This is a spin-off from bug #46944 (MovieClip-based FSCommand issue).

While I was writing tests for Gnash's FSCommand implementation, I noticed that when Gnash is running as a plug-in, FSCommand call made by the SWF with string parameter full of symbols (especially '<' and '>') will not reach JavaScript FSCommand handler, while ones with normal string parameter will.

Upon inspection, I found that Gnash communication module does not escape '<' and '>' in string content of the message. When '<' is present, the message structure became ambiguous and causes problem with receiver/plugin-side's parser, resulting in discarded message (thus the missing FSCommand call).

This problem is not specific to FSCommand: generic `getURL()` instruction, built-in plugin function like `GetVariable()`, and scripting API like `ExternalInterface` are very likely to be affected too; though these will need additional testing to confirm.

Current automated tests tracking this issue (in FSCommand usage) are:

• hostcmd_testrunner_v*: (1) (2)
• hostcmd_htmltest_v*.html: (1) (2)

Gnash: 0.8.11dev (git 8a11e60 8-Mar-2017)
Browser: Iceweasel 10.0.12 (debian)
System: Debian GNU/Linux 7.0 Wheezy i386