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CVE-2026-33490— h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on Unrelated Prefix-Matching Routes

Quick assessment

Affected
h3js h3
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

H3是H3开源的一个HTTP框架。 H3 2.0.2-rc.17之前版本存在安全漏洞，该漏洞源于mount方法使用startsWith检查且未验证路径段边界，可能导致中间件在非预期路径上执行。

CVSS 3.7 · Low EPSS 0.28% · P19

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application
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I. Basic Information for CVE-2026-33490

Vulnerability Information
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Vulnerability Title
h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on Unrelated Prefix-Matching Routes
Source: CVE Program / CVE List V5
Vulnerability Description
H3 is a minimal H(TTP) framework. In versions 2.0.0-0 through 2.0.1-rc.16, the `mount()` method in h3 uses a simple `startsWith()` check to determine whether incoming requests fall under a mounted sub-application's path prefix. Because this check does not verify a path segment boundary (i.e., that the next character after the base is `/` or end-of-string), middleware registered on a mount like `/admin` will also execute for unrelated routes such as `/admin-public`, `/administrator`, or `/adminstuff`. This allows an attacker to trigger context-setting middleware on paths it was never intended to cover, potentially polluting request context with unintended privilege flags. Version 2.0.2-rc.17 contains a patch.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
使用不正确的解析名称或索引
Source: CVE Program / CVE List V5
Vulnerability Title
H3 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
H3是H3开源的一个HTTP框架。 H3 2.0.2-rc.17之前版本存在安全漏洞，该漏洞源于mount方法使用startsWith检查且未验证路径段边界，可能导致中间件在非预期路径上执行。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
h3js h3 >= 2.0.1-alpha.0, < 2.0.1-rc.17 -

II. Public POCs for CVE-2026-33490

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III. Intelligence Information for CVE-2026-33490

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Vendor Advisories for CVE-2026-33490 (1)

IV. Related Vulnerabilities

Same product: h3
Same vendor: h3js
Same weakness: CWE-706

V. Comments for CVE-2026-33490

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