It is incorrect to "normalize" // in HTTP URL paths

NGinx, Kube-NGINX, Apache, Traefik all default to normalizing request paths per reference of RFC 3986 [1]. This behavior can be disabled when requests are proxied to resources on the back-end that require double-slashes. I only reference the RFC to describe what they are talking about, not why they default to merging. They all agreed on a decision as one was not made for them.

To generalize by saying "incorrect" is incorrect. The correct answer is that it depends on the requirements in the given implementation. Making such generalizations will just lead to endless arguing. If there is still any debate then a group must vote to deprecate and replace the existing RFC with a new RFC that requires that merging slashes MUST be either be always enabled or always disabled using verbiage per RFC 2119 [2] and optionally RFC 6919 [3]. Even then one may violate an RFC is there is a need to do so and everyone has verified, documented and signed off that doing so has not introduced any security or other risks in the given implementation and if such a risk is identified that it will be remediated or mitigated in a timely manor.

[Edit] For clarification the reason I am linking to RFC 3986 is that it only defines path characteristics and does not explicitly say what to do or not to do. Arguments will persist until a new RFC is created rather than blog and stack overflow posts. Even then people may violate the RFC if they feel it is safe to do so. I do not know how to reword this to make it less confusing.

[1] - https://datatracker.ietf.org/doc/html/rfc3986

[2] - https://datatracker.ietf.org/doc/html/rfc2119

[3] - https://datatracker.ietf.org/doc/html/rfc6919

> Wait, are there any implementations that wrongly collapse double-slashes?

> nginx with merge_slashes

How can it be wrong if it is server-side? If the server wants to treat those paths equally, it can if it wants to.

It would only be wrong if a client does it and requests a different URL than the user entered, right?

URL parsing/normalisation/escaping/unescaping is a minefield. There are many edge cases where every implementation does things differently. This is a perfect example.

It gets worse if you are mapping URLs to a filesystem (e.g. for serving files). Even though they look similar, URL paths have different capabilities and rules than filesystems, and different filesystems also vary. This is also an example of that (I don't think most filesystems support empty directory names).

We cut those and few others coz historically there were exploits relying on it

Nothing on web is "correct", deal with it

This exact ambiguity causes massive headaches when putting Nginx in front of a Spring Boot backend. Nginx defaults to merge_slashes on, so it silently 'fixes' the path. But Spring Security's strict firewall explicitly rejects URLs with // as a potential directory traversal vector and throws an error. It forces you to explicitly decide which layer in your infrastructure owns path normalization, because if Nginx passes it raw, the Java backend completely panics.

But maybe you should anyway.

Because maybe you use S3, which treats `foo/bar.txt` and `foo//bar.txt` as entirely separate things. Because to S3, directories don't exist and those are literally the exact names of the keys under which data is stored.

So you have script A concatenate "foo" + "/bar" and script B concatenate "foo/" + "/bar", and suddenly you have a weird problem.

I can't imagine a real use case where you'd think this is desirable.

There are still email forms that refuse pluses in email addresses too...

I don't think it's incorrect for distinct paths to point to the same resource.

Of course you shouldn't assume that in a client. If you are implementing against an API don't deviate regarding // and trailing / from the API documentation.

What I’ve learned in doing this type of normalization is whatever the specification says, you will always find some website that uses some insane url tweak to decide what content it should show.

As some others have indirectly pointed out, this article conflates two things:

- URL parsing/normalization; and

- Mapping URLs to resources (e.g. file paths or database entries) to be served from the server, and whether you ever map two distinct URLs to the same resource (either via redirects or just serving the same content).

The former has a good spec these days: https://url.spec.whatwg.org/ tells you precisely how to turn a string (e.g., sent over the network via HTTP requests) into a normalized data structure [1] of (scheme, username, password, host, port, path, query, fragment). The article is correct insofar that the spec's path (which is a list of strings, for HTTP URLs) can contain empty string segments.

But the latter is much more wild-west, and I don't know of any attempt being made to standardize it. There are tons of possible choices you can make here:

- Should `https://example.com/foo//bar` serve the same resource as `https://example.com/foo/bar`? (What the article focuses on.)

- `https://example.com/foo/` vs. `https://example.com/foo`

- `https://example.com/foo/` vs. `https://example.com/FOO`

- `https://example.com/foo` vs. `https://example.com/fo%6f%` vs. `https://example.com/fo%6F%`

- `https://example.com/foo%2Fbar` vs. `https://example.com/foo/bar`

- `https://example.com/foo/` vs. `https://example.com/foo.html`

Note that some things are normalized during parsing, e.g. `/foo\bar` -> `/foo/bar`, and `/foo/baz/../bar` -> `/foo/bar`. But for paths, very few.

Relatedly:

- For hosts, many more things are normalized during parsing. (This makes some sense, for security reasons.)

- For query, very little is normalized during parsing. But unlike for pathname, there is a standardized format and parser, application/x-www-form-urlencoded [2], that can be used to go further and canonicalize from the raw query string into a list of (name, value) string pairs.

Some discussions on the topic of path normalization, especially in terms of mapping the filesystem, in the URL Standard repo:

- https://github.com/whatwg/url/issues/552

- https://github.com/whatwg/url/issues/606

- https://github.com/whatwg/url/issues/565

- https://github.com/whatwg/url/issues/729

-----

[1]: https://url.spec.whatwg.org/#url-representation [2]: https://url.spec.whatwg.org/#application/x-www-form-urlencod...

// is useful if the server needs to serve both static files in the filesystem, and embedded files like a webpage. // can be used for embedded files' URL because they will never conflict with filesystem paths.

I’m going to keep doing it.

i'm gonna do it anyway

Wait until you try http:/example.com and http://////example.com in your browser.

It is probably “incorrect”, but given the established actual usage over the decades, it’s most likely what you need to do nevertheless.

Not doing it is like punishing people for not using Oxford commas, or entering an hour long debate each time someone writes “would of” instead of “would have”. It grinds my gears too, but I have different hills to die on.