What’s Vulnerable › Vulnerabilities

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

• Severity: critical (CVSS 10.0 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-03-20 · updated 2026-06-17
• Weakness: CWE-93

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.

Affected products

• h3js h3: before 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 (+8) — fixed in 1.15.6

How to fix

Upgrade to a fixed version: h3js h3 1.15.6. Check the vendor advisory for mitigations.

References

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

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