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

> CVE-2025-4820: Incorrect congestion window growth by optimistic ACK. CVSS 5.3 (CVSS 3.1).

Canonical: https://rewindex.com/cve/CVE-2025-4820
Last updated: 2026-06-17T09:34:05Z

- Published: 2025-06-18 15:45 UTC · Updated: 2026-06-17 09:34 UTC · Assigner: cloudflare
- CVSS: 5.3 (CVSS 3.1) · Severity: Medium · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- EPSS: 0.86%, 58th percentile (scored 2026-10-25)
- CISA KEV: no
- CWE: 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 vendors and products

- Cloudflare: quiche

## Tracked companies

- [Cloudflare](https://rewindex.com/company/cloudflare.md)

## References

- [CVE record on cve.org](https://www.cve.org/CVERecord?id=CVE-2025-4820)
- [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2025-4820)
- [https://github.com/cloudflare/quiche/security/advisories/GHSA-2v9p-3p3h-w56j](https://github.com/cloudflare/quiche/security/advisories/GHSA-2v9p-3p3h-w56j)

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_Sources: CVE records from the CVE Program (cve.org). CVE® is a registered trademark of The MITRE Corporation. (https://www.cve.org/) This product uses data from the NVD API but is not endorsed or certified by the NVD. (https://nvd.nist.gov/) Known Exploited Vulnerabilities catalog from CISA. (https://www.cisa.gov/known-exploited-vulnerabilities-catalog) EPSS scores from FIRST (Exploit Prediction Scoring System). (https://www.first.org/epss/)_


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Canonical: https://rewindex.com/cve/CVE-2025-4820 · Markdown: https://rewindex.com/cve/CVE-2025-4820.md · JSON: https://rewindex.com/api/v1/cves/CVE-2025-4820 · Data licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) · Generated 2026-10-26T03:00:50.530Z