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GHSA-8423-8fgw-73vq
tornadoDescription
Summary
parse_multipart_form_data (httputil.py:34) calls
data.split(b"--"+boundary+b"\r\n") before the max_parts check (:35).
A 600KB body with 100k parts creates a 100k-element transient list first,
then rejects transient memory amplification (each split element is a copy).
Pre-auth HTTP DoS.
Root cause
parts = data[:final_boundary_index].split(b"--" + boundary + b"\r\n") # :34 huge list first
if len(parts) > config.max_parts: # :35 check after
raise HTTPInputError("multipart/form-data has too many parts")
PoC
gist: https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617
poc.py + output.txt (100k parts from 600KB transient list).
Fix
Count separators without materializing the list (e.g. data.count(b"--"+boundary) first).
Credit
Reported by afldl, 2026-07.
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The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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