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30 March 2021

Yield Skimming: Forcing Bad Swaps on Yield Farming

By the Dedaub team

Yield Skimming

Last week we received bug bounties for disclosing smart contract vulnerabilities to Vesper Finance and BT Finance, via Thank you, all!

(Notice for clients of these services: None of the vulnerabilities drain the original user funds. An attack would have financial impact, but not an overwhelming one. The maximum proceeds in the past few months would have been around $150K, and, once performed, the attack would be likely to alert the service to the vulnerability, making the attack non-repeatable. The vulnerabilities have since been mitigated and, to our knowledge, no funds are currently threatened.)

Both vulnerabilities follow the same pattern and many other services could potentially be susceptible to such attacks (though all others that we checked are not, by design or by circumstance — it will soon be clear what this means). It is, therefore, a good idea to document the pattern and draw some attention to it, as well as to its underlying financials.

Yield Skimming | The Attack

A common pattern in yield farming services is to have strategies that, upon a harvest, swap tokens on an exchange, typically Uniswap. A simplified excerpt from actual deployed code looks like this:

function harvest() public {
  uint256 reward = TokenA.balanceOf(address(this));
  unirouter.swapExactTokensForTokens(reward, 0, pathTokenAB, this, now.add(1800));

Example harvest function, with swapping.

Similar code is deployed in hundreds (if not thousands) of contracts. Typical uses of the pattern are a little more complex, with the harvest and the swap happening in different functions. But the essence remains unchanged. Similar code may also be found at the point where the service rebalances its holdings, rather than at the harvest point. We discuss harvest next, as it is rather more common.

[Short detour: you see that now.add(1800)for the “deadline” parameter of the swap? The add(1800) has no effect whatsoever. Inside a contract, the swap will always happen at time now, or not at all. The deadline parameter is only meaningful if you can give it a constant number.]

Back to our main pattern, the problem with the above code is that the harvest can be initiated by absolutely anyone! “What’s the harm?” — you may ask — “Whoever calls it pays gas, only to have the contract collect its rightful yield.”

The problem, however, is that the attacker can call harvest after fooling the Uniswap pool into giving bad prices for the yield. In this way, the victim contract distorts the pool even more, and the attacker can restore it for a profit: effectively the attacker can steal almost all of the yield, if its value is high enough.

In more detail, the attack goes like this:

a) the attacker distorts the Uniswap pool (the AssetA-to-AssetB pool) by selling a lot of the asset A that the strategy will try to swap. This makes the asset very cheap.

b) the attacker calls harvest. The pool does a swap at very bad prices for the asset.

c) the attacker swaps back the Asset B they got in the first step (plus a tiny bit more for an optimal attack) and gets the original asset A at amounts up to the original swapped (of step (a)) plus what the victim contract put in.

Yield Skimming

Yield Skimming | Example

For illustration, consider some concrete, and only slightly simplified, numbers. (If you are familiar with Uniswap and the above was all you needed to understand the attack, you can skip ahead to the parametric analysis.)

Say the harvest is in token A and the victim wants to swap that to token B. The Uniswap pool initially has
1000 A tokens and 500 B tokens. The “fair” price of an A denominated in Bs is 500/1000 = 0.5. The product k of the amounts of tokens is 500,000: this is a key quantity in Uniswap — the system achieves automatic pricing by keeping this product constant while swaps take place.

In step (a) the attacker swaps 1000 A tokens into Bs. This will give back to the attacker 250 B tokens, since the Uniswap pool now has
2000 A tokens and 250 B tokens (in order to keep the product k constant). The price of an A denominated in Bs has now temporarily dropped to a quarter of its earlier value: 0.125, as far as Uniswap is concerned.

In step (b) the victim’s harvest function tries to swap, say, 100 A tokens into Bs. However, the price the victim will get is now nowhere near a fair price. Instead, the Uniswap pool goes to
2100 A tokens and 238 B tokens, giving back to the victim just 12 B tokens from the swap.

In step (c) the attacker swaps back the 250 B tokens they got in step (a), or, even better, adds another 12 to reap maximum benefit from the pool skew. The pool is restored to balance at the initial
1000 A tokens and 500 B tokens. The attacker gets back 1100 A tokens for a price of 1000 A tokens and 12 B tokens. The attacker effectively got the 100 As that the victim swapped at 1/4th of the fair price.

Yield Skimming | Parametric Analysis

The simplistic example doesn’t capture an important element. The attacker is paying Uniswap fees for every swap they perform, at steps (a) and (c). Uniswap currently charges 0.3% of the swapped amount in fees for a direct swap. The net result is that the attack makes financial sense only when the amounts swapped by the victim are large. How large, you may ask? If the initial amount of token A in the pool is a and the victim will swap a quantity d of A tokens, when can an attacker make a profit, and what values x of A tokens does the attacker need to swap in step (a)? If you crunch the numbers, the cost-benefit analysis comes down to a cubic inequality. Instead of boring you with algebra, let’s ask Wolfram Alpha.

The result that Alpha calculates is that the attack is profitable as long as the number d of A tokens that the victim will swap is more than 0.3% of the number a of A tokens that the pool had initially. In the worst case, is significant (e.g., 10% of a, as in our example) and the attacker’s maximum profit is very close to the entire swapped amount.

Another consideration is gas prices, which we currently don’t account for. For swaps in the thousands of dollars, gas prices will be a secondary cost, anyway.

Yield Skimming | Mitigation

In practice, yield farming services protect against such attacks in one of the following ways:

  • They limit the callers of harvest or rebalance. This also needs care. Some services limit the direct callers of harvest but the trusted callers include contracts that have themselves public functions that call harvest.
  • They have bots that call harvest regularly, so that the swapped amounts never grow too much. Keep3r seems to be doing this consciously. This is fine but costly, since the service incurs gas costs even for harvests that don’t produce much yield.
  • They check the slippage suffered in the swap to ensure that the swap itself is not too large relative to the liquidity of the pool. We mention this to emphasize that it is not valid protection! Note the numbers in our above example. The problem with the victim’s swap in step (b) is not high slippage: the victim gets back 12 B tokens (11.9 to be exact) whereas with zero slippage they would have gotten back 12.5. This difference, of about 5%, may certainly pass a slippage check. The problem is not the 5% slippage but the 4x lower-than-fair price of the asset, to begin with!

There are other factors that can change the economics of this swap. For instance, the attacker could be already significantly vested in the Uniswap pool, thus making the swap fee effectively smaller for them. Also, Uniswap v3 was announced right at the time of this writing, and promises 0.05% fees for some price ranges (i.e., one-sixth of the current fees). This may make similar future attacks a lot more economical even for small swaps.


The pattern we found in different prominent DeFi services offers opportunities for interesting financial manipulation. It is an excellent representative of the joint code analysis (e.g., swap functionality reachable by untrusted callers) and financial analysis that are both essential in the modern Ethereum/DeFi security landscape.