CVE-2026-23527Public exploitNetwork attack vector

Request smuggling via case-sensitive chunked check

Published Jan 15, 2026 · Updated Jan 15, 2026

HTTP request smuggling in UnJS H3 before 1.15.5 allows remote attackers to inject requests via mixed-case Transfer-Encoding values. The readRawBody guard compares the chunked token case-sensitively, treats a mixed-case chunked request as bodyless, and leaves its body bytes unread. Exploitation requires a front end that preserves the value and reuses the connection, enabling WAF bypass or poisoning of another user's request.

CVSS severity8.9
High
EPSS probability0.64%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in UnJS H3 before 1.15.5 allows remote attackers to inject requests via mixed-case Transfer-Encoding values. The readRawBody guard compares the chunked token case-sensitively, treats a mixed-case chunked request as bodyless, and leaves its body bytes unread. Exploitation requires a front end that preserves the value and reuses the connection, enabling WAF bypass or poisoning of another user's request.

The record

CVE
CVE-2026-23527
Published
Jan 15, 2026
Updated
Jan 15, 2026
Vendor
UnJS
Product
H3
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 15, 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=< 1.15.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

UnJS · H3version=< 1.15.5

Published CVSS scores

8.9GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:L
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

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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