Alchemix Transmuter

Summary

The approve function in the strategy contract allows for an unlimited allowance (type(uint256).max) to a new router when switching to it. However, the approval for the old router is not revoked, leaving the old router with maximum approval. This creates an over-allowance vulnerability where unauthorized or unintended actions can exploit the old router's residual approval.

Vulnerability Details

The function does not revoke allowances for the old router when transitioning to a new one.
The first Router address and the Second both will have type(uint256).max approve.

Impact

Unnecessary approve for old router which leaves every old router with max approve.

Tools Used

Solidit:
Defi / General / SOL-Defi-General-6 Does the protocol revert on maximum approval to prevent over-allowance? Setting high allowances can make funds vulnerable to abuse; protocols sometimes set max to prevent this risk. Consider implementing a revert on approval functions when an unnecessarily high allowance is set.

Recommendations

1. Revoke Old Router Approval: Introduce logic to explicitly reset the allowance of the old router to zero (underlying.safeApprove(oldRouter, 0)) before assigning maximum approval to the new router.