CVE-2026-33128: H3 is a minimal H(TTP) framework

This week

HIGH 7.5REMOTEUPGRADE

CVE-2026-33128 is a high-severity flaw (CVSS 7.5) affecting h3. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 52%)
CISA exploited list	No
Weakness	CWE-93
Published	2026-03-20
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: h3

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-20 10:16 UTCPublished CVE record published
2026-06-17 10:36 UTCUpdated Record last modified

h3 h3 UPGRADE

Affected: < 1.15.6 · 2.0.0 · 2.0.1 rc10 · 2.0.1 rc11 · 2.0.1 rc12 · 2.0.1 rc13 · 2.0.1 rc14 · 2.0.1 rc2

Fixed in: 1.15.6

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls h3

2. UPGRADE to 1.15.6

npm install h3@">=1.15.6"

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-33128?

h3: upgrade to 1.15.6. Exact commands for each OS are on this page.

Is CVE-2026-33128 being exploited?

CVE-2026-33128 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-33128 affect?

CVE-2026-33128 affects h3. Severity is high (CVSS 7.5). 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

How urgent is CVE-2026-33128?

Priority 48/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-33490 LOW 3.7 DOWNGRADE H3 is a minimal H(TTP) framework
CVE-2026-33131 HIGH 7.4 DOWNGRADE H3 is a minimal H(TTP) framework
CVE-2026-33129 MEDIUM 5.9 DOWNGRADE H3 is a minimal H(TTP) framework

References

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