CVE-2026-86252Public exploitNetwork attack vector

SSE event injection via unsanitized carriage returns

Published Sep 6, 2026 · Updated Sep 8, 2026

SSE injection in UnJS H3 before 1.15.9 and 2.0.0-beta.0 through 2.0.1-rc.16 allows remote attackers to inject browser-parsed events. EventStream formatting splits data and comments only on newline characters, leaving carriage returns for browsers to treat as SSE line boundaries. Exploitation requires an application to pass attacker-controlled content to push or pushComment; injected directives can spoof event types or split one message into multiple events.

CVSS severity5.3
Medium
EPSS probability0.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSE injection in UnJS H3 before 1.15.9 and 2.0.0-beta.0 through 2.0.1-rc.16 allows remote attackers to inject browser-parsed events. EventStream formatting splits data and comments only on newline characters, leaving carriage returns for browsers to treat as SSE line boundaries. Exploitation requires an application to pass attacker-controlled content to push or pushComment; injected directives can spoof event types or split one message into multiple events.

The record

CVE
CVE-2026-86252
Published
Sep 6, 2026
Updated
Sep 8, 2026
Vendor
UnJS
Product
H3
Classifications
CWE-74, T1659
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 6, 2026CVE publishedPublication date reported by the CVE source.
  2. Sep 8, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=0 <1.15.9
  2. Affected versionversion=2.0.0-beta.0 <2.0.1-rc.17

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

UnJS · H3version=0 <1.15.9; version=2.0.0-beta.0 <2.0.1-rc.17

Published CVSS scores

5.3GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/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-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
T1659Content Injection

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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