CVE-2026-33128Public exploitNetwork attack vector

Client event spoofing via unsanitized SSE newlines

Published Mar 20, 2026 · Updated Mar 20, 2026

Server-Sent Events injection in UnJS H3 before 1.15.6 and 2.0.0 through 2.0.1-rc.14 allows remote attackers to spoof client events. formatEventStreamMessage() places id, event, and data values into protocol framing without neutralizing embedded newlines; formatEventStreamComment() does the same for comments. Reachability requires an application to pass attacker-controlled SSE fields to createEventStream; injected directives let connected clients process spoofed events or aggressive retry settings.

CVSS severity7.5
High
EPSS probability0.57%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Server-Sent Events injection in UnJS H3 before 1.15.6 and 2.0.0 through 2.0.1-rc.14 allows remote attackers to spoof client events. formatEventStreamMessage() places id, event, and data values into protocol framing without neutralizing embedded newlines; formatEventStreamComment() does the same for comments. Reachability requires an application to pass attacker-controlled SSE fields to createEventStream; injected directives let connected clients process spoofed events or aggressive retry settings.

The record

CVE
CVE-2026-33128
Published
Mar 20, 2026
Updated
Mar 20, 2026
Vendor
UnJS
Product
H3
Classifications
CWE-93, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 20, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 20, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.15.6
  2. Affected versionversion=>= 2.0.0, < 2.0.1-rc.15

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

UnJS · H3version=< 1.15.6; version=>= 2.0.0, < 2.0.1-rc.15

Published CVSS scores

10.0NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
7.5GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-93Improper Neutralization of CRLF Sequences ('CRLF Injection')
T1565.002Transmitted Data Manipulation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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