Submitted by HE1M, also found by HollaDieWaldfee
An attacker can apply grieving attack by preventing users from interacting with some of the protocol functions. In other words whenever a user is going to reduce his debt, or buy and reduce his debt in one tx, it can be failed by the attacker.
In the following scenario, I am explaining how it is possible to fail users transaction to reduce their debt fully. Failing other transactions (buy and reduce the debt in one tx) can be done similarly.
https://github.com/with-backed/papr/blob/9528f2711ff0c1522076b9f93fba13f88d5bd5e6/src/PaprController.sol#L148
https://github.com/with-backed/papr/blob/9528f2711ff0c1522076b9f93fba13f88d5bd5e6/src/PaprController.sol#L481
https://github.com/with-backed/papr/blob/9528f2711ff0c1522076b9f93fba13f88d5bd5e6/src/PaprController.sol#L486
https://github.com/with-backed/papr/blob/9528f2711ff0c1522076b9f93fba13f88d5bd5e6/src/PaprController.sol#L487
In summary, Bob could prevent the user from paying her debt fully by just repaying a very small amount of the users debt in advance and as a result causing underflow error. Bob can apply this attack for all other users who are going to repay their debt fully. Please note that if a user is going to repay her debt partially, the attack can be expensive and not financially reasonable, but in case of full repayment of debt, it is very cheap to apply this grieving attack.
This attack can be applied on the transactions that are going to interact with the function _reduceDebt. The transactions interacting with this specific function are:
https://github.com/with-backed/papr/blob/9528f2711ff0c1522076b9f93fba13f88d5bd5e6/src/PaprController.sol#L229
https://github.com/with-backed/papr/blob/9528f2711ff0c1522076b9f93fba13f88d5bd5e6/src/PaprController.sol#L149
It means that the attacker can prevent users from calling the functions above.
The following condition should be added to the function _reduceDebtWithoutBurn:
https://github.com/with-backed/papr/blob/9528f2711ff0c1522076b9f93fba13f88d5bd5e6/src/PaprController.sol#L486
wilsoncusack (Backed) confirmed 
