diff --git a/src/main/java/com/digitalsanctuary/spring/user/security/WebSecurityConfig.java b/src/main/java/com/digitalsanctuary/spring/user/security/WebSecurityConfig.java
index 338710d..15e7b65 100644
--- a/src/main/java/com/digitalsanctuary/spring/user/security/WebSecurityConfig.java
+++ b/src/main/java/com/digitalsanctuary/spring/user/security/WebSecurityConfig.java
@@ -12,6 +12,7 @@
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.env.Environment;
+import org.springframework.http.HttpMethod;
 import org.springframework.security.config.ObjectPostProcessor;
 import org.springframework.security.config.annotation.web.builders.HttpSecurity;
 import org.springframework.security.core.session.SessionRegistry;
@@ -168,6 +169,18 @@ public SecurityFilterChain buildSecurityFilterChain(HttpSecurity http, SessionRe
 			setupMfa(http);
 		}
 
+		// Close Spring Security's built-in WebAuthn credential-delete endpoint (DELETE /webauthn/register/{id}),
+		// registered by http.webAuthn(...). It deletes a passkey after checking only credential ownership, bypassing
+		// the framework's own DELETE /user/webauthn/credentials/{id} safeguards (last-credential lockout protection,
+		// current-password re-authentication, and audit logging). See GHSA-3cv9-vgqh-jwpm. This deny rule must be
+		// registered before the anyRequest() rule below (Spring rejects matchers added after anyRequest()) and applies
+		// regardless of defaultAction, since allow mode's anyRequest().permitAll() would otherwise expose it too.
+		// Consumers relying on this endpoint should migrate to DELETE /user/webauthn/credentials/{id}.
+		if (webAuthnConfigProperties.isEnabled()) {
+			http.authorizeHttpRequests(
+					(authorize) -> authorize.requestMatchers(HttpMethod.DELETE, "/webauthn/register/**").denyAll());
+		}
+
 		// Configure authorization rules based on the default action
 		String defaultAction = userSecurityConfig.getDefaultAction();
 		if (DEFAULT_ACTION_DENY.equals(defaultAction)) {
diff --git a/src/test/java/com/digitalsanctuary/spring/user/api/WebAuthnFeatureEnabledIntegrationTest.java b/src/test/java/com/digitalsanctuary/spring/user/api/WebAuthnFeatureEnabledIntegrationTest.java
index 99ea55c..a03b07f 100644
--- a/src/test/java/com/digitalsanctuary/spring/user/api/WebAuthnFeatureEnabledIntegrationTest.java
+++ b/src/test/java/com/digitalsanctuary/spring/user/api/WebAuthnFeatureEnabledIntegrationTest.java
@@ -181,4 +181,16 @@ void shouldRejectRenameWhenCurrentPasswordIncorrectForPasswordAccount() throws E
 				.isPresent()
 				.hasValueSatisfying(c -> assertThat(c.getLabel()).isEqualTo("My Device"));
 	}
+
+	@Test
+	@DisplayName("should deny Spring Security's built-in DELETE /webauthn/register/{id} endpoint (GHSA-3cv9-vgqh-jwpm)")
+	void shouldDenySpringWebAuthnRegistrationDeleteEndpoint() throws Exception {
+		// http.webAuthn(...) registers WebAuthnRegistrationFilter, whose DELETE /webauthn/register/{id} deletes a
+		// passkey after checking only credential ownership, bypassing the framework's lockout protection,
+		// current-password re-authentication, and audit logging. The filter chain must deny it outright.
+		mockMvc.perform(delete("/webauthn/register/cred-1").with(user(TEST_EMAIL).roles("USER")).with(csrf()))
+				.andExpect(status().isForbidden());
+
+		assertThat(webAuthnCredentialRepository.findByIdWithUser("cred-1")).isPresent();
+	}
 }
diff --git a/src/test/java/com/digitalsanctuary/spring/user/security/WebAuthnRegistrationDeleteAllowModeTest.java b/src/test/java/com/digitalsanctuary/spring/user/security/WebAuthnRegistrationDeleteAllowModeTest.java
new file mode 100644
index 0000000..133b598
--- /dev/null
+++ b/src/test/java/com/digitalsanctuary/spring/user/security/WebAuthnRegistrationDeleteAllowModeTest.java
@@ -0,0 +1,52 @@
+package com.digitalsanctuary.spring.user.security;
+
+import static org.springframework.security.test.web.servlet.request.SecurityMockMvcRequestPostProcessors.csrf;
+import static org.springframework.security.test.web.servlet.request.SecurityMockMvcRequestPostProcessors.user;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.delete;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.test.context.TestPropertySource;
+import org.springframework.test.web.servlet.MockMvc;
+import com.digitalsanctuary.spring.user.test.annotations.SecurityTest;
+
+/**
+ * Regression test for GHSA-3cv9-vgqh-jwpm under {@code user.security.defaultAction=allow}.
+ * <p>
+ * {@link WebAuthnFeatureEnabledIntegrationTest} already locks in the deny under the test profile's default
+ * {@code defaultAction=deny}. That mode alone would not have exposed the vulnerability: under {@code deny}, an
+ * unlisted URI merely requires authentication, so an authenticated attacker could still have reached the endpoint.
+ * The scenario the fix specifically targets is {@code allow} mode, where {@code anyRequest().permitAll()} would
+ * otherwise expose the built-in {@code DELETE /webauthn/register/{id}} endpoint to anonymous callers entirely. This
+ * test proves the deny rule is registered ahead of both {@code defaultAction} branches, not just the {@code deny}
+ * one.
+ * </p>
+ */
+@SecurityTest
+@TestPropertySource(properties = {"user.security.defaultAction=allow", "user.webauthn.enabled=true"})
+@DisplayName("WebAuthn built-in delete endpoint - defaultAction=allow (GHSA-3cv9-vgqh-jwpm)")
+class WebAuthnRegistrationDeleteAllowModeTest {
+
+	@Autowired
+	private MockMvc mockMvc;
+
+	@Test
+	@DisplayName("should deny an anonymous DELETE to the built-in WebAuthn registration endpoint when defaultAction is allow")
+	void shouldDenyAnonymousDeleteWhenAllow() throws Exception {
+		// Under allow mode, anyRequest().permitAll() is the fallback for anything not explicitly matched. Without
+		// the fix, an unlisted DELETE /webauthn/register/{id} would fall through to that permitAll() and be
+		// reachable by anyone. denyAll() still produces an AccessDeniedException for an anonymous principal, but
+		// ExceptionTranslationFilter routes an unauthenticated caller to the login entry point (302) rather than a
+		// bare 403 -- matching WebSecurityAuthorizationAllowTest's anonymous-denied assertions. Either way the
+		// delete never executes.
+		mockMvc.perform(delete("/webauthn/register/cred-1").with(csrf())).andExpect(status().is3xxRedirection());
+	}
+
+	@Test
+	@DisplayName("should deny an authenticated DELETE to the built-in WebAuthn registration endpoint when defaultAction is allow")
+	void shouldDenyAuthenticatedDeleteWhenAllow() throws Exception {
+		mockMvc.perform(delete("/webauthn/register/cred-1").with(user("user@test.com").roles("USER")).with(csrf()))
+				.andExpect(status().isForbidden());
+	}
+}