fix(challenge/metarefresh): ensure that clients have waited long enough

Some admins have noticed that clients are not waiting the right amount
of time in order to access a resource protected by the metarefresh
challenge. This patch adds a check to make sure that clients have waited
at least 95% (difficulty times 950 milliseconds instead of difficulity
times 1000 milliseconds) of the time they should.

If this scales, maybe time is the best way to go for Anubis in the near
future instead of anything else computational.

Signed-off-by: Xe Iaso <me@xeiaso.net>

docs/docs/CHANGELOG.md

@@ -15,7 +15,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add a "proof of React" challenge to prove that the client is able to run a simple JSX app.
 - Added possibility to disable HTTP keep-alive to support backends not properly
-  handling it
+  handling it.
+- Add a server-side check for the meta-refresh challenge that makes sure clients have waited for at least 95% of the time that they should.
 - Added a missing link to the Caddy installation environment in the installation documentation.
 - Downstream consumers can change the default [log/slog#Logger](https://pkg.go.dev/log/slog#Logger) instance that Anubis uses by setting `opts.Logger` to your slog instance of choice ([#864](https://github.com/TecharoHQ/anubis/issues/864)).
 - The [Thoth client](https://anubis.techaro.lol/docs/admin/thoth) is now public in the repo instead of being an internal package.

lib/challenge/metarefresh/metarefresh.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log/slog"
 	"net/http"
+	"time"
 
 	"github.com/TecharoHQ/anubis"
 	"github.com/TecharoHQ/anubis/lib/challenge"
@@ -42,6 +43,12 @@ func (i *Impl) Issue(r *http.Request, lg *slog.Logger, in *challenge.IssueInput)
 }
 
 func (i *Impl) Validate(r *http.Request, lg *slog.Logger, in *challenge.ValidateInput) error {
+	wantTime := in.Challenge.IssuedAt.Add(time.Duration(in.Rule.Challenge.Difficulty) * 950 * time.Millisecond)
+
+	if time.Now().Before(wantTime) {
+		return challenge.NewError("validate", "insufficent time", fmt.Errorf("%w: wanted user to wait until at least %s", challenge.ErrFailed, wantTime.Format(time.RFC3339)))
+	}
+
 	gotChallenge := r.FormValue("challenge")
 
 	if subtle.ConstantTimeCompare([]byte(in.Challenge.RandomData), []byte(gotChallenge)) != 1 {