double-edged sword

Short definition from Wiktionary

Noun

1. 1
   A benefit that is also a liability, or (a benefit) that carries some significant but not-so-obvious cost or risk.
2. 2
   A neutral principle that has applications that may be either positive (beneficial) or negative (adverse) to one's own interests.
3. 3
   Used other than figuratively or idiomatically: see double-edged, sword.