$uri is double-decoded, reporting wrong value

[apps.viva.org.uk@…]

The config file variable $uri is a normalised version of $request_uri with URL entities decoded (e.g. %21 is decoded to !). The bug occurs if $request_uri includes %25 (which decodes to %) because then it is decoded a second time, which causes problems if there are numbers following it.

You can see the correct behaviour if the URL contains an encoded character other than %25:

Example $request_uri: /one-%2126-two
Actual (correct) $uri: one-!26-two

And now see the problem happens when %25 is used:

Example $request_uri: /one-%2526-two
Expected (correct) $uri: one-%26-two
Actual (wrong) $uri: one-&-two

The $uri should only be decoded once and contain %26, but it's actually being decoded twice and returning &.

[Maxim Dounin]

A simple test case:

location / {
    return 200 "request: $request_uri\nuri: $uri\n";
}

Test results:

$ curl http://127.0.0.1:8080/one-%2126-two
request: /one-%2126-two
uri: /one-!26-two
$ curl http://127.0.0.1:8080/one-%2526-two
request: /one-%2526-two
uri: /one-%26-two

As you can see, in both cases $uri is correctly decoded once. Second decoding likely happens for some reason in your particular configuration. In particular, a rewrite which uses $uri in the replacement string might do this, as rewrite assumes that variables in the replacement string are escaped (and unescapes URI part as needed).