OneRing Finance Exploit — Missing Reentrancy Guard + Under-priced `depositSafe`

Source & credit. Exploit reproduction, trace data, and analysis adapted from DeFiHackLabs by SunWeb3Sec — an open registry of reproduced on-chain exploits. Standalone Foundry PoC and full write-up: 2022-03-OneRing_exp in the evm-hack-registry mirror. Upstream DeFiHackLabs PoC: src/test/…/OneRing_exp.sol.

Vulnerability classes: vuln/reentrancy/single-function · vuln/logic/state-update

Reproduction: the PoC compiles & runs in an isolated Foundry project at this project folder. Full verbose trace: output.txt.

Key info#


Loss	~$1.45M USDC (the PoC extracts 1,526,751,528,201 = ~1.526M USDC)
Vulnerable contract	OneRing `Vault` — `0x4e332D616b5bA1eDFd87c899E534D996c336a2FC` (Fantom)
Flash source	USDC/wAnyRING Uniswap-V2-style pair — `0xbcab7d083Cf6a01e0DdA9ed7F8a02b47d125e682`
Chain / block / date	Fantom / 34,041,499 / Mar 2022
Bug class	Reentrancy + accounting flaw — `depositSafe(amount, asset, minOut)` mints vault shares on a flash-deposit, then `withdraw()` redeems them before the vault's oracle/rounding accounting catches up, letting a flash-borrowed deposit be withdrawn for more than deposited.

TL;DR#

OneRing's vault priced shares from a "strategy total value" oracle rounded/computed per-epoch. The depositSafe/withdraw pair had no reentrancy lock, so within a single flash-loan callback the attacker could:

pair.swap(80M USDC, …) — flash-borrow 80,000,000 USDC.
In hook: usdc.approve(vault, max); vault.depositSafe(amount, USDC, 1) — mints vault shares.
Immediately vault.withdraw(shares, USDC) — redeems, but the vault's share-price (derived from the just-deposited amount and its rounding) credits slightly more than was deposited.
Repay the flash loan (amount0/9999*10000 + 10000) and keep the surplus.

After exploit, USDC balance of attacker: 1526751528201 (~1.526M USDC).

Root cause#

A flash-loan-friendly deposit/withdraw without reentrancy guard or settlement-epoch gating. The share price was computed from the vault's current holdings including the just-flashed deposit, and the withdraw in the same transaction captured the inflated/redemption difference. OneRing allowed same-tx deposit→withdraw (no lockup, no nonReentrant), which is the textbook prerequisite for this class of vault drain.

Contributing factors:

minOut = 1 on deposit accepted any (even 0) mint, hiding the rounding issue.
No settlement/epoch boundary between deposit and withdraw, so a single tx can do both.
Flash-loanable capital (the Uniswap pair) supplies the deposit.

Preconditions#

A flash source for the deposit token (USDC pair here).
The vault permits same-transaction deposit + withdraw with no reentrancy/lockup.

Diagrams#

sequenceDiagram autonumber actor A as Attacker participant P as USDC pair participant V as OneRing Vault A->>P: swap(flash 80M USDC) P-->>A: USDC (→ hook) A->>V: depositSafe(80M, USDC, 1) → mint shares A->>V: withdraw(shares, USDC) ⚠️ no guard, accounting lag → +surplus A->>P: repay flash + fee Note over A: +1.526M USDC

flowchart TD D["depositSafe(amount) → shares"] --> W["withdraw(shares) same-tx, no nonReentrant"] W --> AC["share price from inflated holdings → redeem > deposit"] AC --> RP["repay flash, keep surplus"] style W fill:#ffcdd2,stroke:#c62828,stroke-width:2px style RP fill:#c8e6c9,stroke:#2e7d32

Remediation#

nonReentrant on deposit/depositSafe/withdraw.
Settlement epochs: a deposit cannot be withdrawn until the next epoch/round (or with a delay), so same-tx deposit→withdraw is impossible.
Don't price shares from instantaneous holdings that include an unconfirmed deposit; use a committed balance snapshot updated at epoch boundaries.
Enforce minOut/maxOut sanity and reject 0/1 dust outputs.

How to reproduce#

BASH

_shared/run_poc.sh 2022-03-OneRing_exp --mt testExploit -vvvvv

RPC: Fantom archive (block 34,041,499). foundry.toml uses fantom-mainnet.public.blastapi.io.
Result: [PASS] testExploit() — After exploit, USDC balance of attacker: 1526751528201 (~$1.526M).

Reference: OneRing Finance flash-deposit reentrancy, Fantom, Mar 2022 (~$1.45M).

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 2022-03-OneRing_exp (evm-hack-registry mirror).
Upstream DeFiHackLabs PoC: OneRing_exp.sol.

Alerts & third-party analyses

DeFiHackLabs incident explorer: search "OneRing Finance Exploit".
Web3Sec X hacked database: search.
Rekt leaderboard: search.
Solodit incident search: search.

These dashboards index community alerts tweets, post-mortems, and independent write-ups. Reach them through the protocol name above to cross-check this reproduction against other analyses.