CVE-2026-11352: QUIC zero-length UDP datagrams busy-loop

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.0% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-07-03 · updated 2026-09-15
Weakness: CWE-835

Description

An issue in curl’s QUIC UDP receive function allows a malicious HTTP/3 server to trigger a remote denial of service against a curl or libcurl client. Because the helper function discards zero-length UDP datagrams before counting them toward the per-call packet budget, a connected QUIC peer can continuously stream empty datagrams to indefinitely stall the client.

Affected products

curl: from 8.18.0 before 8.21.0 — fixed in 8.21.0

How to fix

Upgrade to a fixed version: curl 8.21.0. Check the vendor advisory for mitigations.

References

https://curl.se/docs/CVE-2026-11352.html
https://curl.se/docs/CVE-2026-11352.json
https://hackerone.com/reports/3783438
https://hackerone.com/reports/3783438

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