Full Steps:
1) Access Moodle, enable TeX filter.
2) Enter some valid LaTeX into Moodle
3) Note URL for generated LaTeX in page (using developer tools or similar)
4) Logout of Moodle
5) Access above URL, LaTeX is returned without being authenticated.

What I expected:
That accessing the generated LaTeX would be unsuccessful due to be unauthenticated.

What actually happened:
LaTeX image was returned to the browser while unauthenticated.

It could be possible to generate random strings that match an MD5 sum and retrieve some (or given enough time, all) of the generated .GIF files stored in moodledata/filter/tex/*