[dev] ?==?utf-8?q? ILIAS 5.2.x: XML issues with PHP 5.6.x

Ralf Mattes rm at mh-freiburg.de
Mi Aug 16 15:59:22 CEST 2017


Am Mittwoch, 16. August 2017 15:13 CEST, Michael Jansen <mjansen at databay.de> schrieb:

> Dear developers and administrators,
>
> the following information may save you some time as it took me a while to figure it out:
>
> Today I noticed some strange issues regarding XML handling in ILIAS with (v5.2.6 2017-07-13, PHP 5.6.31-4+ubuntu16.04.1+deb.sury.org+4). Initially, I discovered a problem in one of my plugins which deals with XML that uses simplexml_load_file() with a local(!) file.
>
> The error message I got from it was:
>
> simplexml_load_file(): I/O warning : failed to load external entity "[MY_FILE]"


Hmm - you should have posted to this list earlier, that one is a really good ol' friend of mine ...

> <...>
>
> When I added ...
>
> libxml_disable_entity_loader(false);
>
> ... in my plugin before using simplexml_load_file() and before the ILIAS SOAP server is instantiated
> in ./webservice/soap/server.php, the issue seems to be fixed for both cases.
>
> libxml_disable_entity_loader() is not thread safe, so this is the root of all evil (including but not
> limited to pineapple on pizza).

To be technically correct, what's bitting you here isn't thread safety (I assume you are not running in
a multi-threaded server environment) but global state. As (almost) always, global state is unfortunate.

> If ...
>
> libxml_disable_entity_loader(true);

> ... is called in another script (or another PHP application) and not resetted to false, the problematic state persists globally. There are even some calls of with a boolean true in ILIAS (PHPExcel, SVG Sanitizer), which is dangerous in case an error occured and the state could not be properly resetted to a boolean false.

Yes, that's exactly why global state is so dangerous even in single threaded environments. But, since external entity loading
is known as a hard to control entry point for vulnerabilities it actually is a good idea to disable it.

BTW, do you now see why I'm so scared by other global state manipulations (umask, for example)?

 Cheers, Ralf Mattes




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