GoKawiilMuPDF and Foxit found to generate weak AES-256 keys for encrypted PDFs
Researchers found that several PDF applications claiming AES-256 encryption actually derive their 32-byte file keys from a tiny or predictable pool of randomness. MuPDF, before version 1.28.5, seeded its random number generator with only the current time in seconds, leaving just 9 bits of effective entropy in the resulting key, while Foxit was found to base its key generation on time-based randomness with similar weaknesses.