Nginx - Decode URL query parameter and forward it as request header

"Pure" nginx solution

Unfortunately nginx does not provide a rich string operations set. I think there isn't a way to do global search-and-replace through some string (which can be a solution if we could replace all %2B with +, %2F with / and %3D with =). However there are circumstances under which nginx performs an urldecoding of some string - when this string becomes a part of an URI which will be forwarded to an upstream proxy server.

So we can add a value of a BASIC_AUTH request argument to the URI and make a proxy request to ourself:

# Main server block
server {
    listen 80 default_server;
    ...

    location / {
        if ($arg_basic_auth) {
            # "basic_auth" request argument is present,
            # append "/decode_basic_auth/<BASE64_token>" to the URI
            # and go to the next location block
            rewrite ^(.*)$ /decode_basic_auth/$arg_basic_auth$1 last;
        }
        # No "basic_auth" request argument present,
        # can do a proxy call from here without setting authorization headers
        ...
    }

    location /decode_basic_auth/ {
        # This will be an internal location only
        internal;
        # Remove "basic_auth" request argument from the list of arguments
        if ($args ~* (.*)(^|&)basic_auth=[^&]*(\2|$)&?(.*)) {
            set $args $1$3$4;
        }
        # Some hostname for processing proxy subrequests
        proxy_set_header Host internal.basic.auth.localhost;
        # Do a subrequest to ourselfs, preserving other request arguments
        proxy_pass http://127.0.0.1$uri$is_args$args;
    }
}

# Additional server block for proxy subrequests processing
server {
    listen 80;
    server_name internal.basic.auth.localhost;

    # Got URI in form "/decode_basic_auth/<BASE64_token>/<Original_URI>"
    location ~ ^/decode_basic_auth/([^/]+)(/.*)$ {
        proxy_set_header Authorization "Basic $1";
        # Setup other HTTP headers here
        ...
        proxy_pass http://<upstream_server>$2$is_args$args;
    }

    # Do not serve other requests
    location / {
        return 444;
    }
}

Maybe this is not a very elegant solution, but it is tested and works.

OpenResty / ngx_http_lua_module

This can be easily solved with openresty or ngx_http_lua_module using ngx.escape_uri function:

server {
    listen 80 default_server;
    ...

    location / {
        set $auth $arg_basic_auth;
        if ($args ~* (.*)(^|&)basic_auth=[^&]*(\2|$)&?(.*)) {
            set $args $1$3$4;
        }
        rewrite_by_lua_block {
            ngx.var.auth = ngx.unescape_uri(ngx.var.auth)
        }
        proxy_set_header Authorization "Basic $auth";
        # Setup other HTTP headers here
        ...
        proxy_pass http://<upstream_server>;
    }
}