Ruby application processes, particularly during DNS operations
• Implement network-based detection for DNS packets with suspicious compression ratios
• Deploy application performance monitoring to identify DNS resolution timeouts or hangs
• Analyze DNS traffic for packets with unusually deep compression pointer hierarchies

Monitoring Recommendations

• Enable detailed logging for DNS resolution activities in Ruby applications
• Set up alerts for sustained high CPU usage in processes utilizing the resolv library
• Monitor for DNS response packets exceeding normal size thresholds
• Track application response time metrics to detect DoS impact early

How to Mitigate CVE-2025-24294

Immediate Actions Required

• Update Ruby to the latest patched version that addresses CVE-2025-24294
• Review applications for dependency on the vulnerable resolv library
• Implement rate limiting for DNS resolution requests in affected applications
• Consider using alternative DNS resolution mechanisms until patches are applied

Patch Information

Ruby has released a security update addressing this vulnerability. Refer to the Ruby Security Advisory for CVE-2025-24294 for detailed patch information and updated versions.

Workarounds

• Implement application-level timeouts for DNS resolution operations to limit the impact of malicious packets
• Deploy network-level filtering to inspect and reject malformed DNS packets before they reach vulnerable applications
• Use a dedicated DNS resolver service that performs validation before passing responses to applications
• Consider implementing DNS response size limits at the network perimeter

# Example: Add timeout wrapper for DNS operations in Ruby applications
# Set appropriate timeout values based on your application requirements
# ruby -e "require 'timeout'; Timeout.timeout(5) { require 'resolv'; Resolv.getaddress('example.com') }"