Summary

During Sunrise, Beanstalk adjusts the amount of available Soil. The Soil supply is based on the number of Pods that ripen and become harvestable at the beginning of the Season, the Temperature, and the Beanstalk debt level. When P > 1 and the Pod Rate is high, Beanstalk issues less Soil. When P > 1 and the Pod Rate is low, Beanstalk issues more Soil. However, there are missing cases and not all the cases taken into account meet this criteria, which results in an inaccurate calculation.

Vulnerability Details

The contracts/beanstalk/sun/SeasonFacet/Sun::stepSun function take the precalculated deltaB and caseId to update the Soil issuance. When above peg (deltaB > 0), Sun:setSoilAbovePeg function is used to determine and update the soil in the system.

On the other hand, the contracts/libraries/LibCases.sol contract handles the cases for Beanstalk.

The parameters that are taken into account to calculate the different cases are:

• the Liquidity to Supply Ratio (L2SR) level (extremely low, reasonably low, reasonably high, extremely high)

• the debt level or Pod Rate level (excessively low, reasonably low, reasonably high, excessively high)

• the peg state (P < 1, P > 1, P > Q)

• the Soil demand behavior (decreasing, steady, increasing).

The LibCases::setCasesV2 function sets the s.sys.casesV2 system variable that stores the 144 cases resulting from the combination of the above parameters.

Given the structure of the elements contained in s.sys.casesV2, it is possible to see that the order of the cases is as indicated in the following table:

Table 1. Description of the 144 possible cases for the Beanstalk status, including their Id number and the corresponding line number in the LibCases::setCasesV2 function.

Comparing the descriptions contained in Table 1 with the cases taken into account in the Sun:setSoilAbovePeg function, it is easy to see that several cases do not match the desired criteria, but are included anyway.

For example, the first 12 cases that meet caseId.mod(36) >= 24 are

However, according to the whitepaper, only cases where there is excessively high debt and P > 1 at the same time should be taken into account.

The first 8 cases that meet caseId.mod(36) < 8 are

Nevertheless, according to the whitepaper, cases where there is excessively low debt and P > 1 at the same time should be taken into account. In this example, the caseId 8 should be included too.

Impact

Impact: Medium

Additional undesirable cases are included in the Soil issuance calculation.

Most of the relevant cases have already been taken into account.

Likelihood: Medium

The inconsistencies found directly affect the Soil issuance calculation only when P > 1.

Tools Used

Manual Review

Recommended Mitigation

According to the whitepaper, a proper implementation of the Sun:setSoilAbovePeg function is