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<a href="https://gitlab.com/dueno">Daiki Ueno</a>
commented:
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<p dir="auto">I would say this is the correct behavior according to the <a href="https://tools.ietf.org/html/rfc8446#section-4.1.3" rel="nofollow noreferrer noopener" target="_blank">spec</a>:</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext"> TLS 1.3 has a downgrade protection mechanism embedded in the server's</span>
<span id="LC2" class="line" lang="plaintext"> random value. TLS 1.3 servers which negotiate TLS 1.2 or below in</span>
<span id="LC3" class="line" lang="plaintext"> response to a ClientHello MUST set the last 8 bytes of their Random</span>
<span id="LC4" class="line" lang="plaintext"> value specially in their ServerHello.</span>
<span id="LC5" class="line" lang="plaintext"></span>
<span id="LC6" class="line" lang="plaintext"> If negotiating TLS 1.2, TLS 1.3 servers MUST set the last 8 bytes of</span>
<span id="LC7" class="line" lang="plaintext"> their Random value to the bytes:</span>
<span id="LC8" class="line" lang="plaintext"></span>
<span id="LC9" class="line" lang="plaintext"> 44 4F 57 4E 47 52 44 01</span>
<span id="LC10" class="line" lang="plaintext"></span>
<span id="LC11" class="line" lang="plaintext"> [...]</span>
<span id="LC12" class="line" lang="plaintext"></span>
<span id="LC13" class="line" lang="plaintext"> TLS 1.3 clients receiving a ServerHello indicating TLS 1.2 or below</span>
<span id="LC14" class="line" lang="plaintext"> MUST check that the last 8 bytes are not equal to either of these</span>
<span id="LC15" class="line" lang="plaintext"> values. [...]</span>
<span id="LC16" class="line" lang="plaintext"> If a match is found, the client MUST abort the handshake with</span>
<span id="LC17" class="line" lang="plaintext"> an "illegal_parameter" alert.</span></code></pre>
<p dir="auto">If you enable TLS 1.3 in the priority string, the peer behaves as a TLS 1.3 server or client so the downgrade protection will be in effect.</p>
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