diff --git a/pkg/oidc/access_token_claims_test.go b/pkg/oidc/access_token_claims_test.go
new file mode 100644
index 0000000000..eccd227922
--- /dev/null
+++ b/pkg/oidc/access_token_claims_test.go
@@ -0,0 +1,117 @@
+package oidc_test
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/MicahParks/keyfunc/v2"
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/opencloud-eu/opencloud/pkg/log"
+	"github.com/opencloud-eu/opencloud/pkg/oidc"
+	"github.com/opencloud-eu/opencloud/services/proxy/pkg/config"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAccessTokenClaimExtraction(t *testing.T) {
+	client, key := newClaimExtractionClient(t, "https://issuer.example")
+	t.Run("preserves arbitrary claims and numeric types", func(t *testing.T) {
+		claims := jwt.MapClaims{
+			"iss": "https://issuer.example", "sub": "alice", "sid": "session",
+			"exp": 4102444800.75, "groups": []any{"users", "engineering"},
+			"profile": map[string]any{"enabled": true, "score": 1.25}, "custom": nil,
+		}
+		registered, all, err := client.VerifyAccessToken(context.Background(), signClaimExtractionToken(t, key, claims))
+		require.NoError(t, err)
+		require.Equal(t, "alice", registered.Subject)
+		require.Equal(t, "session", registered.SessionID)
+		require.EqualValues(t, 4102444800, registered.ExpiresAt.Unix())
+		require.Equal(t, claims, all)
+	})
+	t.Run("preserves malformed map claim errors", func(t *testing.T) {
+		// Typed claims ignore this custom field; decoding MapClaims must still
+		// reject its overflowing number and preserve the JWT and JSON errors.
+		_, _, err := client.VerifyAccessToken(context.Background(), signClaimExtractionToken(t, key, jwt.MapClaims{
+			"iss": "https://issuer.example", "custom": json.Number("1e1000"),
+		}))
+		require.ErrorIs(t, err, jwt.ErrTokenMalformed)
+		var jsonError *json.UnmarshalTypeError
+		require.ErrorAs(t, err, &jsonError)
+		require.Equal(t, "number 1e1000", jsonError.Value)
+	})
+	t.Run("retains empty map for null payload", func(t *testing.T) {
+		// Without an issuer requirement, a signed null payload previously
+		// returned an initialized, non-nil empty map.
+		client, key := newClaimExtractionClient(t, "")
+		registered, all, err := client.VerifyAccessToken(context.Background(), signClaimExtractionToken(t, key, nil))
+		require.NoError(t, err)
+		require.Equal(t, oidc.RegClaimsWithSID{}, registered)
+		require.NotNil(t, all)
+		require.Empty(t, all)
+	})
+	t.Run("rejects invalid tokens before extracting map claims", func(t *testing.T) {
+		_, otherKey := newClaimExtractionClient(t, "https://issuer.example")
+		for _, tt := range []struct {
+			name    string
+			claims  jwt.MapClaims
+			key     *rsa.PrivateKey
+			wantErr error
+		}{
+			{name: "invalid signature", claims: jwt.MapClaims{"iss": "https://issuer.example"}, key: otherKey, wantErr: jwt.ErrTokenSignatureInvalid},
+			{name: "invalid issuer", claims: jwt.MapClaims{"iss": "https://other.example"}, key: key, wantErr: jwt.ErrTokenInvalidIssuer},
+			{name: "expired", claims: jwt.MapClaims{"iss": "https://issuer.example", "exp": time.Now().Add(-time.Hour).Unix()}, key: key, wantErr: jwt.ErrTokenExpired},
+			{name: "not yet valid", claims: jwt.MapClaims{"iss": "https://issuer.example", "nbf": time.Now().Add(time.Hour).Unix()}, key: key, wantErr: jwt.ErrTokenNotValidYet},
