Multiple PDF tools' AES-256 encryption trivially broken—MuPDF key has only 9 bits of entropy
jedisct1 · x · 2026-10-29
Rick de Jager (V12) disclosed that several PDF implementations generate their AES-256 file keys from tiny state spaces, enabling passwordless decryption of protected files. MuPDF versions before 1.28.5 seed drand48 with the epoch in seconds, leaving only 9 bits of random state affecting the actual key—trivially enumerable. Foxit and other editors derive keys from time-based randomness, also easily recovered. The flaw sits in the randomness source protecting the 32-byte file key, not in AES itself.
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