diff --git a/packages/eth-json-rpc-middleware/CHANGELOG.md b/packages/eth-json-rpc-middleware/CHANGELOG.md
index a6823832a4..3e2467f183 100644
--- a/packages/eth-json-rpc-middleware/CHANGELOG.md
+++ b/packages/eth-json-rpc-middleware/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Add strict validation for `eth_sendTransaction` and `eth_signTransaction` params — reject requests whose params contain extraneous top-level keys or nest beyond `MAX_TRANSACTION_PARAM_DEPTH` (10). Prevents downstream normalization / PPOM WASM from crashing on deeply-nested junk fields and silently bypassing security scans. ([#9482](https://github.com/MetaMask/core/pull/9482))
 - Bump `@metamask/utils` from `^11.9.0` to `^11.11.0` ([#9074](https://github.com/MetaMask/core/pull/9074))
 - Bump `@metamask/json-rpc-engine` from `^10.2.4` to `^10.5.0` ([#8661](https://github.com/MetaMask/core/pull/8661), [#8746](https://github.com/MetaMask/core/pull/8746), [#8753](https://github.com/MetaMask/core/pull/8753))
 - Bump `@metamask/message-manager` from `^14.1.1` to `^14.1.2` ([#8755](https://github.com/MetaMask/core/pull/8755))
diff --git a/packages/eth-json-rpc-middleware/src/index.test.ts b/packages/eth-json-rpc-middleware/src/index.test.ts
index ff6ab44352..beece96550 100644
--- a/packages/eth-json-rpc-middleware/src/index.test.ts
+++ b/packages/eth-json-rpc-middleware/src/index.test.ts
@@ -296,6 +296,7 @@ describe('index module', () => {
         "createWalletMiddleware": [Function],
         "providerAsMiddleware": [Function],
         "providerAsMiddlewareV2": [Function],
+        "validateTransactionParams": [Function],
       }
     `);
   });
diff --git a/packages/eth-json-rpc-middleware/src/index.ts b/packages/eth-json-rpc-middleware/src/index.ts
index 8efff9e55d..a933d1669d 100644
--- a/packages/eth-json-rpc-middleware/src/index.ts
+++ b/packages/eth-json-rpc-middleware/src/index.ts
@@ -35,4 +35,5 @@ export {
 } from './methods/wallet-get-supported-execution-permissions';
 export * from './providerAsMiddleware';
 export * from './retryOnEmpty';
+export { validateTransactionParams } from './utils/validation';
 export * from './wallet';
diff --git a/packages/eth-json-rpc-middleware/src/utils/validation.test.ts b/packages/eth-json-rpc-middleware/src/utils/validation.test.ts
index 230b476a4c..198133b3cd 100644
--- a/packages/eth-json-rpc-middleware/src/utils/validation.test.ts
+++ b/packages/eth-json-rpc-middleware/src/utils/validation.test.ts
@@ -5,9 +5,11 @@ import { any, validate } from '@metamask/superstruct';
 
 import type { WalletMiddlewareKeyValues } from '../wallet';
 import {
+  MAX_TRANSACTION_PARAM_DEPTH,
   resemblesAddress,
   validateAndNormalizeKeyholder,
   validateParams,
+  validateTransactionParams,
   validateTypedMessageKeys,
 } from './validation';
 
