C++: Add qhelp for result conflation query.

cpp/ql/src/Security/CWE/CWE-295/SSLResultConflation.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+<overview>
+<p>When checking the result of SSL certificate verification, accepting any error code may allow an attacker to impersonate someone who is trusted.</p>
+
+</overview>
+<recommendation>
+
+<p>When checking an SSL certificate with <code>SSL_get_verify_result</code>, only <code>X509_V_OK</code> is a success code. If there is any other result the certificate should not be accepted.</p>
+
+</recommendation>
+<example>
+
+<p>In this example the error code <code>X509_V_ERR_CERT_HAS_EXPIRED</code> is treated the same as an OK result. An expired certificate should not be accepted as it is more likely to be compromised than a valid certificate.</p>
+
+<sample src="SSLResultConflationBad.cpp" />
+
+<p>In the corrected example, only a result of <code>X509_V_OK</code> is accepted.</p>
+
+<sample src="SSLResultConflationGood.cpp" />
+
+</example>
+<references>
+
+</references>
+</qhelp>

cpp/ql/src/Security/CWE/CWE-295/SSLResultConflationBad.cpp

@@ -0,0 +1,13 @@
+// ...
+
+if (cert = SSL_get_peer_certificate(ssl))
+{
+	result = SSL_get_verify_result(ssl);
+
+	if ((result == X509_V_OK) || (result == X509_V_ERR_CERT_HAS_EXPIRED)) // BAD (conflates OK and a non-OK codes)
+	{
+		do_ok();
+	} else {
+		do_error();
+	}
+}

cpp/ql/src/Security/CWE/CWE-295/SSLResultConflationGood.cpp

@@ -0,0 +1,13 @@
+// ...
+
+if (cert = SSL_get_peer_certificate(ssl))
+{
+	result = SSL_get_verify_result(ssl);
+
+	if (result == X509_V_OK) // GOOD
+	{
+		do_ok();
+	} else {
+		do_error();
+	}
+}