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CVE-2026-53540— Python-Multipart: Negative Content-Length in parse_form buffers the entire body in memory

Quick assessment

Affected
Kludex python-multipart
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.

Kludex python-multipart是Kludex的开源Web中间件流式解析库。 Kludex python-multipart 0.0.31之前版本存在输入验证错误漏洞，该漏洞源于parse_form()函数在将Content-Length标头用于限制请求体的分段读取之前未进行验证，可能导致负数的Content-Length将受限读取变为读取至文件结束符，从而将整个请求体一次性加载到内存中。

CVSS 3.7 · Low EPSS 0.34% · P26

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 1

VendorProduct Version RangeStatus
Kludex python-multipart < 0.0.31 affected
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I. Basic Information for CVE-2026-53540

Vulnerability Information
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Vulnerability Title
Python-Multipart: Negative Content-Length in parse_form buffers the entire body in memory
Source: CVE Program / CVE List V5
Vulnerability Description
Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.31, parse_form() did not validate the Content-Length header before using it to bound its chunked read of the request body. A negative Content-Length turned the bounded read into a read-until-EOF, so the entire body was loaded into memory in a single read instead of in fixed-size chunks. This vulnerability is fixed in 0.0.31.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
CWE-1284
Source: CVE Program / CVE List V5
Vulnerability Title
Kludex python-multipart 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Kludex python-multipart是Kludex的开源Web中间件流式解析库。 Kludex python-multipart 0.0.31之前版本存在输入验证错误漏洞，该漏洞源于parse_form()函数在将Content-Length标头用于限制请求体的分段读取之前未进行验证，可能导致负数的Content-Length将受限读取变为读取至文件结束符，从而将整个请求体一次性加载到内存中。
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
Kludex python-multipart < 0.0.31 -

II. Public POCs for CVE-2026-53540

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III. Intelligence Information for CVE-2026-53540

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Vendor Advisories for CVE-2026-53540 (1)

Same Patch Batch · Kludex · 2026-06-22 · 6 CVEs total

CVE-2026-53539 7.5 HIGH Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU
CVE-2026-54283 7.5 HIGH Starlette: request.form() limits silently ignored for application/x-www-form-urlencoded en
CVE-2026-53537 3.7 LOW Python-Multipart: Content-Disposition parameter smuggling via RFC 2231/5987 extended param
CVE-2026-53538 3.7 LOW Python-Multipart: Semicolon treated as querystring field separator enables parameter smugg
CVE-2026-54282 3.7 LOW Starlette: Unvalidated request path concatenated into authority poisons request.url.hostna

IV. Related Vulnerabilities

Same product: python-multipart
Same vendor: Kludex
Same weakness: CWE-1284

V. Comments for CVE-2026-53540

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