Gocoin incorrectly parsing OP_PUSHDATA4
Gocoin is a full Bitcoin solution written entirely from scratch in Go language with zero dependencies, according to their website. We are differentially fuzzing its VerifyTxScript function against Bitcoin Core’s VerifyScript. These functions are consensus critical and they are responsible for verifying scriptPubKey and scriptSig.
During the execution, we got a crash with the following scripts:
scriptSig: 5b00
scriptPubKey: 00004c020f600100009f9a000102020202004e000064a2a2a2a2a2000000a2a2a2a2a2a2a2a2
The error returned by Bitcoin Core is SCRIPT_ERR_BAD_OPCODE that was being returned due to OP_PUSHDATA4 claiming more bytes than are available.
GetOp() attempts to advance pc by 100 bytes, overshoots the end of the script buffer, and then, returns false. Looking at gocoin’s code, the similar function to Core’s GetOpcode is GetOp:
func GetOpcode(b []byte) (opcode int, ret []byte, pc int, e error) {
// Read instruction
if pc+1 > len(b) {
e = errors.New("GetOpcode error 1")
return
}
opcode = int(b[pc])
pc++
if opcode <= OP_PUSHDATA4 {
size := 0
if opcode < OP_PUSHDATA1 {
size = opcode
}
if opcode == OP_PUSHDATA1 {
if pc+1 > len(b) {
e = errors.New("GetOpcode error 2")
return
}
size = int(b[pc])
pc++
} else if opcode == OP_PUSHDATA2 {
if pc+2 > len(b) {
e = errors.New("GetOpcode error 3")
return
}
size = int(binary.LittleEndian.Uint16(b[pc : pc+2]))
pc += 2
} else if opcode == OP_PUSHDATA4 {
if pc+4 > len(b) {
e = errors.New("GetOpcode error 4")
return
}
size = int(binary.LittleEndian.Uint16(b[pc : pc+4]))
pc += 4
}
if pc+size > len(b) {
e = errors.New(fmt.Sprint("GetOpcode size to fetch exceeds remainig data left: ", pc+size, "/", len(b)))
return
}
ret = b[pc : pc+size]
pc += size
}
return
}
The issue is that gocoin was using Uint16 in the context of OP_PUSHDATA4 instead of using Uint32. The size field is explicitly defined as a 4-byte little-endian unsigned integer.
Thanks Piotr Narewski for responding to my report and fixing the bug so quickly. Also, he told me to go ahead and disclosure it.