Reject Android attestations chaining to the software root

Some older Android devices do not support Android Key Attestation with a hardware-backed key and fall back to using a software-based solution. These attestations chain up to a root certificate with CN = Android Keystore Software Attestation Root. These attestations are not considered trustworthy for use cases requiring hardware-backed keys.

To prevent accidental misconfiguration, this change also adds a check to ensure the software root is never configured as the main trust anchor.

PiperOrigin-RevId: 803547414

Author: agxp

src/main/kotlin/SoftwareRoot.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.keyattestation.verifier
+
+import java.security.cert.X509Certificate
+
+// The software root certificate used by the Android Key Attestation standard.
+// https://android.googlesource.com/platform/system/core/+/refs/heads/main/trusty/keymaster/set_attestation_key/keymaster_soft_attestation_keys.xml#97
+private const val GOOGLE_SOFTWARE_ROOT =
+  """-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----"""
+
+val SOFTWARE_ROOT: X509Certificate by lazy { GOOGLE_SOFTWARE_ROOT.asX509Certificate() }
+
+internal fun X509Certificate.isSoftwareRoot() = this.publicKey == SOFTWARE_ROOT.publicKey

src/main/kotlin/Verifier.kt

@@ -62,6 +62,8 @@ sealed interface VerificationResult {
 
   data class ExtensionConstraintViolation(val cause: String, val reason: KeyAttestationReason) :
     VerificationResult
+
+  data object SoftwareAttestationUnsupported : VerificationResult
 }
 
 /**
@@ -141,6 +143,13 @@ open class Verifier(
 ) {
   init {
     Security.addProvider(KeyAttestationProvider())
+    for (anchor in trustAnchorsSource()) {
+      if (anchor.trustedCert?.isSoftwareRoot() == true) {
+        throw IllegalArgumentException(
+          "Software attestation root cannot be used as a trust anchor."
+        )
+      }
+    }
   }
 
   /**
@@ -244,6 +253,17 @@ open class Verifier(
       try {
         certPathValidator.validate(certPath, certPathParameters) as PKIXCertPathValidatorResult
       } catch (e: CertPathValidatorException) {
+        if (
+          e.message == "No matching trust anchor found" &&
+            certPath.certificatesWithAnchor.last().isSoftwareRoot()
+        ) {
+          return VerificationResult.PathValidationFailure(
+            CertPathValidatorException(
+              "Chain terminates in a software root and no matching trust anchor was found, so the chain was not validated.",
+              e,
+            )
+          )
+        }
         return VerificationResult.PathValidationFailure(e)
       }
 

src/test/kotlin/VerifierTest.kt

@@ -43,6 +43,7 @@ import java.time.LocalDate
 import java.time.ZoneOffset
 import java.util.concurrent.Executors
 import java.util.concurrent.TimeUnit
+import kotlin.test.assertFailsWith
 import kotlin.test.assertIs
 import kotlinx.coroutines.guava.await
 import kotlinx.coroutines.runBlocking
@@ -237,4 +238,11 @@ class VerifierTest {
       verifier.verifyAsync(this, chain, delayedAlwaysTrueChecker).await()
     )
   }
+
+  @Test
+  fun init_softwareRootAsTrustAnchor_fails() {
+    assertFailsWith<IllegalArgumentException> {
+      Verifier({ setOf(TrustAnchor(SOFTWARE_ROOT, null)) }, { setOf<String>() }, { Instant.now() })
+    }
+  }
 }