How severe is CVE-2025-4820?

CVE-2025-4820 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability low.

Is CVE-2025-4820 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (48th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-4820?

CVE-2025-4820 affects Cloudflare Quiche. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-4820?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2025-4820 have an EU (EUVD) identifier?

Yes. CVE-2025-4820 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-18652.

When was CVE-2025-4820 published?

CVE-2025-4820 was published on 2025-06-18 and last updated on 2026-06-17.

CVE-2025-4820

CVE-2025-4820 is a medium-severity vulnerability in Cloudflare Quiche with a CVSS 3.x base score of 5.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-770.

Key facts

Severity: Medium (CVSS 3.x base score 5.3)
EPSS exploit prediction: 1% (48th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-18652
Weakness: CWE-770
Affected product: Cloudflare Quiche
Published: 2025-06-18
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2025-4820?: 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.
How severe is CVE-2025-4820?: CVE-2025-4820 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability low.
Is CVE-2025-4820 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (48th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-4820?: CVE-2025-4820 affects Cloudflare Quiche. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-4820?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2025-4820 have an EU (EUVD) identifier?: Yes. CVE-2025-4820 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-18652.
When was CVE-2025-4820 published?: CVE-2025-4820 was published on 2025-06-18 and last updated on 2026-06-17.

References

https://github.com/cloudflare/quiche/security/advisories/GHSA-2v9p-3p3h-w56j

Affected products (1)

cpe:2.3:a:cloudflare:quiche:*:*:*:*:*:rust:*:*

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