Zerologon (AES-CFB8 Zero-State Keystream Collision)

Updated 19 Sep 2026

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  What it is

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    AES-CFB8 encrypts one byte at a time by encrypting a 16-byte "state" (seeded from the IV) with the block cipher and using only its first output byte as keystream, then shifting the state left one byte and feeding a new byte back in. During decryption, the byte fed back in is the ciphertext byte, not the plaintext byte. If an attacker submits an all-zero ciphertext with an all-zero IV, the state starts at 16 zero bytes and — since it only ever gets zero bytes appended — stays all-zero for the entire decryption. That means every decrypted byte equals the same fixed value: the first byte of $E_K(\mathbf{0})$. That fixed byte is uniformly distributed for a random key, so there's exactly a 1/256 chance it's zero — and when it is, the "decrypted" output (and anything derived from it, like a password) comes out entirely as zero bytes. This is the same vulnerability class as the real-world Zerologon (CVE-2020-1472), where Windows' Netlogon protocol used AES-CFB8 with a zero IV in a way that let an attacker authenticate with roughly 1-in-256 odds per attempt.

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  When to apply

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    A protocol uses AES-CFB8 (or another byte-feedback stream mode) with an attacker-controlled or fixed zero IV, decrypts attacker-supplied all-zero ciphertext, and derives something security-sensitive (a password, a session key) directly from the decrypted bytes.

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  Math

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    For decrypt: $state_{i+1} = state_i[1:] + C_i$​. If $C_i = 0$ for all ii i and $state_0 = \mathbf{0}$, then $state_i = \mathbf{0}$ for all $i$, so every output byte equals $E_K(\mathbf{0})[0]$ — a single fixed value depending only on the key. $P(E_K(\mathbf{0})[0] = 0) = 1/256$ for a uniformly random key.

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  Worked example

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    For one random key, an all-zero CFB8 token under an all-zero IV decrypts to a repeated byte. The attack succeeds when $E_K(0)[0]=0$, with probability about $1/256$ per independent key. After $t$ independent resets, the chance of at least one hit is $1-(255/256)^t$; at $t=177$ it is about $50\%$.

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  Python

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    def cfb8_decrypt_allzero(key, n):
        from Crypto.Cipher import AES
        cipher = AES.new(key, AES.MODE_ECB)
        state = b"\x00" * 16
        out = b""
        for _ in range(n):
            b = cipher.encrypt(state)[0]
            out += bytes([b])
            state = state[1:] + b"\x00"   # ciphertext byte fed back is always 0
        return out
    

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  Related

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    Known-Plaintext Keystream Recovery (Many-Time Pad)

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  Cards

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    Why does the internal CFB8 state stay all-zero throughout decryption of an all-zero ciphertext?

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      Because decryption feeds the ciphertext byte (always 0 here) back into the state, so a state that starts at all-zero never changes.

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    What's the probability a random AES key produces a zero first-output-byte when encrypting an all-zero block?

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      1/256, since AES output bytes are effectively uniformly distributed for a random key.

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