Length-Extension Attack

You hashed a secret-prefix MAC: tag = SHA-256(secret || message). Anyone who knows tag and the message length can forge SHA-256(secret || message || glue || suffix) for ANY suffix they choose — without ever learning secret.

This is why bare SHA-256 (and SHA-1, SHA-512) is unsafe as a MAC, and why HMAC exists.

Why it works

The output of SHA-256 IS the internal state after the last block was absorbed. Given tag = SHA(M), an attacker can:

1. Parse tag into 8 big-endian uint32s — that's the state (a,b,c,d,e,f,g,h).
2. Compute the glue padding that was applied to M: the 0x80 byte, zero padding to 448 mod 512 bits, plus |M| in bits.
3. Continue the SHA-256 algorithm from the recovered state with ANY suffix, padding it as if |M| + |glue| bytes had already been processed.
4. The resulting hash equals SHA(M || glue || suffix).

A concrete attack

secret  = "supersekret"            (unknown to attacker, 11 bytes)
message = "user=guest&role=user"   (visible to attacker)
tag     = SHA-256(secret || message)  (visible — published as the MAC)

The attacker:

state = parse(tag)                 # 8 uint32 words
glue  = pad_for_length(11 + 20)    # 0x80 + zeros + 64-bit (31*8)
forged = continue(state, "&role=admin", processed = 11 + 20 + |glue|)

# Server then computes:
real = SHA-256(secret || message || glue || "&role=admin")
# real == forged. The attacker promoted themselves to admin.

Defenses

Suffix-MAC — SHA-256(M || secret) — is not length-extendable, because the attacker cannot append past the secret. It is still a bad choice: a single collision SHA-256(M) == SHA-256(M') transfers straight to the tags, so its security rests on collision resistance rather than the much weaker assumption HMAC needs.

Always use HMAC when you need a MAC from SHA-2. The double-hash structure of HMAC prevents the attacker from continuing from the published tag.

Real-world incidents

• Flickr API (2009): used MD5(secret || params) for signing — patched by switching to HMAC.
• Ruby on Rails / Rack cookie signing (early versions): signed session cookies with hash(secret || data), so a forged cookie could be extended without the secret; fixed by moving to HMAC.
• Many home-grown auth schemes: any time you see hash(key || ...) in API signing, it's worth checking.