Infinite decoding loop through specially crafted payload

The Candid library causes a Denial of Service while

parsing a specially crafted payload with 'empty' data type. For example,

if the payload is `record { * ; empty }` and the canister interface expects `record { * }` then the Rust candid decoder treats empty as an extra field required by the type. The problem with the type empty is that the candid Rust library wrongly categorizes empty as a recoverable error when skipping the field and thus causing an infinite decoding loop.

Canisters using affected versions of candid

are exposed to denial of service by causing the decoding to run

indefinitely until the canister traps due to reaching maximum

instruction limit per execution round. Repeated exposure to the payload

will result in degraded performance of the canister. Note: Canisters written in Motoko are unaffected.