Specific string causes page edit to fail with 400 Bad Request

Issue Summary

Having a specific string (../) in the body of a page causes a 400 Bad Request error when saving or closing the page from edit mode.

Steps to Reproduce

1. Create a new page.
2. Add anywhere in the page a string containing two periods followed by a forward slash (../).
3. Save the page.
4. Edit the page, and then attempt to close it or save.

Expected Results

Page edit will save or close without any errors.

Actual Results

A 400 Bad Request error is returned from this action.

Workaround

Replace the problematic string in the pages, either saving them on an attachment or removing them temporarily.