Incorrect congestion window growth by optimistic ACK

Medium5.3CVE-2025-4820 · Published Jun 17, 2025 · updated Jul 30, 2026

### 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](https://datatracker.ietf.org/doc/html/rfc9000#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.