Summary
tornado: multipart split() creates huge temp list before max_parts check -> memory amplification DoS (httputil.py:34)
Description
parse_multipart_form_data (httputil.py:34) callsdata.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/649861f25d39b53b7edbe0298e171617poc.py + output.txt (100k parts from 600KB transient list).
Credit
Reported by afldl, 2026-07.
Impact
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Count separators without materializing the list (e.g. data.count(b"--"+boundary) first).
Frequently Asked Questions
- What is GHSA-8423-8FGW-73VQ? GHSA-8423-8FGW-73VQ is a medium-severity allocation of resources without limits or throttling vulnerability in tornado (pip), affecting versions <= 6.5.7. It is fixed in 6.5.8. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- Which versions of tornado are affected by GHSA-8423-8FGW-73VQ? tornado (pip) versions <= 6.5.7 is affected.
- Is there a fix for GHSA-8423-8FGW-73VQ? Yes. GHSA-8423-8FGW-73VQ is fixed in 6.5.8. Upgrade to this version or later.
- Is GHSA-8423-8FGW-73VQ exploitable, and should I be worried? Whether GHSA-8423-8FGW-73VQ is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-8423-8FGW-73VQ is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-8423-8FGW-73VQ? Upgrade
tornadoto 6.5.8 or later.