Security Audit Report: Reentrancy & Access Control Review: PancakeSwap AMM

Target Protocol: PancakeSwap AMM (TVL: $1555.9M)

Security Audit Report – Reentrancy & Access‑Control Review

Protocol: PancakeSwap Automated Market Maker (AMM) – Ethereum & L2 deployments

TVL (as of 28 Oct 2026): ≈ $1.56 B (combined Ethereum + L2)

Audit Window: 01 Oct 2026 – 20 Oct 2026

Prepared by: [Senior DeFi Security Researcher – Confidential]

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1. Executive Summary

PancakeSwap’s AMM contracts are the core liquidity‑routing and swap engine that underpin the platform’s DEX functionality. The contracts have been battle‑tested on Binance Smart Chain (BSC) for several years and were recently ported to Ethereum and multiple L2s (Arbitrum, Optimism, zkSync). This audit focuses exclusively on reentrancy and access‑control weaknesses that could be leveraged to drain user funds, manipulate price feeds, or seize privileged functions.

Key Findings

#	Category	Severity	Brief Description
1	Reentrancy in swapExactTokensForTokens (router)	High (8/10)	External call to ERC‑20 transfer occurs before updating the internal amountOut accounting, enabling a crafted token to re‑enter the router and receive extra output.
2	Reentrancy in removeLiquidity (pair)	Medium‑High (7/10)	transfer of LP tokens to the caller is performed before the pair’s internal burn bookkeeping, opening a “withdraw‑while‑burning” vector.
3	Missing nonReentrant guard on flashSwap	Medium (6/10)	Flash‑swap implementation relies on a “callback” that can be re‑entered to call swap again, potentially creating infinite loops or profit‑extraction attacks.
4	Unrestricted setFeeTo & setFeeToSetter in factory	Critical (9/10)	The feeToSetter role is not protected by a multi‑sig or timelock; a compromised admin key can redirect all protocol fees to an attacker‑controlled address.
5	Improper role checks on addLiquidity for fee‑on‑transfer tokens	Medium (5/10)	The router assumes transferFrom succeeds atomically; fee‑on‑transfer tokens can cause a mismatch between expected and actual amounts, leading to slippage manipulation.
6	Lack of “pause” capability on emergency functions	Low‑Medium (4/10)	No circuit‑breaker to halt swaps or liquidity removal in case of a discovered exploit, increasing exposure window.
7	Upgradeable proxy admin not time‑locked	Medium (6/10)	The proxy admin can upgrade core contracts without a delay, allowing a malicious upgrade to introduce hidden reentrancy bugs.

Overall, the AMM’s design is solid, but the identified gaps—particularly around privileged role management and missing reentrancy guards—represent significant attack surface given the $1.56 B TVL.

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2. Identified Attack Vectors

2.1 Reentrancy in Router swapExactTokensForTokens

Flow:

1. User calls swapExactTokensForTokens → router pulls amountIn from user via transferFrom.
2. Router computes amountOutMin and calls the pair’s swap function.
3. Pair executes tokenOut.transfer(to, amountOut).
4. If tokenOut is a malicious ERC‑20 with a crafted transfer that calls back into the router’s swapExactTokensForTokens, the router will re‑enter before it updates its internal accounting (_amounts array).
5. The attacker can receive additional amountOut on each re‑entry, effectively minting tokens from the pool.

Why it works: The router does not use a checks‑effects‑interactions pattern nor a nonReentrant modifier for the entire swap flow. The external token transfer is the last step, but the router still holds mutable state (_amounts) that is read after the external call.

2.2 Reentrancy in Pair removeLiquidity (burn)

Flow:

1. LP holder calls removeLiquidity → router invokes pair.transfer(address(this), liquidity).
2. Pair’s burn function first transfers the LP tokens back to the caller before updating reserves and minting underlying assets.
3. A malicious LP token can re‑enter removeLiquidity (or any function that reads reserves) before the reserves are updated, allowing the attacker to extract more underlying assets than entitled.

2.3 Flash‑Swap Callback Re‑entrancy

The flashSwap function expects the borrower to implement IUniswapV2Callee.pancakeCall. The borrower can call back into the pair’s swap again before the original flashSwap finishes, creating a recursive flash‑swap that can be used to manipulate price or drain assets if combined with a price oracle attack.

2.4 Unrestricted setFeeTo / setFeeToSetter

The factory contract stores feeTo (address receiving protocol fees) and feeToSetter (admin). The setFeeTo function is public and guarded only by msg.sender == feeToSetter. If the private key of feeToSetter is compromised (phishing, key‑reuse, or insider threat), the attacker can instantly redirect all swap fees (≈ 0.25 % per trade) to an address they control, resulting in continuous revenue siphoning.

2.5 Fee‑On‑Transfer Token Handling

The router assumes that transferFrom returns the exact amountIn. Tokens that charge a transfer fee (e.g., deflationary tokens) will cause the router to over‑estimate the amount received, leading to unfair slippage and potential front‑running opportunities for the attacker who can supply a token that “steals” part of the input.