@@ -278,4 +280,127 @@ describe('Validation Utils', () => {
       });
     });
   });
+
+  describe('validateTransactionParams', () => {
+    const VALID_FROM = '0xbe93f9bacbcffc8ee6663f2647917ed7a20a57bb';
+    const VALID_TO = '0xdAC17F958D2ee523a2206206994597C13D831ec7';
+
+    it('does not throw for minimal valid params', () => {
+      expect(() =>
+        validateTransactionParams({ from: VALID_FROM }),
+      ).not.toThrow();
+    });
+
+    it('does not throw for the full allowlisted param set', () => {
+      expect(() =>
+        validateTransactionParams({
+          accessList: [
+            {
+              address: VALID_TO,
+              storageKeys: ['0x00', '0x01'],
+            },
+          ],
+          authorizationList: [
+            {
+              chainId: '0x1',
+              address: VALID_TO,
+              nonce: '0x0',
+            },
+          ],
+          chainId: '0x1',
+          data: '0x095ea7b3',
+          from: VALID_FROM,
+          gas: '0x5208',
+          gasLimit: '0x5208',
+          gasPrice: '0x1',
+          maxFeePerGas: '0x2',
+          maxPriorityFeePerGas: '0x1',
+          nonce: '0x0',
+          to: VALID_TO,
+          type: '0x2',
+          value: '0x0',
+        }),
+      ).not.toThrow();
+    });
+
+    it.each([
+      ['null', null],
+      ['undefined', undefined],
+      ['a string', 'not-an-object'],
+      ['a number', 42],
+      ['a boolean', true],
+      ['an array', [{ from: VALID_FROM }]],
+    ])('throws when params is %s', (_label, value) => {
+      expect(() => validateTransactionParams(value)).toThrow('Invalid input.');
+    });
+
+    it('throws for an extraneous top-level key', () => {
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          to: VALID_TO,
+          extraKey: 'unexpected',
+        }),
+      ).toThrow('Invalid input.');
+    });
+
+    it('throws for the incident repro payload (deeply-nested junk field)', () => {
+      let junk: Record<string, unknown> = {};
+      for (let i = 0; i < 1200; i++) {
+        junk = { b: junk };
+      }
+
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          to: VALID_TO,
+          value: '0x0',
+          data: '0x095ea7b3',
+          test: junk,
+        }),
+      ).toThrow('Invalid input.');
+    });
+
+    it('throws when an allowlisted field nests beyond the depth limit', () => {
+      let deep: Record<string, unknown> = { leaf: true };
+      for (let i = 0; i < MAX_TRANSACTION_PARAM_DEPTH + 5; i++) {
+        deep = { nested: deep };
+      }
+
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          data: deep as unknown as string,
+        }),
+      ).toThrow('Invalid input.');
+    });
+
+    it('throws when a deeply-nested array exceeds the depth limit', () => {
+      let deepArray: unknown = 'leaf';
+      for (let i = 0; i < MAX_TRANSACTION_PARAM_DEPTH + 5; i++) {
+        deepArray = [deepArray];
+      }
+
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          accessList: deepArray as never,
+        }),
+      ).toThrow('Invalid input.');
+    });
+
+    it('does not throw when params sit exactly at the depth limit', () => {
+      let deep: unknown = 'leaf';
+      for (let i = 0; i < MAX_TRANSACTION_PARAM_DEPTH - 1; i++) {
+        deep = { nested: deep };
+      }
+
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          data: deep,
+        }),
+      ).not.toThrow();
+    });
+  });
 });
diff --git a/packages/eth-json-rpc-middleware/src/utils/validation.ts b/packages/eth-json-rpc-middleware/src/utils/validation.ts
index de3782fa67..04af3e6c6c 100644
--- a/packages/eth-json-rpc-middleware/src/utils/validation.ts
+++ b/packages/eth-json-rpc-middleware/src/utils/validation.ts
@@ -234,3 +234,101 @@ export function validateTypedMessageKeys(data: string): void {
     }
   }
 }
+
+/**
+ * Top-level keys explicitly permitted on `eth_sendTransaction` and
+ * `eth_signTransaction` params. Any additional top-level key causes the
+ * request to be rejected before it reaches downstream consumers such as PPOM
+ * or the Security Alerts API.
+ *
+ * Derived from the dapp-facing subset of `TransactionParams` in
+ * `@metamask/transaction-controller`. Internal-only fields
+ * (`estimateGasError`, `estimatedBaseFee`, `estimateSuggested`,
+ * `estimateUsed`, `gasUsed`) are intentionally omitted — dapps should not be
+ * able to inject them.
+ */
+export const ALLOWED_TRANSACTION_PARAM_KEYS = new Set<string>([
+  'accessList',
+  'authorizationList',
+  'chainId',
+  'data',
+  'from',
+  'gas',
+  'gasLimit',
+  'gasPrice',
+  'maxFeePerGas',
+  'maxPriorityFeePerGas',
+  'nonce',
+  'to',
+  'type',
+  'value',
+]);
+
+/**
+ * Maximum nesting depth permitted anywhere inside a transaction params
+ * object. Legitimate params (including `accessList` and
+ * `authorizationList`) are at most ~4 levels deep. Anything beyond this is
+ * treated as a denial-of-service attempt against downstream normalization
+ * (which recurses and can overflow the call stack in native/WASM code).
+ */
+export const MAX_TRANSACTION_PARAM_DEPTH = 10;
+
+/**
+ * Recursively checks that a value does not nest beyond
+ * `MAX_TRANSACTION_PARAM_DEPTH`.
+ *
+ * @param value - The value to check.
+ * @param depth - The current depth. Callers should pass `0`.
+ * @throws rpcErrors.invalidInput() if the value nests too deeply.
+ */
+function assertMaxDepth(value: unknown, depth: number): void {
+  if (depth > MAX_TRANSACTION_PARAM_DEPTH) {
+    throw rpcErrors.invalidInput();
+  }
+
+  if (value === null || typeof value !== 'object') {
+    return;
+  }
+
+  if (Array.isArray(value)) {
+    for (const item of value) {
+      assertMaxDepth(item, depth + 1);
+    }
+    return;
+  }
+
+  for (const key of Object.getOwnPropertyNames(
+    value as Record<string, unknown>,
+  )) {
+    assertMaxDepth((value as Record<string, unknown>)[key], depth + 1);
+  }
+}
+
+/**
+ * Validates that `eth_sendTransaction` / `eth_signTransaction` params contain
+ * only spec-defined top-level keys and no excessively-nested structures.
+ *
+ * This guards against malicious dapps attaching deeply-nested junk fields
+ * (e.g. `{ from, to, data, test: { b: { b: { b: /* ~1200 levels *\/ } } } }`)
+ * that would otherwise crash downstream normalization or PPOM WASM with a
+ * `RangeError: Maximum call stack size exceeded`, bypassing security checks.
+ *
+ * @param params - The transaction params object supplied by the dapp.
+ * @throws rpcErrors.invalidInput() if params is not a plain object, contains
+ * an extraneous top-level key, or nests beyond `MAX_TRANSACTION_PARAM_DEPTH`.
+ */
+export function validateTransactionParams(params: unknown): void {
+  if (params === null || typeof params !== 'object' || Array.isArray(params)) {
+    throw rpcErrors.invalidInput();
+  }
+
+  const hasExtraneousKey = Object.keys(params).some(
+    (key) => !ALLOWED_TRANSACTION_PARAM_KEYS.has(key),
+  );
+
+  if (hasExtraneousKey) {
+    throw rpcErrors.invalidInput();
+  }
+
+  assertMaxDepth(params, 0);
+}
diff --git a/packages/eth-json-rpc-middleware/src/wallet.test.ts b/packages/eth-json-rpc-middleware/src/wallet.test.ts
index a9964c0d3a..25ed91e977 100644
--- a/packages/eth-json-rpc-middleware/src/wallet.test.ts
+++ b/packages/eth-json-rpc-middleware/src/wallet.test.ts
@@ -1,5 +1,6 @@
 import { MessageTypes, TypedMessage } from '@metamask/eth-sig-util';
 import { JsonRpcEngineV2 } from '@metamask/json-rpc-engine/v2';
+import type { Json } from '@metamask/utils';
 
