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CVE-2025-4821: Cloudflare Quiche Incorrect Congestion Window Growth Vulnerability

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Overview

This report highlights the vulnerability CVE-2025-4821 discovered in Cloudflare quiche, which could lead to incorrect congestion window growth. This vulnerability, if exploited, could lead to potential system compromise or data leakage. This report aims to provide an understanding of the severity, attack vector, impact, and potential mitigation tactics of this cybersecurity threat.

Vulnerability Summary

CVE ID: CVE-2025-4821
Severity: High (CVSS: 7.5)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Potential system compromise or data leakage

Affected Products

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Product | Affected Versions

Cloudflare Quiche | Versions prior to 0.24.4

How the Exploit Works

An unauthenticated remote attacker can exploit the vulnerability by first completing a handshake and initiating a congestion-controlled data transfer towards itself. Then, the attacker could manipulate the victim’s congestion control state by sending ACK frames covering a large range of packet numbers, causing the congestion window to grow beyond typical expectations and allow more bytes in flight than the path might actually support. In extreme cases, the window might grow beyond the limit of the internal variable’s type, leading to an overflow panic.

Conceptual Example Code

Below is a conceptual example of how the vulnerability might be exploited:

# Begin handshake
init_handshake(target="target.example.com")
# Initiate congestion-controlled data transfer
data_transfer = start_data_transfer(target="target.example.com")
# Send ACK frames with large range of packet numbers
for i in range(1, 1000000):
send_ack_frame(data_transfer, packet_number=i)

This script starts a handshake, initiates a data transfer, and then sends a large range of ACK frames to the victim, potentially causing an overflow panic.

Mitigation Guidance

It is recommended to apply the patch provided by the vendor. The earliest version containing the fix for this issue is quiche 0.24.4. As a temporary mitigation, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can provide some level of protection. However, this should not replace the need for applying the patch.

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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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