CVE-2026-33128

HighPublic PoC

Published: 20 March 2026

Published

20 March 2026

Modified

20 March 2026

KEV Added

—

Patch

—

CVSS Score 7.5 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N

EPSS Score 0.0002 6.9th percentile

Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-33128 is a high-severity CRLF Injection (CWE-93) vulnerability in H3 H3. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 6.9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-15 (Information Output Filtering).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 2 other techniques. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Requires identification, reporting, and timely remediation of the specific flaw in H3's createEventStream via patching to sanitized versions 1.15.6 or 2.0.1-rc.15.

SI-10 Information Input Validation good match

prevent

Mandates validation of attacker-controlled SSE fields (id, event, data, comment) to neutralize CRLF sequences before formatting into event streams.

SI-15 Information Output Filtering good match

prevent

Enforces filtering of generated SSE output prior to client transmission to block arbitrary event injection.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1659 Content Injection Initial Access

Adversaries may gain access and continuously communicate with victims by injecting malicious content into systems through online network traffic.

attack.mitre.org →

T1565.002 Transmitted Data Manipulation Impact

Adversaries may alter data en route to storage or other systems in order to manipulate external outcomes or hide activity, thus threatening the integrity of the data.

attack.mitre.org →

Why these techniques?

CVE describes a remotely exploitable injection flaw in a public-facing HTTP framework (enables T1190). The core effect is injection of arbitrary events into an SSE stream delivered to clients, directly mapping to content injection (T1659) and manipulation of transmitted application data (T1565.002).

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

NVD 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.

Deeper analysisAI

CVE-2026-33128 is a Server-Sent Events (SSE) injection vulnerability in the H3 minimal HTTP framework, specifically affecting the createEventStream function due to missing newline sanitization in formatEventStreamMessage() and formatEventStreamComment(). The flaw impacts H3 versions prior to 1.15.6 and those between 2.0.0 through 2.0.1-rc.14. It is rated 7.5 on the CVSS 3.1 scale (AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N) and maps to CWE-93 (Improper Neutralization of CRLF Sequences).

An attacker who controls any part of an SSE message field—such as id, event, data, or comment—can inject arbitrary SSE events that are delivered to connected clients. Exploitation requires network access and high attack complexity but no privileges or user interaction, with scope changing to enable low confidentiality impact and high integrity impact through injected events.

The H3 GitHub security advisory (GHSA-22cc-p3c6-wpvm) and related commit detail the fix, implemented in versions 1.15.6 and 2.0.1-rc.15 via proper sanitization. Security practitioners should upgrade affected H3 instances to these patched releases to mitigate the issue.