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H3Application

CVE-2026-33128

CRITICAL · 10.0 CVSS v3.1 Published 2026-03-20
Fix available
A fix is available. Upgrade to 1.15.6 or later.
See remediation →
100/100
Remediation priority · Urgent
Public exploit Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

H3 framework's createEventStream function has an SSE injection vulnerability due to missing newline sanitization in formatEventStreamMessage() and formatEventStreamComment(). An attacker who controls any SSE message field (id, event, data, or comment) can inject arbitrary newlines to split the message and inject malicious additional SSE events to connected clients.

MitigationUpgrade H3 to version 1.15.6 or 2.0.1-rc.15 or later, which includes proper newline sanitization in SSE formatting functions.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
H3Application
Affected:< 1.15.6= 2.0.0= 2.0.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed H3 version
    Run 'npm list h3' or check package.json dependencies to find the installed version of the h3 package
    Affected if The version is < 1.15.6, or exactly 2.0.0 or 2.0.1 (including 2.0.1-rc versions before rc.15)
  2. Locate createEventStream usage
    Search codebase for imports or usage of 'createEventStream' from h3 (e.g., grep 'createEventStream' or review server code that handles SSE endpoints)
    Affected if The function is used in your application to serve SSE endpoints
  3. Check SSE message field sources
    Review the code that calls the SSE send method (e.g., event.stream.send() or similar) to identify what values are passed to id, event, data, or comment fields
    Affected if Any of these fields receive input from untrusted sources (query parameters, headers, body data, databases, or external APIs) without sanitization
  4. Verify newline sanitization is applied
    Search for sanitization logic before SSE message construction - look for code that removes or replaces newline characters (\n, \r) in values before passing them to the SSE stream
    Affected if No sanitization is performed on user-controlled input before it is used in SSE messages

You are affected if you use H3 version < 1.15.6 or exactly 2.0.0/2.0.1 AND your application passes untrusted input to SSE message fields without newline sanitization.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 1.15.6 or later
Fixed in 1.15.6
Vendor patch github.com →
Interim mitigation

Upgrade H3 to version 1.15.6 or 2.0.1-rc.15 or later, which includes proper newline sanitization in SSE formatting functions.

Recommended fix High confidence

1.15.6 (for 1.x branch) or 2.0.1-rc.15 (for 2.x branch)

  1. Check current H3 version by running: npm list h3 or pnpm list h3
  2. For H3 1.x users: Run npm install [email protected] (or yarn add [email protected] / pnpm add [email protected])
  3. For H3 2.x users: Run npm install [email protected] (or yarn add [email protected] / pnpm add [email protected])
  4. Verify the upgrade was successful by running the version check command again
  5. Test that Server-Sent Events functionality works correctly after the upgrade

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in H3 Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,984.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-33128 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data