_resolve_media_to_data_urls's ad-hoc _MEDIA_TAG_RE matched any bare
token after MEDIA: (no absolute-path anchor) and read the resolved
path directly with no denylist. A relative/traversal path like
MEDIA:../../../../etc/passwd.png slipped through, and any image-
suffixed file the process could read (including under ~/.ssh, ~/.aws,
etc.) was base64-inlined into the API response if its path merely
appeared in the model's own final reply text.
Every other platform adapter's MEDIA: handling already goes through
two shared primitives in gateway/platforms/base.py:
- MEDIA_TAG_CLEANUP_RE, which anchors the path to ~/, /, or a
Windows drive letter plus a known deliverable extension.
- validate_media_delivery_path, which resolves symlinks and rejects
paths under the credential/system-path denylist.
Reuse both here instead of the local unanchored pattern and naive
Path().expanduser() resolution.
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