First off I'll preface this question by saying after seeing the base64 code in my client's theme I immediately contacted the host about a possible breach - they did a scan and found nothing. So that being clarified here's the full story.
My client has been using Wordpress without issue for a year now (unfortunately without updating the code until now) and she is having a weird issue where out of nowhere a couple days ago, when she modifies or creates a page, after saving the content when she tries accessing the page in the editor, it locks up and crashes the browser.
Everything works on the front end. The theme being used is SlashWP http://themeforest.net/item/slash-wp/701905
So when I took a look at her site, I managed to view her pages in the editor and when I clicked the HTML, low and behold, they entire page content (not just images) started with "base64(" and continued on with gibberish for literally pages and pages.
Now I'm not sure what can be triggering this if it's not malicious code but I've enclosed a screenshot from my client of the script error she's dealing with.
I'm actually thinking of auditing the theme code because I'm not sure how good the web host did with the scan, but as that will cost my client a bit of money I really want to avoid that expense if possible.
Thanks in advance for any insights,