diff --git a/packages/core/auth-js/src/lib/helpers.ts b/packages/core/auth-js/src/lib/helpers.ts
index 7402899537..fefad1ca62 100644
--- a/packages/core/auth-js/src/lib/helpers.ts
+++ b/packages/core/auth-js/src/lib/helpers.ts
@@ -259,13 +259,12 @@ export function retryable<T>(
 }
 
 function dec2hex(dec: number) {
-  return ('0' + dec.toString(16)).substr(-2)
+  return ('0' + dec.toString(16)).slice(-2)
 }
 
 // Functions below taken from: https://stackoverflow.com/questions/63309409/creating-a-code-verifier-and-challenge-for-pkce-auth-on-spotify-api-in-reactjs
 export function generatePKCEVerifier() {
   const verifierLength = 56
-  const array = new Uint32Array(verifierLength)
   if (typeof crypto === 'undefined') {
     const charSet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~'
     const charSetLen = charSet.length
@@ -275,6 +274,11 @@ export function generatePKCEVerifier() {
     }
     return verifier
   }
+  // Each byte maps to two hex characters, so a 56-byte buffer yields a
+  // 112-character verifier (within RFC 7636's 43-128 range). A Uint8Array
+  // requests exactly the entropy consumed; a Uint32Array would draw 4x the
+  // bytes and discard three of every four.
+  const array = new Uint8Array(verifierLength)
   crypto.getRandomValues(array)
   return Array.from(array, dec2hex).join('')
 }
diff --git a/packages/core/auth-js/test/helpers.test.ts b/packages/core/auth-js/test/helpers.test.ts
index d0a62eb2f2..8db5d9e7a4 100644
--- a/packages/core/auth-js/test/helpers.test.ts
+++ b/packages/core/auth-js/test/helpers.test.ts
@@ -4,6 +4,7 @@ import {
   decodeJWT,
   generateCallbackId,
   generatePKCEFlowId,
+  generatePKCEVerifier,
   getAlgorithm,
   getItemAsync,
   parseParametersFromURL,
@@ -275,6 +276,24 @@ describe('getAlgorithm', () => {
   })
 })
 
+describe('generatePKCEVerifier', () => {
+  it('should generate a verifier within the RFC 7636 length range', () => {
+    const verifier = generatePKCEVerifier()
+    expect(verifier.length).toBeGreaterThanOrEqual(43)
+    expect(verifier.length).toBeLessThanOrEqual(128)
+  })
+
+  it('should only use RFC 7636 unreserved characters', () => {
+    const verifier = generatePKCEVerifier()
+    // unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
+    expect(verifier).toMatch(/^[A-Za-z0-9\-._~]+$/)
+  })
+
+  it('should generate a unique verifier on each call', () => {
+    expect(generatePKCEVerifier()).not.toEqual(generatePKCEVerifier())
+  })
+})
+
 describe('getCodeChallengeAndMethod', () => {
   const testCases = [
     {