Are USB extension cords safe to use for overnight charging?

Yes, if you buy a half-decent one. A UL-listed or certified extension cord with USB ports is safe to leave plugged in, as the power conversion has built-in overcurrent and overheating protection. The risk comes from cheap knockoffs that skip safety certifications, so stick with known brands like Belkin, Anker, GE, or Leviton. Don’t bury it under blankets or run it through a wall, and avoid it if it feels hot.