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

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

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 18 Jun 2025
- **Updated:** 18 Jun 2025
- **Also known as:** GHSA-6m38-4r9r-5c4m

## 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 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2025-4821?

It affects 1 product.


Products: 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.

## Other vulnerabilities in Cloudflare quiche

- [CVE-2025-7054](https://insights.sbomify.com/CVE-2025-7054/): Infinite loop triggered by connection ID retirement (high 8.7)
- [CVE-2026-12523](https://insights.sbomify.com/CVE-2026-12523/): Resource exhaustion in quiche HTTP/3 and QPACK layers (high 7.5)
- [CVE-2024-1765](https://insights.sbomify.com/CVE-2024-1765/): Unlimited resource allocation by QUIC CRYPTO frames flooding in quiche (medium 5.9)
- [CVE-2025-4820](https://insights.sbomify.com/CVE-2025-4820/): Incorrect congestion window growth by optimistic ACK (medium 5.3)
- [CVE-2023-6193](https://insights.sbomify.com/CVE-2023-6193/): Unbounded queuing of path validation messages in cloudflare-quiche (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-4821 and fix it if it is affected.

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

Affected product: Cloudflare quiche. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-4821/ (Markdown: https://insights.sbomify.com/CVE-2025-4821/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-4821
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-4821
- OSV: https://osv.dev/vulnerability/CVE-2025-4821

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-4821/
Summaries are generated and may contain mistakes; the official description above is the record.