The Lark/Feishu webhook handler (both the current lark-omni adapter and
the legacy lark adapter) parsed and dispatched inbound HTTP callback
events without checking Lark's own X-Lark-Signature header, and the
Encrypt Key config field used to decrypt payloads is optional and empty
by default. Any caller who reached a bot's webhook URL could inject
fabricated message/card-action events with an attacker-chosen sender,
chat, and content.
Add signature verification matching Lark's documented
event-security-verification scheme (sha256 of timestamp + nonce +
encrypt_key + body, compared to X-Lark-Signature) to both
implementations, rejecting any callback that lacks a valid signature or
whose bot has no Encrypt Key configured. Adds regression tests covering
the rejection paths and updates the two existing webhook tests to
present a valid signature.
Hyu