The vulnerability stems from the package's core get/set functionality handling user-supplied paths without prototype pollution protections. The commit diff shows added validation for proto, prototype and constructor keys in both get and set operations. Prior to patch 1.1.1, the functions: 1) In get(): Line 14 lacked isSafeKey check, allowing prototype property access 2) In set(): Line 26 lacked early return for unsafe keys, enabling prototype modification. The main exported 'deep' function acts as a router to these vulnerable get/set implementations based on argument count.