CAP Labs — a same-block delegation takes 750 of 1000 RWD

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 61538-reward-distribution-enables-front-running-attacks-and-reward. Standalone Foundry PoC and full write-up: 61538-reward-distribution-enables-front-running-attacks-and-reward_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/reward-calculation · vuln/logic/missing-check

Reproduction: a self-contained Foundry PoC that compiles and runs offline with only forge-std — no fork and no RPC. Trace: output.txt. PoC: test/61538-reward-distribution-enables-front-running-attacks-and-reward_exp.sol.

Key info#


Impact	HIGH — vault B delegates in the repayment block and takes 750 of 1000 RWD from vault A
Protocol	CAP Labs `Delegation`
Vulnerable code	`distribute` pays `pot * delegated[vault] / total` with no time weight
Bug class	Reward share ignores how long the stake has been delegated
Finding	Trail of Bits (Benjamin Samuels) — CAP Labs, May 2025 · id 61538
Report	Trail of Bits CAP Labs review
Source	AuditVault finding
Status	Reproduced as a local delegation pot.
Compiler	solc 0.8.27, cancun, via IR

TL;DR#

Vault A already delegates 100 behind an agent. The repayment interest, 1000 RWD, sits on the delegation contract.

Vault B delegates 300 and calls distribute in the same transaction. The pot is split by current delegation: A receives 250 RWD and B receives 750 RWD. B provided none of the backing before this block.

The control locks eligibility after A's deposit. B's later delegation is ignored. A receives the full 1000 RWD and B receives 0.

The vulnerable code#

SOLIDITY

// @> VULN: the share uses delegation at distribute time, including this block
uint256 pay = pot * delegated[vault] / base;
if (pay != 0) reward.transfer(vault, pay);

Preconditions#

Vault A is already delegated for 100.
1000 RWD of interest is ready to distribute.
Vault B can delegate before distribute in the same block. No warp is required.

Attack walkthrough#

Vault B delegates 300. Total delegation becomes 400.
distribute reads the 1000 RWD balance.
A is paid 1000 * 100 / 400 = 250.
B is paid 1000 * 300 / 400 = 750.

Control: eligibility is frozen at A's 100 before B delegates. distributeTo(A) transfers 1000 RWD. B's balance stays 0.

Diagrams#

flowchart TD A["Vault A already delegates 100"] --> B["1000 RWD of interest is ready"] B --> C["Vault B delegates 300 in this block"] C --> D["distribute splits by the new total of 400"] D --> E["A receives 250 RWD"] E --> F["B receives 750 RWD"]

Impact#

Vault A, the existing backer, receives 250 RWD instead of the 1000 RWD interest. Vault B takes the other 750 RWD without having backed the loan before the repayment.

Remediation#

Exclude collateral delegated in the current epoch from the distribution, or pay from a checkpoint of delegation taken before the repayment.

Sources#

Taxonomy#

severity/high
vuln/logic/reward-calculation · vuln/logic/missing-check

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 61538-reward-distribution-enables-front-running-attacks-and-reward_exp (evm-hack-registry mirror).
AuditVault finding: 61538-reward-distribution-enables-front-running-attacks-and-reward.

Alerts & third-party analyses

Web3Sec X hacked database: search.
Rekt leaderboard: search.
Solodit incident search: search.

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