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CVE-2025-5399: High Severity DoS Vulnerability in Libcurl’s WebSocket Code

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Overview

This report discusses the recent discovery of a high-severity vulnerability, designated as CVE-2025-5399, which affects the libcurl’s WebSocket code. If exploited, this vulnerability allows a malicious server to trap libcurl in an endless busy-loop, potentially leading to a Denial-of-Service (DoS) attack. This vulnerability is of critical concern to any applications or systems using libcurl, due to the potential for data leakage or system compromise.

Vulnerability Summary

CVE ID: CVE-2025-5399
Severity: High (CVSS 7.5)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Denial of service leading to potential system compromise or data leakage

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Product | Affected Versions

libcurl | All versions prior to patch

How the Exploit Works

The exploit takes advantage of a flaw in the WebSocket code of libcurl. By sending a specially crafted packet, a malicious server can cause libcurl to enter an endless busy-loop. This loop cannot be exited by the application unless the process or thread is forcibly terminated. In effect, this can lead to a DoS attack, where the application becomes unresponsive and inaccessible.

Conceptual Example Code

While we don’t have the exact exploit code, a conceptual attack might involve a WebSocket handshake request with a malicious payload. This could look something like:

GET /chat HTTP/1.1
Host: target.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13
Origin: http://example.com
{ "malicious_payload": "..." }

In the above example, the “malicious_payload” could be the specially crafted packet which triggers the endless loop in libcurl.

Mitigation

As a mitigation measure, users are advised to apply the vendor patch as soon as it becomes available. Until then, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) may help to identify and block malicious packets. However, these are temporary measures and cannot completely secure the system from the vulnerability.

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Disclaimer:

The information and code presented in this article are provided for educational and defensive cybersecurity purposes only. Any conceptual or pseudocode examples are simplified representations intended to raise awareness and promote secure development and system configuration practices.

Do not use this information to attempt unauthorized access or exploit vulnerabilities on systems that you do not own or have explicit permission to test.

Ameeba and its authors do not endorse or condone malicious behavior and are not responsible for misuse of the content. Always follow ethical hacking guidelines, responsible disclosure practices, and local laws.

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