Submitted by gpersoon, also found by 0xRajeev, cmichel, and nikitastupin.
The solidity function ecrecover is used, however the error result of 0 is not checked for.
See documentation:
https://docs.soliditylang.org/en/v0.8.9/units-and-global-variables.html?highlight=ecrecover#mathematical-and-cryptographic-functions
recover the address associated with the public key from elliptic curve signature or return zero on error. 
Now you can supply invalid input parameters to the Sig.recover function, which will then result 0.
If you also set o.maker to be 0 then this will match and an invalid signature is not detected.
So you can do all kinds of illegal & unexpected transactions.
https://github.com/Swivel-Finance/gost/blob/v2/test/swivel/Swivel.sol#L476-L484
https://github.com/Swivel-Finance/gost/blob/v2/test/swivel/Sig.sol#L16-L23
Verify that the result from ecrecover isnt 0
JTraversa (Swivel) acknowledged
JTraversa (Swivel) commented:
