Design/Logic Flaw

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.