PDF download path improperly sanitised

When visiting the PDF download link which the original PDF generation
link redirects to, the file path is constructed from a combination of
fixed strings and the strings provided via the query string of the
download URL. The download URL has the form
https://www.example.com/wp-content/plugins/article2pdf/article2pdf_getfile.php?p=xxx&r=yyy&d=zzz
where xxx is a base64 encoded absolute string, xxx is a short hex hash
and zzz is the base64 encoded URL title slug of the post the PDF was
generated from. While the plugin attempts to sanitise these input
parameters to not allow path traversal, this sanitisation is
insufficient and can be fully or partially circumvented depending on
the PHP version the WordPress instance is running on.

In the case of PHP version <5.3 it is possible to read any file the
user the plugin is executed under has read access to by just encoding
the full file path in the parameter “d” and terminating that string
with a null-byte. The parameter “p” must not be empty but can contain
any value. The parameter “r” may be empty but its value is of no
significance. If the user that the script is executed as has write
access to the file or the directory it is stored in, the file will be
deleted after it has been downloaded. If the user has no write access,
an error message may be shown at the end of the file contents
offered which discloses the WordPress instance’s install directory on
the server.

In the case of PHP version >=5.3, null-termination will no longer cut
off the string. As the generated file name ends with a fixed string
“.pdf”, only files with that file ending can be read. The parameter “d”
may be any directory on the server. The parameter “p” needs to contain
8 backspace characters to delete a prepended fixed string from the file
name while the parameter “r” must contain exactly one backspace. The
actual file name (without the “.pdf”) can then be appended to the
backspaces in either parameter “p” or parameter “r”. It is also
possible to have “p” contain one random character and then have 10
backspace characters followed by the actual file name (again,
without the “.pdf”) stored in parameter “r”.