What’s Vulnerable › Vulnerabilities
CVE-2025-4820: Incorrect congestion window growth by optimistic ACK
- Severity: medium (CVSS 5.3 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.9% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2025-06-18 · updated 2026-06-17
- Weakness: CWE-770
Description
Impact Cloudflare quiche was discovered to be vulnerable to incorrect congestion window growth, which could cause it to send data at a rate faster than the path might actually support. An unauthenticated remote attacker can exploit the vulnerability by first completing a handshake and initiating a congestion-controlled data transfer towards itself. Then, it could manipulate the victim's congestion control state by sending ACK frames exercising an opportunistic ACK attack; see RFC 9000 Section 21.4. The victim could grow the congestion window beyond typical expectations and allow more bytes in flight than the path might really support. Patches quiche 0.24.4 is the earliest version containing the fix for this issue.
Affected products
- Cloudflare quiche: before 0.24.4 — fixed in 0.24.4
How to fix
Upgrade to a fixed version: Cloudflare quiche 0.24.4. Check the vendor advisory for mitigations.
References
Get alerted when CVE-2025-4820 or the next vulnerability affects your stack