diff --git a/crates/handlers/src/compat/login_sso_complete.rs b/crates/handlers/src/compat/login_sso_complete.rs
index 84c8f8525..df059cd36 100644
--- a/crates/handlers/src/compat/login_sso_complete.rs
+++ b/crates/handlers/src/compat/login_sso_complete.rs
@@ -83,7 +83,7 @@ pub async fn get(
             Some(CompatLoginSsoAction::Register) => {
                 url_builder.redirect(&mas_router::Register::and_continue_compat_sso_login(id))
             }
-            Some(CompatLoginSsoAction::Login) | None => {
+            Some(CompatLoginSsoAction::Login | CompatLoginSsoAction::Unknown) | None => {
                 url_builder.redirect(&mas_router::Login::and_continue_compat_sso_login(id))
             }
         };
@@ -224,7 +224,7 @@ pub async fn post(
             Some(CompatLoginSsoAction::Register) => {
                 url_builder.redirect(&mas_router::Register::and_continue_compat_sso_login(id))
             }
-            Some(CompatLoginSsoAction::Login) | None => {
+            Some(CompatLoginSsoAction::Login | CompatLoginSsoAction::Unknown) | None => {
                 url_builder.redirect(&mas_router::Login::and_continue_compat_sso_login(id))
             }
         };
diff --git a/crates/handlers/src/compat/login_sso_redirect.rs b/crates/handlers/src/compat/login_sso_redirect.rs
index 1ad47c55e..8edb868fd 100644
--- a/crates/handlers/src/compat/login_sso_redirect.rs
+++ b/crates/handlers/src/compat/login_sso_redirect.rs
@@ -13,7 +13,6 @@ use mas_router::{CompatLoginSsoAction, CompatLoginSsoComplete, UrlBuilder};
 use mas_storage::{BoxRepository, compat::CompatSsoLoginRepository};
 use rand::distributions::{Alphanumeric, DistString};
 use serde::Deserialize;
-use serde_with::serde;
 use thiserror::Error;
 use url::Url;
 
@@ -23,12 +22,21 @@ use crate::impl_from_error_for_route;
 pub struct Params {
     #[serde(rename = "redirectUrl")]
     redirect_url: Option<String>,
+
     action: Option<CompatLoginSsoAction>,
 
     #[serde(rename = "org.matrix.msc3824.action")]
     unstable_action: Option<CompatLoginSsoAction>,
 }
 
+impl Params {
+    fn action(&self) -> Option<CompatLoginSsoAction> {
+        self.action
+            .filter(CompatLoginSsoAction::is_known)
+            .or(self.unstable_action.filter(CompatLoginSsoAction::is_known))
+    }
+}
+
 #[derive(Debug, Error)]
 pub enum RouteError {
     #[error(transparent)]
@@ -62,6 +70,8 @@ pub async fn get(
     State(url_builder): State<UrlBuilder>,
     Query(params): Query<Params>,
 ) -> Result<impl IntoResponse, RouteError> {
+    let action = params.action();
+
     // Check the redirectUrl parameter
     let redirect_url = params.redirect_url.ok_or(RouteError::MissingRedirectUrl)?;
     let redirect_url = Url::parse(&redirect_url).map_err(|_| RouteError::InvalidRedirectUrl)?;
@@ -84,10 +94,7 @@ pub async fn get(
 
     repo.save().await?;
 
-    Ok(url_builder.absolute_redirect(&CompatLoginSsoComplete::new(
-        login.id,
-        params.action.or(params.unstable_action),
-    )))
+    Ok(url_builder.absolute_redirect(&CompatLoginSsoComplete::new(login.id, action)))
 }
 
 #[cfg(test)]
@@ -121,4 +128,29 @@ mod tests {
         assert!(location.contains("org.matrix.msc3824.action=register"));
         assert!(location.contains("action=register"));
     }
+
+    #[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+    async fn test_unknown_action(pool: PgPool) {
+        let state: TestState = TestState::from_pool(pool).await.unwrap();
+
+        let request = Request::get(
+            "/_matrix/client/v3/login/sso/redirect?\
+             redirectUrl=http://example.com/\
+             &org.matrix.msc3824.action=undefinedaction",
+        )
+        .empty();
+
+        let response = state.request(request).await;
+
+        response.assert_status(StatusCode::SEE_OTHER);
+
+        let location = response
+            .headers()
+            .get("Location")
+            .unwrap()
+            .to_str()
+            .unwrap();
+        assert!(!location.contains("org.matrix.msc3824.action"));
+        assert!(!location.contains("action"));
+    }
 }
diff --git a/crates/router/src/endpoints.rs b/crates/router/src/endpoints.rs
index 37c200aac..6aa18f13d 100644
--- a/crates/router/src/endpoints.rs
+++ b/crates/router/src/endpoints.rs
@@ -628,6 +628,16 @@ impl SimpleRoute for CompatLoginSsoRedirectIdp {
 pub enum CompatLoginSsoAction {
     Login,
     Register,
+    #[serde(other)]
+    Unknown,
+}
+
+impl CompatLoginSsoAction {
+    /// Returns true if the action is a known action.
+    #[must_use]
+    pub fn is_known(&self) -> bool {
+        !matches!(self, Self::Unknown)
+    }
 }
 
 #[derive(Debug, Serialize, Deserialize, Clone, Copy)]