# #37300 \[BC-Insight] Incorrect Encoding of Negative \*big.Int Values in MakeTopics

**Submitted on Dec 2nd 2024 at 00:47:26 UTC by @CertiK for** [**Attackathon | Ethereum Protocol**](https://immunefi.com/audit-competition/ethereum-protocol-attackathon)

* **Report ID:** #37300
* **Report Type:** Blockchain/DLT
* **Report severity:** Insight
* **Target:** <https://github.com/ledgerwatch/erigon>
* **Impacts:**
  * (Specifications) A bug in specifications with no direct impact on client implementations

## Description

## Brief/Intro

A vulnerability has been identified in the `MakeTopics` function within the `accounts/abi/topics.go` file of the codebase. The issue stems from the incorrect encoding of negative \*big.Int values when generating topics for contract events.

## Vulnerability Details

Affected Codebase:\
<https://github.com/erigontech/erigon/tree/v2.61.0-beta1>

In the `MakeTopics` function, the handling of `*big.Int` types is as follows:

<https://github.com/erigontech/erigon/blob/v2.61.0-beta1/accounts/abi/topics.go#L45>

```go
			case *big.Int:
				blob := rule.Bytes()
				copy(topic[length.Hash-len(blob):], blob)
```

The `rule.Bytes()` method returns the absolute value of the `*big.Int` as a big-endian byte slice, disregarding the sign of the number. This means that negative integers are incorrectly encoded. For example, `big.NewInt(-1).Bytes()` returns `[1]`, leading to the topic being encoded as `0x000...0001` instead of the correct two's complement representation `0xFFFF...FFFF`.

<https://github.com/golang/go/blob/master/src/math/big/int.go#L520>

```go
// Bytes returns the absolute value of x as a big-endian byte slice.
//
// To use a fixed length slice, or a preallocated one, use [Int.FillBytes].
func (x *Int) Bytes() []byte {
   // This function is used in cryptographic operations. It must not leak
   // anything but the Int's sign and bit size through side-channels. Any
   // changes must be reviewed by a security expert.
   buf := make([]byte, len(x.abs)*_S)
   return buf[x.abs.bytes(buf):]
}

```

## Impact Details

The failure to correctly encode negative `*big.Int` values affects any functionality that relies on event topics generated by the `MakeTopics` function.

## References

* <https://github.com/erigontech/erigon/blob/v2.61.0-beta1>
* <https://github.com/ethereum/go-ethereum/pull/28764/>

## Proof of Concept

## Proof of Concept

For simplicity, we can add the following test in `TestMakeTopics`\
<https://github.com/erigontech/erigon/blob/v2.61.0-beta1/accounts/abi/topics\\_test.go#L30>

```
       {
           "support negative *big.Int types in topics",
           args{[][]interface{}{
               {big.NewInt(-1)},
           }},
           [][]libcommon.Hash{
               {libcommon.HexToHash("0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff")},
           },
           false,
       },
```

The test will raise the error:

```
--- FAIL: TestMakeTopics (0.00s)
   --- FAIL: TestMakeTopics/support_negative_*big.Int_types_in_topics (0.00s)
       /Users/xxx/erigon/accounts/abi/topics_test.go:144: makeTopics() = [[0x0000000000000000000000000000000000000000000000000000000000000001]], want [[0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff]]
FAIL
FAIL    github.com/erigontech/erigon/accounts/abi   0.508s
FAIL
```


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