I found (and fixed) a vulnerability in Python

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I found (and fixed) a vulnerability in Python

Following analysis done by James Kettle on web cache poisoning, I decided to deepen my data in this field and detect these vulnerabilities in the originate offer domain. I focused my analysis on the most standard web frameworks, akin to Flask, Bottle, and Tornado. I may no longer imagine that this analysis would stop up in me fixing a security vulnerability in Python 3.9.


Nonetheless wait – let’s start before all the pieces. As section of my analysis, I situation up local instances of those frameworks so I can strive to exploit them. Many of them were deemed susceptible, however the Tornado one caught my consideration. It used to be because Tornado’s maintainer advised me that they were utilizing Python’s fashioned library for parsing the URL.

Python’s offer code

After I looked at Python’s offer code, it turned particular to me that the vulnerability used to be noteworthy extra extreme and profound than I thought it used to be – all packages that primitive Python’s fashioned library were susceptible.

The urlparse module handled semicolon as a separator – whereas most proxies only took ampersands as separators. That supposed that after the attacker may presumably separate inquire parameters utilizing a semicolon (;), they are able to appreciate precipitated a distinction in the interpretation of the quiz between the proxy (working with default configuration) and the server, ensuing in malicious requests being cached as protected ones.

Exploitation instance

GET /?hyperlink=http://google.com&utm_content=1;hyperlink='>alert(1) HTTP/1.1

Host: somesite.com

Upgrade-Timid-Requests: 1		

Particular person-Agent: Mozilla/5.0 (Windows NT 10.0; Steal64; x64) AppleWebKit/537.36 (KHTML, love Gecko) Chrome/85.0.4183.83 Safari/537.36

Catch: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,imag e/webp,image/apng,*/*;q=0.8,application/signed-alternate;v=b3;q=0.9 Catch-Encoding: gzip, deflate			

Catch-Language: en-US,en;q=0.9 Connection: close	

urlparse saw 3 parameters right here: hyperlink, utm_content, and then hyperlink again. On the utterly different hand, the proxy thought to be this corpulent string: 1;hyperlink='>alert(1) as the designate of utm_content, which is why the cache key would appreciate only contained somesite.com/?hyperlink=http://google.com .

I straight away contacted the Python security group of workers and opened a worm model. I additionally created a pull quiz on the CPython repository. It took just a few month of going from aspect to aspect with the PR, for the length of which I with out a doubt appreciate learned to adhere to Python’s contributors’ guidelines – and it bought merged 🎉 on Feb 15 and launched on Feb 19. The fix used to be backported to older variations of Python as properly.


The prison of the myth is to continually strive to dig deeper. You imagine you came across something attention-grabbing? relate your speculation and view the root reason, strive to get it additional alongside the chain, which may additionally lead to noteworthy extra attention-grabbing outcomes.

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