lib/anubis: actually check the result with the correct difficulty (#180)

* cmd/anubis actually check the result with the correct difficulty

* chore: changelog

* test(cmd/anubis): make test check for difficulty

* lib: add regression test for CVE-2025-24369

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

* bump VERSION and CHANGELOG

Tracks #181

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

---------

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

VERSION

@@ -1 +1 @@
-1.15.0
+1.15.1

docs/docs/CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
 - Added a periodic cleanup routine for the decaymap that removes expired entries, ensuring stale data is properly pruned.
 - Added a no-store Cache-Control header to the challenge page
 - Hide the directory listings for Anubis' internal static content
@@ -28,6 +29,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed a typo in the challenge page title.
 - Disabled running integration tests on Windows hosts due to it's reliance on posix features (see [#133](https://github.com/TecharoHQ/anubis/pull/133#issuecomment-2764732309)).
 
+## v1.15.1
+
+Zenos yae Galvus: Echo 1
+
+Fixes a recurrence of [CVE-2025-24369](https://github.com/Xe/x/security/advisories/GHSA-56w8-8ppj-2p4f)
+due to an incorrect logic change in a refactor. This allows an attacker to mint a valid
+access token by passing any SHA-256 hash instead of one that matches the proof-of-work
+test.
+
+This case has been added as a regression test. It was not when CVE-2025-24369 was released
+due to the project not having the maturity required to enable this kind of regression testing.
+
 ## v1.15.0
 
 Zenos yae Galvus

lib/anubis.go

@@ -152,7 +152,6 @@ type Server struct {
 	policy     *policy.ParsedConfig
 	opts       Options
 	DNSBLCache *decaymap.Impl[string, dnsbl.DroneBLResponse]
-	ChallengeDifficulty int
 }
 
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -441,9 +440,9 @@ func (s *Server) PassChallenge(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// compare the leading zeroes
-	if !strings.HasPrefix(response, strings.Repeat("0", s.ChallengeDifficulty)) {
+	if !strings.HasPrefix(response, strings.Repeat("0", rule.Challenge.Difficulty)) {
 		s.ClearCookie(w)
-		lg.Debug("difficulty check failed", "response", response, "difficulty", s.ChallengeDifficulty)
+		lg.Debug("difficulty check failed", "response", response, "difficulty", rule.Challenge.Difficulty)
 		templ.Handler(web.Base("Oh noes!", web.ErrorPage("invalid response")), templ.WithStatus(http.StatusForbidden)).ServeHTTP(w, r)
 		failedValidations.Inc()
 		return

lib/anubis_test.go

@@ -34,6 +34,79 @@ func spawnAnubis(t *testing.T, opts Options) *Server {
 	return s
 }
 
+type challenge struct {
+	Challenge string `json:"challenge"`
+}
+
+func makeChallenge(t *testing.T, ts *httptest.Server) challenge {
+	t.Helper()
+
+	resp, err := ts.Client().Post(ts.URL+"/.within.website/x/cmd/anubis/api/make-challenge", "", nil)
+	if err != nil {
+		t.Fatalf("can't request challenge: %v", err)
+	}
+	defer resp.Body.Close()
+
+	var chall challenge
+	if err := json.NewDecoder(resp.Body).Decode(&chall); err != nil {
+		t.Fatalf("can't read challenge response body: %v", err)
+	}
+
+	return chall
+}
+
+// Regression test for CVE-2025-24369
+func TestCVE2025_24369(t *testing.T) {
+	pol := loadPolicies(t, "")
+	pol.DefaultDifficulty = 4
+
+	srv := spawnAnubis(t, Options{
+		Next:   http.NewServeMux(),
+		Policy: pol,
+
+		CookieDomain:      "local.cetacean.club",
+		CookiePartitioned: true,
+		CookieName:        t.Name(),
+	})
+
+	ts := httptest.NewServer(internal.RemoteXRealIP(true, "tcp", srv))
+	defer ts.Close()
+
+	chall := makeChallenge(t, ts)
+	calcString := fmt.Sprintf("%s%d", chall.Challenge, 0)
+	calculated := internal.SHA256sum(calcString)
+	nonce := 0
+	elapsedTime := 420
+	redir := "/"
+
+	cli := ts.Client()
+	cli.CheckRedirect = func(req *http.Request, via []*http.Request) error {
+		return http.ErrUseLastResponse
+	}
+
+	req, err := http.NewRequest(http.MethodGet, ts.URL+"/.within.website/x/cmd/anubis/api/pass-challenge", nil)
+	if err != nil {
+		t.Fatalf("can't make request: %v", err)
+	}
+
+	q := req.URL.Query()
+	q.Set("response", calculated)
+	q.Set("nonce", fmt.Sprint(nonce))
+	q.Set("redir", redir)
+	q.Set("elapsedTime", fmt.Sprint(elapsedTime))
+	req.URL.RawQuery = q.Encode()
+
+	resp, err := cli.Do(req)
+	if err != nil {
+		t.Fatalf("can't do challenge passing")
+	}
+
+	if resp.StatusCode == http.StatusFound {
+		t.Log("Regression on CVE-2025-24369")
+		t.Errorf("wanted HTTP status %d, got: %d", http.StatusForbidden, resp.StatusCode)
+	}
+}
+
 func TestCookieSettings(t *testing.T) {
 	pol := loadPolicies(t, "")
 	pol.DefaultDifficulty = 0
@@ -72,8 +145,9 @@ func TestCookieSettings(t *testing.T) {
 	nonce := 0
 	elapsedTime := 420
 	redir := "/"
+	calculated := ""
 	calcString := fmt.Sprintf("%s%d", chall.Challenge, nonce)
-	calculated := internal.SHA256sum(calcString)
+	calculated = internal.SHA256sum(calcString)
 
 	req, err := http.NewRequest(http.MethodGet, ts.URL+"/.within.website/x/cmd/anubis/api/pass-challenge", nil)
 	if err != nil {