CVE-2026-33128

h3 has a Server-Sent Events Injection via Unsanitized Newlines in Event Stream Fields

H3 is a minimal H(TTP) framework. In versions prior to 1.15.6 and between 2.0.0 through 2.0.1-rc.14, createEventStream is vulnerable to Server-Sent Events (SSE) injection due to missing newline sanitization in formatEventStreamMessage() and formatEventStreamComment(). An attacker who controls any part of an SSE message field (id, event, data, or comment) can inject arbitrary SSE events to connected clients. This issue is fixed in versions 1.15.6 and 2.0.1-rc.15.

Affected

h3js · h3

CVSS v3.1 NVD

10.0 CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

CVSS v3.1 GITHUB

7.5 HIGHCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N

EPSS

0.71% chance of exploitation in the next 30 days, 53rd percentile

Published

2026-03-20

Proof-of-concept exploits (1)

• advisories/GHSA-22cc-p3c6-wpvm

Security advisories

• h3js/h3/security/advisories/GHSA-22cc-p3c6-wpvm
• h3js/h3/commit/7791538e15ca22437307c06b78fa155bb73632a6

References

• CVE Record
• NVD entry