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
Escape the Surveillance Era
Most apps won’t tell you the truth.
They’re part of the problem.
Phone numbers. Emails. Profiles. Logs.
It’s all fuel for surveillance.
Ameeba Chat gives you a way out.
- • No phone number
- • No email
- • No personal info
- • Anonymous aliases
- • End-to-end encrypted
Chat without a trace.
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.
