[critical] mpgResponse XML parse bug

[cyveros]

@MonerisSolutions
I find a critical bug within mpgResponse class.

The XML parser "failed" on identifying special XML entity character '&'. For example, we expect

<ACSUrl>https://host/path?item1=1&amp;item2=2</ACSUrl>

to be parsed as:

['ACSUrl' => 'https://host/path?item1=1&item2=2']

However, the actual outcome is:

['ACSUrl' => 'item2=2']

The cause of this bug is within method mpgResponse::characterHandler($parser, $data)

Due to '&' as an special XML entity character, the 'data node':

https://host/path?item1=1&item2=2

is tokenized into 3 separate fields:

https://host/path?item1=1
&
item2=2

It means mpgResponse::characterHandler would be invoked three times by php xml parser although the currentTag remain unchanged across these 3 times function calls.

Near the end of mpgResponse::characterHandler, there is a statement

$this->responseData[$this->currentTag] = $data;

So, the later tokenized string will overwrite the previous one. This explain the bug.
In fact, all 5 special xml entity characters will cause this bug.

Thus, I propose to concatenate the tokenized data string instead

$this->responseData[$this->currentTag]  .= $data;