diff --git a/doc/api/webcrypto.md b/doc/api/webcrypto.md
index 59e876d4f19185..5597132f9b2f04 100644
--- a/doc/api/webcrypto.md
+++ b/doc/api/webcrypto.md
@@ -2383,11 +2383,16 @@ added:
 added:
  - v25.9.0
  - v24.18.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/64557
+    description: Limit customization to 512 bytes.
 -->
 
 * Type: {ArrayBuffer|TypedArray|DataView|Buffer|undefined}
 
-The optional customization string for KangarooTwelve.
+The optional customization string for KangarooTwelve. It must not exceed 512
+bytes.
 
 #### `kangarooTwelveParams.name`
 
diff --git a/lib/internal/crypto/webidl.js b/lib/internal/crypto/webidl.js
index c886a4a2280ad2..d0d6036a1ab3ce 100644
--- a/lib/internal/crypto/webidl.js
+++ b/lib/internal/crypto/webidl.js
@@ -272,10 +272,6 @@ function validateCShakeFunctionName(V) {
     'NotSupportedError');
 }
 
-function validateCShakeCustomization(V) {
-  validateMaxBufferLength(V, 'CShakeParams.customization', 512);
-}
-
 converters.RsaPssParams = createDictionaryConverter(
   'RsaPssParams', [
     dictAlgorithm,
@@ -428,7 +424,7 @@ converters.CShakeParams = createDictionaryConverter(
       {
         key: 'customization',
         converter: converters.BufferSource,
-        validator: validateCShakeCustomization,
+        validator: (V, opts) => validateMaxBufferLength(V, 'CShakeParams.customization', 512),
       },
     ],
   ]);
@@ -750,6 +746,7 @@ converters.KangarooTwelveParams = createDictionaryConverter(
       {
         key: 'customization',
         converter: converters.BufferSource,
+        validator: (V, opts) => validateMaxBufferLength(V, 'KangarooTwelveParams.customization', 512),
       },
     ],
   ]);
diff --git a/test/fixtures/webcrypto/supports-modern-algorithms.mjs b/test/fixtures/webcrypto/supports-modern-algorithms.mjs
index a3e5fc54976483..acb0e249dd3886 100644
--- a/test/fixtures/webcrypto/supports-modern-algorithms.mjs
+++ b/test/fixtures/webcrypto/supports-modern-algorithms.mjs
@@ -48,11 +48,15 @@ export const vectors = {
     [false, 'KT128'],
     [true, { name: 'KT128', outputLength: 128 }],
     [true, { name: 'KT128', outputLength: 128, customization: Buffer.alloc(0) }],
+    [true, { name: 'KT128', outputLength: 128, customization: Buffer.alloc(512) }],
+    [false, { name: 'KT128', outputLength: 128, customization: Buffer.alloc(513) }],
     [false, { name: 'KT128', outputLength: 0 }],
     [false, { name: 'KT128', outputLength: 127 }],
     [false, 'KT256'],
     [true, { name: 'KT256', outputLength: 256 }],
     [true, { name: 'KT256', outputLength: 256, customization: Buffer.alloc(0) }],
+    [true, { name: 'KT256', outputLength: 256, customization: Buffer.alloc(512) }],
+    [false, { name: 'KT256', outputLength: 256, customization: Buffer.alloc(513) }],
     [false, { name: 'KT256', outputLength: 0 }],
     [false, { name: 'KT256', outputLength: 255 }],
   ],
diff --git a/test/parallel/test-webcrypto-digest-turboshake-rfc.js b/test/parallel/test-webcrypto-digest-turboshake-rfc.js
index 43762fecc2c41e..271fde76ab23ef 100644
--- a/test/parallel/test-webcrypto-digest-turboshake-rfc.js
+++ b/test/parallel/test-webcrypto-digest-turboshake-rfc.js
@@ -326,6 +326,15 @@ async function checkDigest(name, vectors) {
       else
         algorithm.domainSeparation = rest[0];
     }
+
+    if (isKT && algorithm.customization?.byteLength > 512) {
+      await assert.rejects(subtle.digest(algorithm, input), {
+        name: 'OperationError',
+        message: 'KangarooTwelveParams.customization must be at most 512 bytes',
+      });
+      continue;
+    }
+
     const result = await subtle.digest(algorithm, input);
     assert.deepStrictEqual(
       Buffer.from(result).toString('hex'),
diff --git a/test/parallel/test-webcrypto-digest-turboshake.js b/test/parallel/test-webcrypto-digest-turboshake.js
index 0b5586b19286be..a6f4b2d50f948f 100644
--- a/test/parallel/test-webcrypto-digest-turboshake.js
+++ b/test/parallel/test-webcrypto-digest-turboshake.js
@@ -155,11 +155,22 @@ async function testDigest(size, alg) {
 
 // KT128 with customization string
 (async () => {
-  const digest = await subtle.digest(
-    { name: 'KT128', outputLength: 256, customization: Buffer.from('test') },
-    Buffer.from('hello'));
+  const digest = await subtle.digest({
+    name: 'KT128',
+    outputLength: 256,
+    customization: Buffer.alloc(512),
+  }, Buffer.from('hello'));
   assert(digest instanceof ArrayBuffer);
   assert.strictEqual(digest.byteLength, 32);
+
+  await assert.rejects(subtle.digest({
+    name: 'KT128',
+    outputLength: 256,
+    customization: Buffer.alloc(513),
+  }, Buffer.from('hello')), {
+    name: 'OperationError',
+    message: 'KangarooTwelveParams.customization must be at most 512 bytes',
+  });
 })().then(common.mustCall());
 
 // TurboSHAKE domain separation out of range
diff --git a/test/wpt/status/WebCryptoAPI.cjs b/test/wpt/status/WebCryptoAPI.cjs
index 4572f7e1dbcb94..8ec27109eeca41 100644
--- a/test/wpt/status/WebCryptoAPI.cjs
+++ b/test/wpt/status/WebCryptoAPI.cjs
@@ -91,6 +91,9 @@ if (process.features.openssl_is_boringssl) {
     ['supports-modern.tentative.https.any.js', /ml-kem-512/i]);
 }
 
+skipSubtests(
+  ['digest/kangarootwelve.tentative.https.any.js', /C=(?:\d{4,}|5(?:1[3-9]|[2-9]\d)|[6-9]\d{2}) bytes/]);
+
 function assertNoOverlap(fileSkips, subtestSkips) {
   const subtestSkipFiles = new Set(Object.keys(subtestSkips));
   const overlap = Object.keys(fileSkips).filter((file) => subtestSkipFiles.has(file));