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

> Attackers can inject arbitrary Server-Sent Events into client streams via unsanitized newlines in H3 framework event fields.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Mar 2026
- **Updated:** 20 Mar 2026
- **Also known as:** GHSA-22cc-p3c6-wpvm

## What is CVE-2026-33128?

CVE-2026-33128 is a high-severity vulnerability in h3js h3, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-93 (Improper Neutralization of CRLF Sequences)

## Which versions are affected by CVE-2026-33128?

4 open source packages ship an affected version, including h3. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `h3` | npm | All versions | 1.15.6 |
| `h3` | npm | All versions | 1.15.6 |
| `h3` | npm | From 2.0.0 | 2.0.1-rc.15 |
| `h3` | npm | From 2.0.0 | 2.0.1-rc.15 |

Products: H3 H3.

## How do I fix CVE-2026-33128?

Upgrade h3 to a fixed version for your release line: 1.15.6 or 2.0.1-rc.15.

## Is CVE-2026-33128 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33128 exists.

## Other vulnerabilities in h3js h3

- [CVE-2026-23527](https://insights.sbomify.com/CVE-2026-23527/): h3 v1 has Request Smuggling (TE.TE) issue (high 8.9)
- [CVE-2026-33131](https://insights.sbomify.com/CVE-2026-33131/): h3 has a middleware bypass with one gadget (high 7.4)
- [CVE-2026-33129](https://insights.sbomify.com/CVE-2026-33129/): h3 has an observable timing discrepancy in basic auth utils (medium 5.9)
- [CVE-2026-33490](https://insights.sbomify.com/CVE-2026-33490/): h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on Unrelated Prefix-Matching Routes (low 3.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33128 and fix it if it is affected.

What it is: CVE-2026-33128 is a high-severity vulnerability in h3js h3, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33128 exists.

Affected packages and fixed versions:
- h3 (npm): affected all versions; fixed in 1.15.6
- h3 (npm): affected all versions; fixed in 1.15.6
- h3 (npm): affected from 2.0.0; fixed in 2.0.1-rc.15
- h3 (npm): affected from 2.0.0; fixed in 2.0.1-rc.15

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33128/ (Markdown: https://insights.sbomify.com/CVE-2026-33128/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33128
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33128
- OSV: https://osv.dev/vulnerability/CVE-2026-33128

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33128/
Summaries are generated and may contain mistakes; the official description above is the record.