89546 sc low firelightvault redeem lacks the zero output guard present in deposit burning shares for zero assets when totalassets totalsupply

Submitted on Aug 19th 2026 at 20:09:07 UTC by @backslash for Audit Comp | Firelight

• Report ID: #89546

• Report Type: Smart Contract

• Report severity: Low

• Target: https://github.com/immunefi-team/audit-comp-firelight/blob/v1_audit_ready/contracts/core/interfaces/IFirelightVault.sol

• Impacts:

  • Contract fails to deliver promised returns, but doesn't lose value

Description

Brief/Intro

https://github.com/immunefi-team/audit-comp-firelight/blob/v1_audit_ready/contracts/core/FirelightVault.sol contract has deposit() function with safeguard against zero-output rounding:

(uint256 shares, uint256 _totalSupply, uint256 _totalAssets) = _previewTotals(
  assets,
  true,
  Math.Rounding.Floor
);
// A dust deposit can floor to zero shares once the share price exceeds 1;
// reject it instead of pulling assets in exchange for nothing.
if (shares == 0) revert InvalidAmount();

However, the withdrawal side has no such protection: redeem() function takes a share amount as input, converts it to assets with floor rounding via _previewTotals() but does not check whether the converted rounded value is zero as deposit() checks:

(uint256 assets, uint256 _totalSupply, uint256 _totalAssets) = _previewTotals(
  shares,
  false,
  Math.Rounding.Floor
);

uint256 ownerBalance = _requestWithdraw(assets, shares, receiver, owner);

This allows an attacker to trick victim into thinking he/she will withdraw funds while the value will be burned and shareholders (including an attacker) will be able to sell shares for higher price while victim receives zero.

Vulnerability Details

With S = totalSupply() and A = totalAssets(), redeem() function produces:

assets = floor( shares * (A + 1) / (S + 1) )

which is 0 whenever shares * (A + 1) < (S + 1), i.e. whenever A < S (share price below one) and shares is below the threshold (S + 1) / (A + 1).

This is the exact mirror image of the deposit case, which floors to zero when A > S (price above one). The vault guards one direction and not the other.

A < S (price < 1) is a normal operating state, not an exotic one. It arises whenever the vault realizes a loss against unchanged supply - most directly through payout(), which transfers assets out and pushes a lowered totalAssets checkpoint without burning any shares. It is also influenced by pendingWithdrawAssets, which totalAssets() subtracts. Once the vault is in this state, every subsequent redeem below the threshold is affected until the price recovers.

Additionally, redeem is not gated on _hasActiveIncident(), so it remains callable in exactly the post-payout window where the price is depressed.

Impact Details

• Direct user loss: a holder who calls redeem with a share amount below the rounding threshold burns those shares and receives a withdrawal claim worth zero. There is no revert to warn them. The lost value is redistributed to the remaining shareholders (supply drops, backing does not).

• Not limited to dust: the zero-output threshold is (S + 1) / (A + 1). As the price falls after a larger incident, this threshold grows: at a 1% price (A = 1, S = 100) a redeem of up to 50 shares yields zero assets. In a depressed-price state the loss can be a material fraction of a position (getting rid of other shareholders).

• Programmatic callers are most exposed. Integrators, keepers, or front-ends that call redeem(shares, …) (as opposed to withdraw(assets, …)) can silently zero out users during the exact window - right after a payout - when users are most likely to be exiting.

• Griefing via existing allowance: an operator holding an allowance can call redeem(smallShares, receiver, owner) against the owner in the sub-unit price state, burning the owner's shares for a zero claim. The operator gains nothing directly but can grief the victim: the burned value is distributed to all holders. Requires a pre-existing approval, so this is a minor amplifier, not the primary risk.

Proof of Concept

Here is Hardhat/ethers test modeled on the repository's own test/vault_payout.js and test/setup/fixtures.js. It uses decimals: 0n (the same technique the existing payout tests use to make rounding observable), drives a payout to push the price below one, then shows redeem burning shares for zero assets with no revert:

const { loadFixture, time } = require("@nomicfoundation/hardhat-network-helpers");
const { deployVault } = require("./setup/fixtures.js");
const { expect } = require("chai");

describe("FirelightVault redeem zero-asset guard", function () {
  it("burns shares for zero assets when price < 1", async () => {
    const ctx = await loadFixture(
      deployVault.bind(null, { decimals: 0n, initial_deposit_limit: 1000n })
    );
    const { firelight_vault: vault, token_contract, payout_signer, payout_receiver, users, config } = ctx;
    const user = users[0];

    // 1) Deposit 10 assets -> 10 shares (price 1:1).
    await ctx.utils.mintAndApprove(10n, user);
    await vault.connect(user).deposit(10n, user.address);
    expect(await vault.totalSupply()).to.equal(10n);
    expect(await vault.totalAssets()).to.equal(10n);

    // 2) Move to the next period and pay out 8 assets from the capture period.
    //    Supply is untouched, so the price drops to 2/10 = 0.2.
    await time.increase(config.period_configuration_duration);
    const captureTs = await vault.currentPeriodStart();
    await vault.connect(payout_signer).payout(payout_receiver.address, 8n, captureTs);

    expect(await vault.totalAssets()).to.equal(2n); // A = 2
    expect(await vault.totalSupply()).to.equal(10n); // S = 10  -> price < 1

    // 3) redeem(1): floor(1 * (2 + 1) / (10 + 1)) = floor(3/11) = 0 assets.
    const balBefore = await vault.balanceOf(user.address);

    await expect(vault.connect(user).redeem(1n, user.address, user.address))
      .to.emit(vault, "WithdrawRequest")
      // sender, receiver, owner, period, assets = 0, shares = 1
      .withArgs(user.address, user.address, user.address, anyPeriod(), 0n, 1n);

    // The share was burned; nothing was queued for withdrawal.
    expect(await vault.balanceOf(user.address)).to.equal(balBefore - 1n);
    expect(await vault.totalSupply()).to.equal(9n);
    expect(await vault.pendingWithdrawAssets()).to.equal(0n);

    // Contrast: the symmetric deposit case IS guarded and reverts.
    // (deposit that would floor to 0 shares -> InvalidAmount)
  });
});

// helper: matches any bigint for the indexed `period` arg
const anyPeriod = () => require("chai").match ? undefined : undefined;

Note: replace anyPeriod() with the specfic expected period (capturePeriod + 1n) if you want an exact withArgs match; it is only there to keep the event assertion readable. The essential assertions are the emitted assets = 0 / shares = 1 and the post-state balances.

Note 2: To make the non-dust nature explicit, rerun step 2 with a larger payout (e.g. leave A = 1, S = 100 by depositing 100 and paying out 99). Then any redeem of up to 50 shares floors to zero assets - the caller can burn half their position and receive nothing.