Resource exhaustion in Cloudflare Quiche
CVE-2025-4821
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.009 (59.3th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 59th percentile — 59.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.008 · newest: 0.009 · change: +0.001
Affected products
- Cloudflare Quiche — versions <0.24.4
Weakness classification (CWE)
References
- github.com/cloudflare/quiche/security/advisories/GHSA-6m38-4r9r-5c4m (Vendor Advisory)
Frequently asked questions
- What is CVE-2025-4821?
- CVE-2025-4821 is a high-severity vulnerability in Cloudflare Quiche, classified under Allocation of Resources Without Limits or Throttling. CVSS score: 7.5/10. Published 2025-06-18.
- How severe is CVE-2025-4821?
- High severity. CVSS v3 base score is 7.5 out of 10.