{"id":86507,"date":"2026-02-20T00:18:59","date_gmt":"2026-02-20T00:18:59","guid":{"rendered":""},"modified":"-0001-11-30T00:00:00","modified_gmt":"-0001-11-30T00:00:00","slug":"cve-2025-4821-cloudflare-quiche-incorrect-congestion-window-growth-vulnerability","status":"publish","type":"post","link":"https:\/\/www.ameeba.com\/blog\/cve-2025-4821-cloudflare-quiche-incorrect-congestion-window-growth-vulnerability\/","title":{"rendered":"CVE-2025-4821: Cloudflare Quiche Incorrect Congestion Window Growth Vulnerability<\/strong>"},"content":{"rendered":"

Overview<\/strong><\/p>\n

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.<\/p>\n

Vulnerability Summary<\/strong><\/p>\n

CVE ID: CVE-2025-4821
\nSeverity: High (CVSS: 7.5)
\nAttack Vector: Network
\nPrivileges Required: None
\nUser Interaction: None
\nImpact: Potential system compromise or data leakage<\/p>\n

Affected Products<\/strong><\/p>\n

Product | Affected Versions<\/p>\n

Cloudflare Quiche | Versions prior to 0.24.4<\/p>\n

How the Exploit Works<\/strong><\/p>\n

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.<\/p>\n

Conceptual Example Code<\/strong><\/p>\n

Below is a conceptual example of how the vulnerability might be exploited:<\/p>\n

# Begin handshake\ninit_handshake(target=\"target.example.com\")\n# Initiate congestion-controlled data transfer\ndata_transfer = start_data_transfer(target=\"target.example.com\")\n# Send ACK frames with large range of packet numbers\nfor i in range(1, 1000000):\nsend_ack_frame(data_transfer, packet_number=i)<\/code><\/pre>\n
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.<\/p>\n
Mitigation Guidance<\/strong><\/p>\n
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.<\/p>\n","protected":false},"excerpt":{"rendered":"
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 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"footnotes":""},"categories":[],"tags":[],"vendor":[],"product":[],"attack_vector":[],"asset_type":[],"severity":[],"exploit_status":[],"class_list":["post-86507","post","type-post","status-publish","format-standard","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t