> This is why calling str.lower() represents a difference in the implementation and the specification, and therefore a vulnerability:

I wish there was some explanation how this is a vulnerability and not just a bug generating erroneous data.

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.

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tptacek 5 hours ago | parent | next [-]

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.

This is pretty situational, though, isn't it? You still have to be dealing with IDN names.

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tialaramex 5 hours ago | parent | next [-]

It is situational, but it very much seems like a thing you'd squirrel away and bring out when you find a system where the differential is helpful.

DNS names are a thing where Sales is going to tell the Engineer that they can't issue the customers randomized ASCII names like abxuewrf.my-thing.example because real customers want to write our-brand-name.my-thing.example instead - even though you already know bad guys will choose billing.my-thing.example and name-of-bank.my-thing.example and every other unintended bad choice even before we realise about likelihood of these confusion bugs in software like Python.

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xp84 5 hours ago | parent [-]

That's why for that type of 'semi-white-label' thing, since the main risk is one of impersonating the platform owner (like the billing.my-thing.example) or possibly lending the credibility of "our" brand to some rando UGC, I always push for the most boring and generic second-level domain, like if it's the travel business, 'travel-systems dot us' or in edtech, mylearningplatform dot net... Then push all customers who know what they're doing into 'BYO DNS name' anyway.

I also like how sites like github use githubusercontent.com or something like that when linking to UGC assets directly, to avoid someone direct linking to something with the implication that it's coming from GitHub.

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chuckadams 4 hours ago | parent [-]

It's mostly about not sharing a security context with github.com.

	▲	cwillu 4 hours ago \| parent [-]
		Yes, but github.cc or whatever would have sufficed for that; there's a reason the string they chose to register for prominent public use contains “usercontent”

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TZubiri 36 minutes ago | parent | prev [-]

I can see how this would be a vulnerability in the context of a security researcher that wants to exaggerate their findings in order to get paid a bounty.

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SethMLarson 5 hours ago | parent | prev | next [-]

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.

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.

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jerf 2 hours ago | parent | next [-]

I wouldn't call this a "vulnerability", I'd call it "a thing that can potentially turn into a vulnerability, more often it can turn into an obscure bug, and most often it is just a quirk".

In particular, if my corporate security team started just mass-flagging all instances of "str.lower" as "security bugs" I would be having a talk with their manager about their threshold for what constitutes a "security bug". Their job is arguably to be more sensitive to that than most engineers, but not that sensitive. It would be like flagging all instances of string concatenation as a vulnerability... and I say that as the guy who would like to eliminate simple string concatenation from programming languages, already a very extreme position on that operation, because of it being at the root cause of so many vulnerabilities... but simply flagging every use as a "vulnerability" is way too sensitive. A demonstration of the ability to use it to bypass some sort of security barrier is necessary to call any specific instance a "vulnerability".

And string concatenation has caused orders of magnitude more actual, verified vulnerabilities than incorrect case folding has.

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gcr 2 hours ago | parent | prev | next [-]

That would be a vulnerability in the IDNA filter that they’re responsible for fixing.

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AgentOrange1234 5 hours ago | parent | prev [-]

"if you have a software system that contains two different implementations of IDNA 2003 processing user input"

Is that a real thing though? Is someone doing that?

	▲	kccqzy 5 hours ago \| parent \| next [-]
		It could be an implementation written in the buggy Python and another written in a different language. For example you might use a ready-made WAF written in a non-Python language in front of a Python app.
	▲	rcxdude 4 hours ago \| parent \| prev \| next [-]
		With web applications it's not particularly unusual, because the whole system stack can be quite heterogeneous. If one part of the system is doing authentication and the other part is actually doing the action then it can be a real problem when they interpret the input differently. Differences between proxy and web server interpretations of HTTP headers have been a source of multiple vulnerabilities, for example.
	▲	cwillu 4 hours ago \| parent \| prev \| next [-]
		Consider the case where your system has components in python and another language without the bug, both of which process that input.
	▲	gchamonlive 5 hours ago \| parent \| prev [-]
		It isn't until it is, until during a crunch someone adds a package with that condition and eventually that gets exploited or halts the system. It's never a nitpick to shed your system from undesired state because of how complex systems behave.

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floxy 5 hours ago | parent | prev | next [-]

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.

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wren6991 5 hours ago | parent | prev | next [-]

My favourite example of this is the Chromium bug where enabling floating point flush-to-zero for WebAudio was used to cause deliberate heap corruption: https://issues.chromium.org/issues/382005099

> We have a working exploit (OOB access in the V8 heap), our security folks put one together based on the example I posted above (and they're cleaning it up to post it here). In general, we find that correctness issues like this are pretty much always exploitable with a bit of effort (not even that much effort normally, just gluing together a few gadgets), so we treat correctness issues as security issues until they are proven not to be, rather than the other way around.

The floating-point-to-heap-corruption chain here is... uniquely JavaScript, but in general getting two different implementations to disagree is the start of lots of interesting inconsistent behaviour.

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zaphar 5 hours ago | parent | prev [-]

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.