Bind initializeValidatorSet to the manager's configured subnet ID

icm-contracts/avalanche/validator-manager/ValidatorManager.sol

@@ -214,8 +214,17 @@ contract ValidatorManager is IValidatorManager, Initializable, OwnableUpgradeabl
             revert InvalidInitializationStatus();
         }
 
-        // Check that the blockchainID and validator manager address in the ConversionData correspond to this contract.
-        // Other validation checks are done by the P-Chain when converting the L1, so are not required here.
+        // Check that the subnetID, blockchainID and validator manager address in the ConversionData
+        // correspond to this contract. Other validation checks are done by the P-Chain when converting
+        // the L1, so are not required here.
+        //
+        // The subnetID check is required because the P-Chain places no restriction on which
+        // (blockchainID, address) pair a subnet names as its manager: anyone can convert their own
+        // subnet naming this contract, and the resulting SubnetToL1ConversionMessage would otherwise
+        // pass every check below with an attacker-chosen initial validator set.
+        if (conversionData.subnetID != $._subnetID) {
+            revert InvalidSubnetID(conversionData.subnetID);
+        }
         if (conversionData.validatorManagerBlockchainID != WARP_MESSENGER.getBlockchainID()) {
             revert InvalidValidatorManagerBlockchainID(conversionData.validatorManagerBlockchainID);
         }

icm-contracts/avalanche/validator-manager/interfaces/IValidatorManager.sol

@@ -16,6 +16,7 @@ interface IValidatorManager is IACP99Manager {
     error InvalidValidatorManagerAddress(address validatorManagerAddress);
     error InvalidWarpOriginSenderAddress(address senderAddress);
     error InvalidValidatorManagerBlockchainID(bytes32 blockchainID);
+    error InvalidSubnetID(bytes32 subnetID);
     error InvalidWarpSourceChainID(bytes32 sourceChainID);
     error InvalidInitializationStatus();
     error InvalidMaximumChurnPercentage(uint8 maximumChurnPercentage);

icm-contracts/avalanche/validator-manager/tests/ValidatorManagerTests.t.sol

@@ -387,6 +387,46 @@ abstract contract ValidatorManagerTest is Test {
         manager.initializeValidatorSet(conversionData, 0);
     }
 
+    // The P-Chain lets any subnet name an arbitrary (blockchainID, address) pair as its manager, so a
+    // conversion of an unrelated subnet naming this contract is a genuinely signed P-Chain message.
+    // It must not be accepted as this manager's initial validator set.
+    function testInitializeValidatorSetForeignSubnet() public {
+        vm.prank(address(0x123));
+        IACP99Manager manager = _setUp();
+
+        bytes32 foreignSubnetID =
+            bytes32(hex"8765432187654321876543218765432187654321876543218765432187654321");
+        ConversionData memory conversionData = _defaultConversionData();
+        conversionData.subnetID = foreignSubnetID;
+
+        vm.mockCall(
+            WARP_PRECOMPILE_ADDRESS,
+            abi.encodeWithSelector(IWarpMessenger.getBlockchainID.selector),
+            abi.encode(DEFAULT_SOURCE_BLOCKCHAIN_ID)
+        );
+        vm.mockCall(
+            WARP_PRECOMPILE_ADDRESS,
+            abi.encodeWithSelector(IWarpMessenger.getVerifiedWarpMessage.selector, uint32(0)),
+            abi.encode(
+                WarpMessage({
+                    sourceChainID: validatorManager.P_CHAIN_BLOCKCHAIN_ID(),
+                    originSenderAddress: address(0),
+                    payload: ValidatorMessages.packSubnetToL1ConversionMessage(
+                        sha256(ValidatorMessages.packConversionData(conversionData))
+                    )
+                }),
+                true
+            )
+        );
+
+        vm.expectRevert(
+            abi.encodeWithSelector(IValidatorManager.InvalidSubnetID.selector, foreignSubnetID)
+        );
+        manager.initializeValidatorSet(conversionData, 0);
+
+        assertFalse(validatorManager.isValidatorSetInitialized());
+    }
+
     function testRemoveValidatorTotalWeight5() public {
         // Use prank here, because otherwise each test will end up with a different contract address, leading to a different subnet conversion hash.
         vm.prank(address(0x123));