AK: Reject invalid Base64 encoded string lengths

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/7e38653492
Pull-request: https://github.com/SerenityOS/serenity/pull/23699
Issue: https://github.com/SerenityOS/serenity/issues/23689

AK/Base64.cpp

@@ -34,6 +34,9 @@ static ErrorOr<ByteBuffer> decode_base64_impl(StringView input, ReadonlySpan<i16
 {
     input = input.trim_whitespace();
 
+    if (input.length() % 4 != 0)
+        return Error::from_string_literal("Invalid length of Base64 encoded string");
+
     auto get = [&](size_t offset, bool* is_padding) -> ErrorOr<u8> {
         if (offset >= input.length())
             return 0;

Tests/AK/TestBase64.cpp

@@ -40,6 +40,11 @@ TEST_CASE(test_decode_invalid)
 
     EXPECT(decode_base64("aGVsbG8_d29ybGQ="sv).is_error());
     EXPECT(decode_base64url("aGVsbG8/d29ybGQ="sv).is_error());
+
+    EXPECT(decode_base64("Y"sv).is_error());
+    EXPECT(decode_base64("YQ"sv).is_error());
+    EXPECT(decode_base64("YQ="sv).is_error());
+    EXPECT(decode_base64("PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxMC42MDUiIGhlaWdodD0iMTUuNTU1Ij48cGF0aCBmaWxsPSIjODg5IiBkPSJtMi44MjggMTUuNTU1IDcuNzc3LTcuNzc5TDIuODI4IDAgMCAyLjgyOGw0Ljk0OSA0Ljk0OEwwIDEyLjcyN2wyLjgyOCAyLjgyOHoiLz48L3N2Zz4"sv).is_error());
 }
 
 TEST_CASE(test_decode_only_padding)