URL with multiple forward slashes, does it break anything?

Question

http://example.com/something/somewhere//somehow/script.js

Does the double slash break anything on the server side? I have a script that parses URLs and I was wondering if it would break anything (or change the path) if I replaced multiple slashes with a single slash. Especially on the server side, some frameworks like CodeIgniter and Joomla use segmented URL schemes and routing. I would just want to know if it breaks anything.

Answer 1

We had a specific instance of this causing a bug. In Spring Framework, Java. We had the same path mapping to two functions of the same name, one is a GET, the other a POST. The function parameters were different. The GET takes a ?accountId=1234 parameter on the URL, and the POST had the same info but in a JSON body. Then in PostMan with the path like GET /foo/bar/getaccount?accountId=1234 worked fine.
And POST /foo/bar/getaccount. with the appropriate JSON body worked fine. BUT... POST /foo//bar/getaccount. called the GET version, and showed an error that the parameter accountId was missing.

Likely due to the URL rewriter 'fixing' the URL and thus a redirect.

Answer 2

The correct answer to this question is it depends upon the implementation of the server!

Preface: Double-slash is syntactically valid according to RFC 2396, which defines URL path syntax. As amn explains, it therefore implies an empty URI segment. Note however that RFC 2396 only defines the syntax, not semantics of paths, including empty path segments, so it is up to your server to decide the semantics of the empty path.

You didn't mention the server software stack you're using, perhaps you're even rolling your own? So please use your imagination as to what the semantics could be!

Practically, I would like to point out some everyday semantic-related reasons which mean you should avoid double slashes even though they are syntactically valid:

1. When not everyone assumes that empty should be valid, it can cause bugs! And even though your server technology of today might be compatible with it, your server technology of tomorrow or the next version of your server technology of today might not be. (Example: ASP.NET MVC Web API library throws an error when you try to specify a route template with a double slash.)

2. Some servers might interpret // as indicating the root. This can become a directory traversal bug - and then usually it is a security bug (look up 'directory traversal vulnerability').

3. Because it is sometimes a bug, and a security bug, defensively designed server stacks and firewalls will assume the substring '//', in any incoming request is a possible attempt to exploit a bug, and therefore they will block it by returning 403 Forbidden, 404 Not Found, or 400 Bad Request - without ever actually further processing the URI or request.

Answer 3

Your question is "does it break anything". In terms of the URL specification, extra slashes are allowed. Don't read the RFC, here is a quick experiment you can try to see if your browser silently mangles the URL:

echo '<?= $_SERVER['REQUEST_URI'];' > tmp.php                                   
php -S localhost:4000 tmp.php

I tested macOS 10.14 (18A391) with Safari 12.0 (14606.1.36.1.9) and Chrome 69.0.3497.100 and both get the result:

/hello//world

This indicated that using an extra slash is visible to the web application.

Certain use cases will be broken when using a double slash. This includes URL redirects/routing that are expecting a single-slashed URL or other CGI applications that are analyzing the URI directly.

But for normal cases of serving static content, such as your example, this will still get the correct content. But the client will get a cache miss against the same content accessed with different slashes.

Answer 4

Yes, it can most definitely break things.

The spec considers http://host/pages/foo.html and http://host/pages//foo.html to be different URIs, and servers are free to assign different meanings to them. However, most servers will treat paths /pages/foo.html and /pages//foo.html identically (because the underlying file system does too). But even when dealing with such servers, it's easily possible for extra slash to break things. Consider the situation where a relative URI is returned by the server.

http://host/pages/foo.html  + ../images/foo.png = http://host/images/foo.png
http://host/pages//foo.html + ../images/foo.png = http://host/pages/images/foo.png

Let me explain what that means. Say your server returns an HTML document that contains the following:

<img src="../images/foo.png">

If your browser obtained that page using

http://host/pages/foo.html          # Path has 2 segments: "pages" and "foo.html"

your browser will attempt to load

http://host/images/foo.png          # ok

However, if your browser obtained that page using

http://host/pages//foo.html         # Path has 3 segments: "pages", "" and "foo.html"

you'll probably get the same page (because the server probably doesn't distinguish /pages//foo.html from /pages/foo.html), but your browser will erroneously try to load

http://host/pages/images/foo.png    # XXX

Answer 5

Consider the declaration of the relevant path-absolute non-terminal in "RFC3986: Uniform Resource Identifier (URI): Generic Syntax" (specified, as is typical, in ABNF syntax):

path-absolute = "/" [ segment-nz *( "/" segment ) ]

Then consider the segment declaration a few lines further down in the same document:

segment       = *pchar

If you can read ABNF, the asterisk (*) specifies that the following element pchar may be repeated multiple times to make up a segment, including zero times. Learning this and re-reading the path-absolute declaration above, you can see that a potentially empty segment imples that the second "/" may repeat indefinitely, hence allowing valid combinations like ////// (arbitrary length of at least one /) as part of path-absolute (which itself is used in specifying the rule describing a URI).

As all URLs are URIs we can conclude that yes, URLs are allowed multiple consecutive forward slashes, per quoted RFC.

But it's not like everyone follows or implements URI parsers per specification, so I am fairly sure there are non-compliant URI/URL parsers and all kinds of software that stacks on top of these where such corner cases break larger systems.

Answer 6

You may be surprised for example when building links for resources in your app.

<script src="mysite.com/resources/jquery//../angular/script.js"></script>

will not resolve to mysite.com/resources/angular/script.js but to mysite.com/resources/jquery/angular/script.js what you probably didn't want

Double slashes are evil, try to avoid them.

Answer 7

One thing you may want to consider is that it might affect your page indexing in a search engine. According to this web page,

A URL with the same path repeated 3 times will not be indexed in Google

The example they use is:

example.com/path/path/path/

I haven't confirmed this would also be true if you used example.com///, but I would certainly want to find out if SEO optimization was critical for my website.

They mention that "This is because Google thinks it has hit a URL trap." If anyone else knows the answer for sure, please add a comment to this answer; otherwise, I thought it relevant to include this case for consideration.

Answer 8

HTTP RFC 2396 defines path separator to be single slash.

However, unless you're using some kind of URL rewriting (in which case the rewriting rules may be affected by the number of slashes), the uri maps to a path on disk, but in (most?) modern operating systems (Linux/Unix, Windows), multiple path separators in a row do not have any special meaning, so /path/to/foo and /path//to////foo would eventually map to the same file.

An additional thing that might be affected is caching. Since both your browser and the server cache individual pages (according to their caching settings), requesting same file multiple times via slightly different URIs might affect the caching (depending on server and client implementation).

Answer 9

URLs don't have to map to filesystem paths. So even if // in a filesystem path is equivalent to /, you can't guarantee the same is true for all URLs.