 import type {
   MessageParams,
@@ -150,6 +151,64 @@ describe('wallet', () => {
       );
     });
 
+    it('throws when params contain an extraneous top-level key', async () => {
+      const getAccounts = async (): Promise<string[]> =>
+        testAddresses.slice(0, 2);
+      const processTransaction = async (): Promise<string> => testTxHash;
+      const engine = JsonRpcEngineV2.create({
+        middleware: [
+          createWalletMiddleware({ getAccounts, processTransaction }),
+        ],
+      });
+      const payload = {
+        method: 'eth_sendTransaction',
+        params: [
+          {
+            from: testAddresses[0],
+            to: testAddresses[1],
+            extraKey: 'unexpected',
+          },
+        ],
+      };
+
+      await expect(
+        engine.handle(...createHandleParams(payload)),
+      ).rejects.toThrow('Invalid input.');
+    });
+
+    it('throws for the incident repro payload with deeply-nested junk', async () => {
+      const getAccounts = async (): Promise<string[]> =>
+        testAddresses.slice(0, 2);
+      const processTransaction = async (): Promise<string> => testTxHash;
+      const engine = JsonRpcEngineV2.create({
+        middleware: [
+          createWalletMiddleware({ getAccounts, processTransaction }),
+        ],
+      });
+
+      let junk: Json = {};
+      for (let i = 0; i < 1200; i++) {
+        junk = { b: junk };
+      }
+
+      const payload = {
+        method: 'eth_sendTransaction',
+        params: [
+          {
+            from: testAddresses[0],
+            to: testAddresses[1],
+            value: '0x0',
+            data: '0x095ea7b3',
+            test: junk,
+          },
+        ] as Json[],
+      };
+
+      await expect(
+        engine.handle(...createHandleParams(payload)),
+      ).rejects.toThrow('Invalid input.');
+    });
+
     it('should not override other request params', async () => {
       const getAccounts = async (): Promise<string[]> =>
         testAddresses.slice(0, 2);
@@ -287,6 +346,64 @@ describe('wallet', () => {
         'The requested account and/or method has not been authorized by the user.',
       );
     });
+
+    it('throws when params contain an extraneous top-level key', async () => {
+      const getAccounts = async (): Promise<string[]> =>
+        testAddresses.slice(0, 2);
+      const processSignTransaction = async (): Promise<string> => testTxHash;
+      const engine = JsonRpcEngineV2.create({
+        middleware: [
+          createWalletMiddleware({ getAccounts, processSignTransaction }),
+        ],
+      });
+      const payload = {
+        method: 'eth_signTransaction',
+        params: [
+          {
+            from: testAddresses[0],
+            to: testAddresses[1],
+            extraKey: 'unexpected',
+          },
+        ],
+      };
+
+      await expect(
+        engine.handle(...createHandleParams(payload)),
+      ).rejects.toThrow('Invalid input.');
+    });
+
+    it('throws for the incident repro payload with deeply-nested junk', async () => {
+      const getAccounts = async (): Promise<string[]> =>
+        testAddresses.slice(0, 2);
+      const processSignTransaction = async (): Promise<string> => testTxHash;
+      const engine = JsonRpcEngineV2.create({
+        middleware: [
+          createWalletMiddleware({ getAccounts, processSignTransaction }),
+        ],
+      });
+
+      let junk: Json = {};
+      for (let i = 0; i < 1200; i++) {
+        junk = { b: junk };
+      }
+
+      const payload = {
+        method: 'eth_signTransaction',
+        params: [
+          {
+            from: testAddresses[0],
+            to: testAddresses[1],
+            value: '0x0',
+            data: '0x095ea7b3',
+            test: junk,
+          },
+        ] as Json[],
+      };
+
+      await expect(
+        engine.handle(...createHandleParams(payload)),
+      ).rejects.toThrow('Invalid input.');
+    });
   });
 
   describe('signTypedData', () => {
diff --git a/packages/eth-json-rpc-middleware/src/wallet.ts b/packages/eth-json-rpc-middleware/src/wallet.ts
index b220be16e0..eed733a639 100644
--- a/packages/eth-json-rpc-middleware/src/wallet.ts
+++ b/packages/eth-json-rpc-middleware/src/wallet.ts
@@ -23,6 +23,7 @@ import { normalizeTypedMessage, parseTypedMessage } from './utils/normalize';
 import {
   resemblesAddress,
   validateAndNormalizeKeyholder as validateKeyholder,
+  validateTransactionParams,
   validateTypedDataForPrototypePollution,
   validateTypedDataV1ForPrototypePollution,
   validateTypedMessageKeys,
@@ -249,6 +250,7 @@ export function createWalletMiddleware({
     }
 
     const params = request.params[0] as TransactionParams | undefined;
+    validateTransactionParams(params);
     const txParams: TransactionParams = {
       ...params,
       // Not using nullish coalescing, since `params` may be `null`.
@@ -282,6 +284,7 @@ export function createWalletMiddleware({
     }
 
     const params = request.params[0] as TransactionParams | undefined;
+    validateTransactionParams(params);
     const txParams: TransactionParams = {
       ...params,
       // Not using nullish coalescing, since `params` may be `null`.