2.6 Missing Emergency Pause

No Pausable modifier exists on critical functions (swap, addLiquidity, removeLiquidity). In the event of a discovered exploit, the protocol cannot quickly halt operations, extending the window for attackers to drain funds.

2.7 Upgradeable Proxy Admin Without Delay

Core contracts (factory, router, pair) are deployed behind a Transparent Upgradeable Proxy. The admin can call upgradeTo instantly. Without a time‑lock or multi‑sig, a compromised admin key can push a malicious implementation that introduces hidden backdoors (e.g., hidden selfdestruct, privileged mint).

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3. Prioritized Technical Recommendations

Priority	Recommendation	Affected Contracts	Rationale & Implementation Details
P1	Add nonReentrant (or custom reentrancy guard) to all external‑facing entry points (swapExact*, removeLiquidity, flashSwap).	Router, Pair	Use OpenZeppelin’s ReentrancyGuard. Ensure the guard wraps the entire function, not just the external call.
P1	Refactor router swap flow to follow Checks‑Effects‑Interactions – compute amounts, update internal state, then perform external token transfers after state changes.	Router	Move the _amounts array updates before any transfer/transferFrom.
P1	Restrict setFeeTo / setFeeToSetter with multi‑signature and timelock (e.g., Gnosis Safe + 48‑hour delay).	Factory	Replace onlyOwner with onlyTimelockedOwner. Emit events on proposal and execution.
P2	Introduce a circuit‑breaker (Pausable) for swaps and liquidity functions.	Router, Pair	Add whenNotPaused modifier; only admin (multi‑sig) can pause/unpause.
P2	Validate fee‑on‑transfer tokens – after transferFrom, read the actual balance delta to compute the real amountIn. Reject tokens that do not conform to the ERC‑20 standard or require explicit whitelisting.	Router	Implement balanceBefore/After pattern or use IERC20Metadata.decimals() to detect non‑standard behavior.
P2	Add explicit reserve update before LP token transfer in burn – move pair.transfer(address(this), liquidity) after reserve adjustments.	Pair	Align with Uniswap V2’s burn implementation.
P3	Add a timelock (e.g., 72 h) on proxy admin upgrades and require a multi‑sig to execute.	Proxy Admin	Deploy a TimelockController that holds the admin role.
P3	Implement a “flash‑swap re‑entrancy guard” – a simple bool inFlashSwap flag that reverts if a nested flash‑swap is attempted.	Pair	Set flag at entry, clear at exit, revert on re‑entry.
P4	Perform a formal verification of the pair’s swap invariant (x·y = k) under re‑entrancy scenarios.	Pair	Use tools like Certora, Slither, or Echidna to fuzz re‑entrancy paths.
P4	Add comprehensive unit‑test coverage for fee‑on‑transfer tokens (e.g., Tether, SafeMoon clones).	Router	Include tests for transferFrom returning less than requested.
P4	Deploy a “bug‑bounty” program targeting re‑entrancy and access‑control vectors with a minimum payout of $250k for successful exploits on mainnet.	–	Incentivize external security research.

Priorities are ordered by potential financial impact and exploitability given the current TVL.

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4. Risk Score

Category	Score (1 = Negligible, 10 = Critical)	Comments
Reentrancy	8 / 10	Direct fund extraction possible; mitigations are straightforward but currently missing on several entry points.
Access‑Control	9 / 10	Admin key compromise would reroute all protocol fees; lack of multi‑sig/timelock is a systemic governance risk.
Overall Protocol Risk	8.5 / 10	Combined effect of high‑TVL and identified gaps yields a high‑risk posture.
Residual Risk after P1‑P2 fixes	4 / 10	Remaining risk stems from upgradeability and emergency response latency.

The **aggregate risk score* for the audited scope (reentrancy & access control) is 8.5/10.*

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5. Conclusion

PancakeSwap’s AMM contracts are fundamentally robust and have withstood extensive real‑world usage on BSC. However, the migration to Ethereum/L2 environments introduces new threat vectors—particularly around reentrancy in the router/pair interaction and privileged role management in the factory.

The most severe issues are:

1. Unprotected admin functions (setFeeTo, setFeeToSetter) that could divert ongoing fee revenue.
2. Reentrancy gaps in swap and liquidity removal flows that enable token‑level attacks to mint excess output.

Implementing the high‑priority recommendations (reentrancy guards, checks‑effects‑interactions, multi‑sig/timelock for admin actions) will eliminate the majority of exploitable pathways and bring the protocol’s risk profile in line with industry best practices for a platform managing > $1.5 B in assets.

A disciplined post‑deployment monitoring regime—combined with the suggested bug‑bounty program—will further reduce the likelihood of a successful attack. Once the P1 and P2 mitigations are live, we recommend a follow‑up audit focusing on upgradeability and oracle integration to ensure the protocol remains secure as it scales across additional L2s.

Prepared for internal use by PancakeSwap’s security & governance teams. The findings herein are confidential and should not be disclosed without prior written consent.

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