+		} {
+			t.Run(tt.name, func(t *testing.T) {
+				_, all, err := client.VerifyAccessToken(context.Background(), signClaimExtractionToken(t, tt.key, tt.claims))
+				require.ErrorIs(t, err, tt.wantErr)
+				require.Empty(t, all, "unverified map claims must not be returned")
+			})
+		}
+	})
+}
+
+func BenchmarkVerifyAccessTokenClaims(b *testing.B) {
+	client, key := newClaimExtractionClient(b, "https://issuer.example")
+	token := signClaimExtractionToken(b, key, jwt.MapClaims{
+		"iss": "https://issuer.example", "sub": "alice", "sid": "session",
+		"exp": time.Now().Add(time.Hour).Unix(), "preferred_username": "alice",
+		"email": "alice@example.com", "groups": []any{"users", "engineering"},
+		"profile": map[string]any{"enabled": true, "score": 1.25},
+	})
+	ctx := context.Background()
+	b.ReportAllocs()
+	for b.Loop() {
+		if _, _, err := client.VerifyAccessToken(ctx, token); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func newClaimExtractionClient(t testing.TB, issuer string) (oidc.OIDCClient, *rsa.PrivateKey) {
+	t.Helper()
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	require.NoError(t, err)
+	jwks := keyfunc.NewGiven(map[string]keyfunc.GivenKey{
+		"1": keyfunc.NewGivenRSA(&key.PublicKey, keyfunc.GivenKeyOptions{Algorithm: jwt.SigningMethodRS256.Alg()}),
+	})
+	return oidc.NewOIDCClient(
+		oidc.WithLogger(log.NopLogger()),
+		oidc.WithOidcIssuer(issuer),
+		oidc.WithAccessTokenVerifyMethod(config.AccessTokenVerificationJWT),
+		oidc.WithJWKS(jwks),
+		oidc.WithProviderMetadata(&oidc.ProviderMetadata{}),
+	), key
+}
+
+func signClaimExtractionToken(t testing.TB, key *rsa.PrivateKey, claims jwt.MapClaims) string {
+	t.Helper()
+	token := jwt.NewWithClaims(jwt.SigningMethodRS256, claims)
+	token.Header["kid"] = "1"
+	signed, err := token.SignedString(key)
+	require.NoError(t, err)
+	return signed
+}
diff --git a/pkg/oidc/client.go b/pkg/oidc/client.go
index b6065ec66f..8700719c65 100644
--- a/pkg/oidc/client.go
+++ b/pkg/oidc/client.go
@@ -305,10 +305,22 @@ func (c *oidcClient) verifyAccessTokenJWT(token string) (RegClaimsWithSID, jwt.M
 	if err != nil {
 		return claims, mapClaims, err
 	}
-	_, _, err = new(jwt.Parser).ParseUnverified(token, mapClaims)
+	// The token's structure, encoding and signature have already been verified.
+	// Decode only the payload to retain arbitrary claims without parsing the
+	// header and signature again. Keep typed claims above for validation.
+	_, payloadAndSignature, _ := strings.Cut(token, ".")
+	payload, _, _ := strings.Cut(payloadAndSignature, ".")
+	claimBytes, err := new(jwt.Parser).DecodeSegment(payload)
+	if err != nil {
+		return claims, mapClaims, fmt.Errorf("%w: could not base64 decode claim: %w", jwt.ErrTokenMalformed, err)
+	}
+	// Match ParseUnverified's map value semantics, including a null payload.
+	decodedMapClaims := mapClaims
+	err = json.Unmarshal(claimBytes, &decodedMapClaims)
 	// TODO: decode mapClaims to sth readable
 	c.Logger.Debug().Interface("access token", &claims).Msg("parsed access token")
 	if err != nil {
+		err = fmt.Errorf("%w: could not JSON decode claim: %w", jwt.ErrTokenMalformed, err)
 		c.Logger.Info().Err(err).Msg("Failed to parse/verify the access token.")
 		return claims, mapClaims, err
 	}