item_id and image_type reached the cache filename unsanitised while tag was
already being sanitised, and Path::join neither folds .. nor keeps the base
when handed an absolute path. Applies the tag's existing rule to all three
parts and adds a starts_with(cache_dir) check at the point of use, modelled on
media_server::resolve_path.
Not exploitable as shipped — server URLs must be HTTPS (auth/mod.rs) and
Android blocks cleartext, so the id would have to come from a server the user
chose to trust. This makes the write path consistent with how the rest of the
codebase already handles caller-supplied paths.
TRACES: | DR-210 | UT-204
dtourolle
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