> This is why calling str.lower() represents a difference in the implementation and the specification, and therefore a vulnerability:<p>I wish there was some explanation how this is a vulnerability and not just a bug generating erroneous data.<p>Vulnerability for me sounds like there’s a reasonable way to create an exploit from the bug, and I don’t see one here as someone who’s not very familiar with the topic.
Author here, that's a good idea. A straightforward way to exploit an implementation differential like this is if you have a software system that contains two different implementations of IDNA 2003 processing user input. One part of the process processes the domain correctly, the other incorrectly, and in this case you can have one part of a system (such as a policy/filter) "see" the data one way and the other part of the system (such as, taking an action as a result of the data) see the data in another way.<p>Server-side Request Forgery (SSRF) is an example of such an exploit targeting a differential in implementations of URL parsers, which is similar to this implementation difference.
It creates a parser differential; two different components of the system can treat the same string as different hostnames. Things that have trusted hostnames, or privileged/admin hostnames that are screened out, or SSRF filters all depend on accurately comparing presented hostnames.<p>This is pretty situational, though, isn't it? You still have to be dealing with IDN names.
I suppose I could envision a scenario like: Service A has a "reset your password" option. Someone with a "user@popular-unicode-domain.com" tries to use this option. Normally this fails when mangled-popular-unicode-domain-plus-garbage.com can't be found. Enterprising malicious actor registers mangled-popular-unicode-domain-plus-garbage.com, and now gets a hold of user password reset requests.
I'm too lazy to investigate further but my guess is that if there is a vulnerability here it has something to do with dns name spoofing.