Filenames in forwardAttachedFiles arrived from the source agent's
messages_out content and were used directly in path.join on both
source outbox read and target inbox write. A value like `../evil.sh`
could escape `inbox/<a2a-id>/` on the target session (and similarly
the source outbox on read), breaking session isolation — an
adversarial or hallucinating sub-agent could overwrite files in
a sibling session.
Adds isSafeAttachmentName(name) — exported so it's unit-testable —
which rejects empty, `.`, `..`, anything containing `/`, `\`, or
NUL, and anything path.basename would strip. Guard runs before any
I/O. Unsafe names are dropped with a warning log, same pattern as
missing-source-file handling; a bad filename in one attachment
doesn't kill the whole route's text delivery.
Addresses Codex Review P1 on qwibitai/nanoclaw#1967.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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