&gt;This particular text isn&#x27;t a vulnerability (after all, HTTP is full of known plaintext).<p>This isn&#x27;t quite a proof. You&#x27;d need to show that HTTP is not vulnerable to such an attack.<p>I don&#x27;t think HTTP <i>requires</i> a starting block.<p>HTTP2 apparently <i>requires</i> a client&#x27;s first message to be&#x2F;start with:<p><pre><code>    0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
</code></pre>
I&#x27;d guess that is much longer than any block that HTTP mandates.<p>Maybe I am reading things wrong?