Submitted by 0xluk3, also found by 0xcb90f054, den-sosnowsky, gss1, Heavyweight_hunters, and p4y4b13
https://github.com/code-423n4/2025-01-initia-move/blob/main/usernames-module/sources/name_service.move#L603
Payment for domains (registration, extensions) relies on direct spot price from the Dex module which is directly related to pool reserves. This can be manipulated with a flash loan or a large amount deposit, resulting in: 
Calculating the price based directly on a liquidity pool reserves is a well known insecure pattern.
In usernames module, in function get_cost_amount, it calls dex module in line 603:
let spot_price = dex::get_spot_price(object::address_to_object<PairConfig>(@pair), get_init_metadata());
Function get_spot_price in dex:
The function uses the pool reserves amounts to calculate the price. Please note, that even if that dex module would implement any lock during the loan, the funds used for manipulation might come from other source, e.g. direct deposit or another dex existing in the future, allowing flash loans.
Use a TWAP price source instead, or use an oracle, e.g. Slinky to calculate the price.
andrew (Initia) confirmed and commented:
