CVE-2025-4821 Incorrect congestion window growth by invalid ACK ranges

High 7.5 Published 18 Jun 2025 Updated 18 Jun 2025

An attacker can cause a denial of service in Cloudflare quiche by manipulating congestion control state to trigger an integer overflow panic.

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-2025-4821?

CVE-2025-4821 is a high-severity 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 covering a large range of packet numbers (including packet numbers that had never been sent); see RFC 9000 Section 19.3. The victim could grow the congestion window beyond typical expectations and allow more bytes in flight than the path might really support. In extreme cases, the window might grow beyond the limit of the internal variable's type, leading to an overflow panic.

Patches

quiche 0.24.4 is the earliest version containing the fix for this issue.

Weakness: CWE-770

Which versions are affected by CVE-2025-4821?

It affects 1 product.

Products include Cloudflare Quiche.

How do I fix CVE-2025-4821?

No fix has been published for CVE-2025-4821 yet. Limit exposure of the affected software until one is available.

Is CVE-2025-4821 being exploited?

There is no known exploitation of CVE-2025-4821 in the wild and no public exploit code.

Sources

Also known as GHSA-6m38-4r9r-5c4m.