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Intro: How CVE-2026-53539 Works
The vulnerability resides in the `QuerystringParser` within python_multipart/multipart.py, which processes `application/x-www-form-urlencoded` POST data. The issue arises from an inefficient algorithm used to locate field separators.
To parse each field, the parser employs a two-step lookup:
1. It first scans the entire remaining buffer for an ampersand (&).
2. If no `&` is found, it then scans for a semicolon (;) to find the field boundary.
This approach is catastrophically inefficient for request bodies that use the semicolon as a separator. For a request containing a payload like `a;a;a;…` (repeating `a;` 500,000 times, totaling 1 MB), the payload contains no `&` characters.
Consequently, for every single field in the payload, the parser performs a full scan of the remaining data searching for &. This results in `O(B²)` byte comparisons, where `B` is the buffer size. In the 1 MB example, the parser performs approximately `10¹¹` comparisons, leading to several seconds of CPU consumption for a single HTTP request.
This issue is particularly dangerous because the parser is synchronous. A single attacker-controlled request can block an entire worker process for seconds, making it unavailable to serve legitimate traffic. Frameworks like Starlette and FastAPI, which call request.form(), are directly affected.
DailyCVE Form:
Platform: `python-multipart`
Version: `<0.0.30`
Vulnerability : `CPU-DoS`
Severity: `High`
date: `2026-06-05`
Prediction: `2026-05-31`
What Undercode Say:
Simulate an attack by sending a small, malicious request.
This creates a 1MB payload of "a;aaaaaa...".
Run this against a vulnerable server.
for i in {1..500000}; do echo -n "a;"; done > payload.txt
curl -X POST https://victim.com/login -H "Content-Type: application/x-www-form-urlencoded" --data-binary "@payload.txt"
Monitor server CPU usage while the request is being processed.
top -p $(pgrep -f "uvicorn|gunicorn")
Exploit:
An attacker can simply send an HTTP POST request with a `Content-Type: application/x-www-form-urlencoded` header and a body consisting of repeated `a;` patterns. As the server processes the semicolons, the `O(B²)` complexity will cause high CPU usage, effectively creating a Denial of Service (DoS).
Protection:
Upgrade to `python-multipart` version 0.0.30 or later. This version changes the parser to treat only `&` as a field separator, aligning with the WHATWG URL standard. Consequently, `;` is no longer recognized as a separator and is parsed as ordinary field data, making the parsing algorithm run in linear `O(B)` time.
Impact:
A successful exploit leads to uncontrolled CPU consumption, resulting in a Denial of Service (DoS). Because the parsing is synchronous, a single malicious request can block a worker process for seconds, halting all other request processing on that worker. Sustained concurrent attacks can keep all workers busy, rendering the service unavailable to legitimate users.
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Sources:
Reported By: github.com
Extra Source Hub:
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