diff --git a/src/bases/base.ts b/src/bases/base.ts
index 54cea234..53bb8bda 100644
--- a/src/bases/base.ts
+++ b/src/bases/base.ts
@@ -218,6 +218,14 @@ function createAlphabetIdx (alphabet: string, caseInsensitive: boolean): Record<
   // Build the character lookup table:
   const alphabetIdx: Record<string, number> = {}
   for (let i = 0; i < alphabet.length; ++i) {
+    // The trailing '=' of a padded alphabet is a padding marker, not a data
+    // symbol. Skip it so an interior '=' has no index and is rejected as a
+    // non-base character in decode, instead of being folded into the output as
+    // bogus data. Trailing padding is stripped by decode before the lookup, so
+    // valid input round-trips unchanged.
+    if (alphabet[i] === '=') {
+      continue
+    }
     alphabetIdx[alphabet[i]] = i
     // For case-insensitive codecs, map the opposite case to the same index so
     // differently cased input decodes without errors (multibase spec).
diff --git a/test/test-multibase.spec.ts b/test/test-multibase.spec.ts
index de1bc596..5931c1a2 100644
--- a/test/test-multibase.spec.ts
+++ b/test/test-multibase.spec.ts
@@ -199,6 +199,20 @@ describe('multibase', () => {
     assert.throws(() => base64.decode(b64.substring(0, b64.length - 1)), 'Unexpected end of data')
   })
 
+  it('rejects interior padding (RFC 4648 section 3.2)', () => {
+    const padded = { ...b32, ...b64 }
+    for (const base of [padded.base64pad, padded.base64urlpad, padded.base32pad, padded.base32hexpad]) {
+      const value = Uint8Array.from([0x41, 0x42])
+      // A valid encoding, with legitimate trailing padding, still round-trips.
+      const encoded = base.encode(value)
+      assert.deepStrictEqual(base.decode(encoded), value)
+      // A '=' spliced into the data run (padding may only be a trailing run) is
+      // invalid and must be rejected, not silently decoded to bogus bytes.
+      const tampered = encoded.slice(0, 2) + '=' + encoded.slice(2)
+      assert.throws(() => base.decode(tampered), `Non-${base.name} character`)
+    }
+  })
+
   it('infers prefix and name correctly', () => {
     const name = base32.name