void test_base64_decode(void)
{
  char buffer[16];
  int len = mutt_b64_decode(buffer, encoded, sizeof(buffer));
  if (!TEST_CHECK(len == sizeof(clear) - 1))
  {
    TEST_MSG("Expected: %zu", sizeof(clear) - 1);
    TEST_MSG("Actual  : %zu", len);
  }
  buffer[len] = '\0';
  if (!TEST_CHECK(strcmp(buffer, clear) == 0))
  {
    TEST_MSG("Expected: %s", clear);
    TEST_MSG("Actual  : %s", buffer);
  }
}
The program copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow.