abi.encodePacked Collision

abi.encodePacked writes its arguments back to back in the tightest form, with no padding for short types and no length prefix for dynamic ones such as bytes, string and arrays. When two or more of those arguments are variable-length, the boundary between them is not part of the output. ("ab", "c") and ("a", "bc") pack to the same bytes.

That becomes a vulnerability whenever the packed result is hashed and then signed or compared. A signature over one set of fields also validates every other set that packs to the same bytes. In a cross-chain adapter that receives amount and recipient as raw bytes, an attacker can move bytes from one field into the other: in July 2026, an authorization for about 3,110 tokens was turned into a release of more than 203M this way.

The fix is to hash with boundaries. abi.encode stores each dynamic field with its length, so no bytes can move between fields, and EIP-712 typed data gives signed messages the same property along with domain separation. abi.encodePacked remains safe when every argument has a fixed size, or when there is at most one dynamic argument.