CVE-2025-5399: Haxx Curl WebSocket DoS Vulnerability

CVE-2025-5399 Overview

CVE-2025-5399 is a Denial of Service vulnerability in libcurl's WebSocket implementation that allows a malicious server to trap libcurl in an endless busy-loop by sending a specially crafted packet. Once triggered, there is no way for the application to escape or exit this loop other than killing the thread or process, making this a severe availability threat for any application relying on libcurl for WebSocket communications.

Critical Impact

Applications using libcurl for WebSocket connections can be rendered completely unresponsive through a malicious server response, requiring process termination to recover.

Affected Products

• haxx curl (libcurl WebSocket implementation)
• Applications using libcurl WebSocket functionality
• Systems with vulnerable libcurl versions

Discovery Timeline

• 2025-06-04 - Vulnerability disclosed via OpenWall OSS Security Discussion
• 2025-06-07 - CVE-2025-5399 published to NVD
• 2025-07-30 - Last updated in NVD database

Technical Details for CVE-2025-5399

Vulnerability Analysis

This vulnerability stems from a flaw in libcurl's WebSocket code (CWE-835: Loop with Unreachable Exit Condition). When processing WebSocket frames, a malicious server can send a particularly crafted packet that exploits a logic error in the frame parsing routine, causing libcurl to enter an infinite busy-loop.

The attack is network-accessible and requires no authentication or user interaction to exploit. Once the vulnerable code path is triggered, the affected thread enters a tight loop that consumes CPU resources indefinitely. The only recovery mechanism is to terminate the affected process or thread externally, making this particularly dangerous for long-running services and daemons that depend on libcurl for WebSocket communications.

Root Cause

The root cause is a logic error in libcurl's WebSocket frame processing code that fails to properly handle certain malformed or specially crafted packets. This results in a loop condition that can never be satisfied, trapping the execution flow indefinitely. The flaw represents a classic infinite loop vulnerability where the exit condition becomes unreachable under specific input conditions.

Attack Vector

The attack is conducted over the network, where a malicious WebSocket server sends a specially crafted packet to a connecting libcurl client. The attack scenario involves:

1. A victim application initiates a WebSocket connection using libcurl
2. The malicious server accepts the connection and sends a crafted WebSocket frame
3. libcurl's WebSocket parsing code enters an infinite loop while processing the frame
4. The application becomes unresponsive, consuming CPU resources until the process is terminated

This attack requires the victim to connect to a malicious server, which could be achieved through various means including DNS hijacking, man-in-the-middle attacks, or simply luring users to connect to attacker-controlled endpoints.

Detection Methods for CVE-2025-5399

Indicators of Compromise

• Processes using libcurl exhibiting abnormally high CPU utilization (near 100% on a single core)
• Applications with WebSocket connections becoming completely unresponsive
• Threads stuck in libcurl WebSocket-related function calls for extended periods
• System logs showing connection timeouts or watchdog kills for libcurl-dependent services

Detection Strategies

• Monitor for unusual CPU consumption patterns in applications utilizing libcurl WebSocket functionality
• Implement watchdog timers around WebSocket operations to detect and recover from hung states
• Audit network connections to identify suspicious WebSocket servers or unexpected connection behavior
• Use application performance monitoring to flag sudden responsiveness degradation in libcurl-dependent services

Monitoring Recommendations

• Deploy endpoint detection solutions capable of identifying infinite loop patterns and abnormal CPU utilization
• Implement connection timeout policies for WebSocket operations to limit exposure duration
• Monitor process state and responsiveness for critical services using libcurl
• Configure alerting on sustained high CPU usage by specific application processes

How to Mitigate CVE-2025-5399

Immediate Actions Required

• Identify all systems and applications using libcurl with WebSocket functionality
• Review the cURL CVE-2025-5399 Documentation for official patch information
• Prioritize updates for internet-facing applications and services that connect to external WebSocket endpoints
• Consider temporarily disabling WebSocket functionality in critical applications until patching is complete

Patch Information

Haxx has released security information regarding this vulnerability. Administrators should consult the official cURL CVE-2025-5399 Documentation for specific patch versions and upgrade instructions. Additional technical details are available in the HackerOne Report #3168039 which documents the original vulnerability discovery.

Workarounds

• Implement application-level timeouts for WebSocket operations to automatically terminate connections that exceed expected processing time
• Use process supervisors or watchdog mechanisms to automatically restart services that become unresponsive
• Restrict WebSocket connections to trusted, known-good servers where possible
• Consider network-level filtering to limit connections to untrusted WebSocket endpoints

# Example: Implementing process supervision with systemd
# Add watchdog timeout to service unit file
[Service]
WatchdogSec=30
Restart=always
RestartSec=5