FWIW it happens even with

• http://localhost:8878//
• http://localhost:8878/fdsfklasdjfldsajfldksajf/sdfssdf//sdfdsaf

The key aspect being two consecutive /s. It's redirected to a path without two /s.

Redirecting http://localhost:8878// to http://localhost:8878/ is correct.
But redirecting http://localhost:8878/%2F to http://localhost:8878/ is not correct.
I think this is a bug to be fixed.

CC @bradfitz @tombergan

I fixed similar bug in the past, so I will confirm this.

Change https://golang.org/cl/103696 mentions this issue: net/http: avoid incorrect redirection on paths containing encoded slashes

I'm not sure if this actually a bug in the first place. The URL reported in the original comment is:

http://localhost:8878/path:a%2F%2Fgoogle.com%2F

The decoded path is:

/path:a//google.com/

ServerMux.Handler uses path.Clean. That path clearly has a double slash and path.Clean translates double slashes to single slashes. Also see here. The godoc for ServerMux.Handler says:

// If the path is not in its canonical form, the
// handler will be an internally-generated handler that redirects
// to the canonical path.

If there's an open question here, the question is around the definition of "canonical path". Does the canonical path convert double slashes to single slashes? If so, does it also convert %2F%2F to "/"? If yes to both questions, there's no bug. Otherwise, there's a bug. AFAICT, these questions are not answered in any documentation. In net/http/server_tests.go, serverMuxTests does not have a case for double slashes, so I suspect this question hasn't come up until now.

@tombergan Thank you for the clear explanation. I'll investigate this problem again.

@tombergan @bradfitz

As a countermeasure against this similar problem, there is nginx merge_slashes syntax.
Even with nginx, the above options are used for cases where you do not want to integrate encoded duplicate slashes into one.

And then, I have take a look at RFC-3986 for confirming the encoded double slashes validity.
According to it, the encoded string pchar is considered valid.

   path          = path-abempty    ; begins with "/" or is empty
                 / path-absolute   ; begins with "/" but not "//"
                 / path-noscheme   ; begins with a non-colon segment
                 / path-rootless   ; begins with a segment
                 / path-empty      ; zero characters

   path-abempty  = *( "/" segment )
   path-absolute = "/" [ segment-nz *( "/" segment ) ]
   path-noscheme = segment-nz-nc *( "/" segment )
   path-rootless = segment-nz *( "/" segment )
   path-empty    = 0<pchar>

   segment       = *pchar
   segment-nz    = 1*pchar
   segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
                 ; non-zero-length segment without any colon ":"

   pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

   query         = *( pchar / "/" / "?" )

   fragment      = *( pchar / "/" / "?" )

   pct-encoded   = "%" HEXDIG HEXDIG

Therefore, I think, if we follow RFC-3986, the current behavior is buggy.

@bradfitz ping? If my survey is reasonable, I'm going to try to improve the patch based on this survey result.
Do you have any opinion?

@bradfitz @tombergan ping?

Change https://golang.org/cl/234657 mentions this issue: net/http: prevent incorrect redirections when the path contains %2F%2F

To expand on @tombergan's comment (#21955 (comment)), the ServeMux documentation states:

ServeMux also takes care of sanitizing the URL request path and the Host header, stripping the port number and redirecting any request containing . or .. elements or repeated slashes to an equivalent, cleaner URL.

The documentation is not explicit on whether %2f is considered a slash, but the implementation consistently treats it as such. ServeMux.Handle("/foo/", f) matches the request /foo%2fbar.

I believe the current behavior is consistent with the documentation and probably can't be changed without violating compatibility. Perhaps the documentation should explicitly mention the treatment of escaped path components.

I'm suffering the same problem trying to use an encoded full URL in the path.

For example I'm trying to use this URL (an image from Wikimedia):

https://upload.wikimedia.org/wikipedia/commons/4/49/Mosuo_Woman_-_42723465920.jpg

Once encoded we get the following URL:

https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F4%2F49%2FMosuo_Woman_-_42723465920.jpg

And this is the request to my server using the previous URL as path:

http://localhost:8080/https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F4%2F49%2FMosuo_Woman_-_42723465920.jpg

I'm receiving a 301 status code with a Moved Permanently body message linking to:

http://localhost:8080/https:/upload.wikimedia.org/wikipedia/commons/4/49/Mosuo_Woman_-_42723465920.jpg

I understand that ServeMux is cleaning paths and redirecting in the case of differences but what I don't understand it's why that is done for escaped characters like %2F%2F.

Apart from the possible bug, anybody knows any solution/workaround that doesn't involve creating from scratch a custom serve mux or using another one available? Any possible way to avoid that cleaning?

This is clearly a bug.

The encoding method can encode arbitrary data in a Uniform Resource Identifier (URI), which is the point of the encoding.

"/" is the reserved character slash. "%2f" is an arbitrary string.
"/" and "%2f" are two different things.

Is "%2f" is a slash?
No, of course it is not a slash. and "%25" is not a percent sign
Otherwise "https://www.google.com/" will be equivalent to "https:%252f%252525252fwww.google.com%2f".

Just faced this issue in the latest go 1.18.3

curl http://localhost/%2Fhello is decoded internally to //hello (r.URL.Path) and the decision for a 301 redirect based on r.URL.Path instead of r.URL.RawPath.

A dirty hack that fixed useless 301 redirect issue for me:

--- go/src/net/http/server.go   2022-06-01 18:44:51.000000000 +0200
+++ go_/src/net/http/server.go  2022-06-15 23:44:02.602151436 +0200
@@ -2412,7 +2412,7 @@
        // All other requests have any port stripped and path cleaned
        // before passing to mux.handler.
        host := stripHostPort(r.Host)
-       path := cleanPath(r.URL.Path)
+       path := cleanPath(r.URL.RawPath)
 
        // If the given path is /tree and its handler is not registered,
        // redirect for /tree/.
@@ -2420,7 +2420,7 @@
                return RedirectHandler(u.String(), StatusMovedPermanently), u.Path
        }
 
-       if path != r.URL.Path {
+       if path != r.URL.RawPath {
                _, pattern = mux.handler(host, path)
                u := &url.URL{Path: path, RawQuery: r.URL.RawQuery}
                return RedirectHandler(u.String(), StatusMovedPermanently), pattern

Change https://go.dev/cl/528356 mentions this issue: net/http: ServeMux matches escaped paths

Change https://go.dev/cl/530575 mentions this issue: net/http: unescape paths and patterns by segment