Signal, the mobile messaging app recommended by NSA leaker Edward Snowden and a large number of security professionals, just fixed a bug that allowed attackers to tamper with the contents of encrypted messages sent by Android users.

The authentication-bypass vulnerability was one of two weaknesses found by researchers Jean-Philippe Aumasson and Markus Vervier in an informal review of the Java code used by the Android version of Signal. The bug made it possible for attackers who compromised a Signal server or were otherwise able to monitor data passing between Signal users to replace a valid attachment with a fraudulent one. A second bug possibly would have allowed attackers to remotely execute malicious code, but a third bug made limited exploits to a simple remote crash.

“The results are not catastrophic, but show that, like any piece of software, Signal is not perfect,” Aumasson wrote in an e-mail. “Signal drew the attention of many security researchers, and it’s impressive that no vulnerability was ever published until today. This pleads in favor of Signal, and we’ll keep trusting it.”