Attempt to fix #356

[schuelermine]

This is an attempt to fix #356 in case any implementation from upstream (deno std library) is not to be expected

[kitsonk]

We should try to find a way to test this, as I am not 100% clear under what conditions it fails. A specific example in the issue and a test would really help in understanding if the fix is correct.

[schuelermine]

@kitsonk What’s happening is that the Deno standard library’s path.parse function spits out an object that separates out the root of the path, which is the initial slash on Linux/macOS, and the drive letter on Windows. send then strips out this segment. However, it doesn’t strip out multiple slashes, because parse only recognizes the first slash as the root. This stripping out is done because resolvePath (from oak) expects a relative path without an initial slash as its second argument, and throws a BadRequestError with message “Malicious Path”, because it thinks the user is trying to access the root directory.

Example REPL session (click to expand)

$ deno
Deno 1.11.2
exit using ctrl+d or close()
> const Path = await import("https://deno.land/std/path/mod.ts");
undefined
> const Send = await import("https://deno.land/x/oak/send.ts");
undefined
> const Util = await import("https://deno.land/x/oak/util.ts");
undefined
> Path.parse("/home");
{ root: "/", dir: "/", base: "home", ext: "", name: "home" }
> Path.parse("home");
{ root: "", dir: "", base: "home", ext: "", name: "home" }
> Path.parse("//home");
{ root: "/", dir: "/", base: "home", ext: "", name: "home" }
> Util.resolvePath("/", ".");
"/"
> Util.resolvePath("/", "home");
"/home"
> Util.resolvePath("/", "/home");
Uncaught BadRequestError: Malicious Path
    at createHttpError (https://deno.land/x/oak@v7.7.0/httpError.ts:130:10)
    at Module.resolvePath (https://deno.land/x/oak@v7.7.0/util.ts:313:11)
    at <anonymous>:2:6

On Linux, // is a valid alias for /, even though / is not a child of /. A webserver essentially imitates a changed root, evidenced by the use of / to refer to the root of the webserver’s virtual filesystem, and should therefore accept //. What’s more, most prominent websites seem to recognize // as simply pointing to the root.

This PR addresses this by stripping out any remaining initial slashes via a regex match. This is safe, since Windows doesn’t allow slashes in paths. An alternative solution would be to prepend ./ to the path before passing it into resolvePath, since resolvePath tolerates double slashes inside the path.

I believe that the fact that // is not reported as the root by parse may be an upstream bug, which is why I noted this in the body of the PR.

[schuelermine]

This is a test comment