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CVE-2026-34827— Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser

Quick assessment

Affected
rack rack
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.

Rack是Rack开源的一个模块化的Ruby web服务器界面。 Rack 3.1.21之前版本和3.2.6之前版本存在安全漏洞，该漏洞源于Rack::Multipart::Parser#handle_mime_head解析带引号的多部分参数时处理不当，可能导致拒绝服务。

CVSS 7.5 · High EPSS 0.47% · P39

Possible ATT&CK Techniques 1 AI

T1496 · Resource Hijacking

Affected Version Matrix 2

VendorProduct Version RangeStatus
rack rack >= 3.0.0.beta1, < 3.1.21 affected
>= 3.2.0, < 3.2.6 affected
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I. Basic Information for CVE-2026-34827

Vulnerability Information
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Vulnerability Title
Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser
Source: CVE Program / CVE List V5
Vulnerability Description
Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
算法复杂性
Source: CVE Program / CVE List V5
Vulnerability Title
Rack 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Rack是Rack开源的一个模块化的Ruby web服务器界面。 Rack 3.1.21之前版本和3.2.6之前版本存在安全漏洞，该漏洞源于Rack::Multipart::Parser#handle_mime_head解析带引号的多部分参数时处理不当，可能导致拒绝服务。
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
rack rack >= 3.0.0.beta1, < 3.1.21 -

II. Public POCs for CVE-2026-34827

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III. Intelligence Information for CVE-2026-34827

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Vendor Advisories for CVE-2026-34827 (1)

Same Patch Batch · rack · 2026-04-02 · 13 CVEs total

CVE-2026-34785 7.5 HIGH Rack: Local file inclusion in `Rack::Static` via URL Prefix Matching
CVE-2026-34829 7.5 HIGH Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length
CVE-2026-34830 5.9 MEDIUM Rack: Rack::Sendfile regex injection via HTTP_X_ACCEL_MAPPING header allows arbitrary file
CVE-2026-34826 5.3 MEDIUM Rack: Unbounded Range Count in get_byte_ranges Enables DoS
CVE-2026-34786 5.3 MEDIUM Rack: Rack::Static header_rules bypass via URL-encoded paths
CVE-2026-34763 5.3 MEDIUM Rack: Rack::Directory info disclosure and DoS via unescaped regex interpolation
CVE-2026-34230 5.3 MEDIUM Rack: Quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encodin
CVE-2026-26962 4.8 MEDIUM Rack: Header injection in multipart requests
CVE-2026-32762 4.8 MEDIUM Rack: Forwarded Header semicolon injection enables Host and Scheme spoofing
CVE-2026-34835 4.8 MEDIUM Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.
CVE-2026-34831 4.8 MEDIUM Rack: Content-Length mismatch in Rack::Files error responses
CVE-2026-26961 3.7 LOW Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass

IV. Related Vulnerabilities

Same product: rack
Same vendor: rack
Same weakness: CWE-407

V. Comments for CVE-2026-34827

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