CVE-2025-4820 Incorrect congestion window growth by optimistic ACK

Medium 5.3 Published 18 Jun 2025 Updated 18 Jun 2025

An attacker can cause excessive congestion window growth in Cloudflare quiche, leading to network congestion and potential denial of service.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2025-4820?

CVE-2025-4820 is a medium-severity denial of service vulnerability in Cloudflare quiche, published on 18 Jun 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-770

Which versions are affected by CVE-2025-4820?

It affects 1 product.

Products include Cloudflare Quiche.

How do I fix CVE-2025-4820?

No fix has been published for CVE-2025-4820 yet. Limit exposure of the affected software until one is available.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2025-4820 being exploited?

There is no known exploitation of CVE-2025-4820 in the wild and no public exploit code.

Sources

Also known as GHSA-2v9p-3p3h-w56j.