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CVE-2026-86251— h3 before 1.15.9 Path Traversal via Double Decoding

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组织开源的一个HTTP框架。 H3 1.15.9之前版本存在路径遍历漏洞，该漏洞源于serveStatic工具的双重解码缺陷，请求路径中的双重编码点序列（如%252e%252e）被解码为%2e%2e，绕过resolveDotSegments()函数仅检查字面“.”字符的限制，当结果资产ID由基于URL的后端（CDN、S3、对象存储）解析时，%2e%2e按RFC 3986被解释为“..”，导致路径遍历，从而读取后端任意文件。

CVSS 5.9 · Medium EPSS 0.43% · P36

Possible ATT&CK Techniques 1 AI

T1083 · File and Directory Discovery

Affected Version Matrix 2

VendorProduct Version RangeStatus
h3js h3 < 1.15.9 affected
1.15.9 unaffected
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I. Basic Information for CVE-2026-86251

Vulnerability Information
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Vulnerability Title
h3 before 1.15.9 Path Traversal via Double Decoding
Source: CVE Program / CVE List V5
Vulnerability Description
h3 versions before 1.15.9 contain a path traversal vulnerability in the serveStatic utility. A double-decoding flaw allows a request path containing double-encoded dot sequences (e.g. %252e%252e) to be decoded to %2e%2e, which survives resolveDotSegments() because that function only checks for literal '.' characters. When the resulting asset ID is resolved by URL-based backends (CDN, S3, object storage), %2e%2e is interpreted as '..' per RFC 3986, enabling path traversal to read arbitrary files from the backend.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
对路径名的限制不恰当(路径遍历)
Source: CVE Program / CVE List V5
Vulnerability Title
H3 路径遍历漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
H3是H3组织开源的一个HTTP框架。 H3 1.15.9之前版本存在路径遍历漏洞，该漏洞源于serveStatic工具的双重解码缺陷，请求路径中的双重编码点序列（如%252e%252e）被解码为%2e%2e，绕过resolveDotSegments()函数仅检查字面“.”字符的限制，当结果资产ID由基于URL的后端（CDN、S3、对象存储）解析时，%2e%2e按RFC 3986被解释为“..”，导致路径遍历，从而读取后端任意文件。
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 0 ~ 1.15.9 -

II. Public POCs for CVE-2026-86251

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III. Intelligence Information for CVE-2026-86251

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Vendor Advisories for CVE-2026-86251 (2)

Same Patch Batch · h3js · 2026-09-06 · 5 CVEs total

CVE-2026-86250 7.5 HIGH h3 before 2.0.1-rc.18 Denial of Service via Unbounded Chunked Cookie
CVE-2026-86253 5.9 MEDIUM h3 before 1.15.6 Path Traversal via Percent-Encoded Dot Segments
CVE-2026-86205 5.4 MEDIUM h3 before 2.0.1-rc.18 Open Redirect via redirectBack()
CVE-2026-86252 5.3 MEDIUM h3 before 1.15.9 SSE Event Injection via Carriage Return

IV. Related Vulnerabilities

Same product: h3
Same vendor: h3js
Same weakness: CWE-22

V. Comments for CVE-2026-86251

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