Don't report DROWN from TLS 1.0 and TLS 1.1 alone

src/tls_analyzer.py

@@ -146,8 +146,7 @@ def probe_protocols(host: str, port: int) -> tuple[list[str], list[str]]:
 
     ``deprecated`` contains only protocols whose support was *confirmed* by a
     successful, version-verified handshake — never an untestable placeholder.
-    This keeps downstream counts (DROWN heuristic, grading) free of false
-    positives.
+    This keeps grading (and any SSLv2-gated checks) free of false positives.
     """
     supported = []
     deprecated = []
@@ -410,17 +409,22 @@ def check_crime(sock: ssl.SSLSocket) -> Optional[TLSFinding]:
     return None
 
 def check_drown(deprecated: list[str]) -> Optional[TLSFinding]:
-    """DROWN: SSLv2 support on same key."""
-    # Can't reliably test SSLv2 from Python; flag if very old protocols found
-    if len(deprecated) >= 2:
-        return TLSFinding(
-            severity="CRITICAL", cvss=9.8, cve="CVE-2016-0800",
-            title="DROWN — Decrypting RSA with Obsolete and Weakened eNcryption",
-            detail="Multiple deprecated protocol versions detected. If SSLv2 is also "
-                   "enabled on this host or shares a key with an SSLv2-enabled server, "
-                   "DROWN attack can decrypt RSA-encrypted TLS sessions."
-        )
-    return None
+    """DROWN (CVE-2016-0800) requires SSLv2 on the host or a shared RSA key.
+
+    Python cannot negotiate SSLv2, so this check must not infer DROWN from
+    TLS 1.0+TLS 1.1 (the only deprecated versions we can actually probe).
+    Those are common on legacy stacks and are already reported as MEDIUM
+    deprecated-protocol findings. Treating them as confirmed DROWN produced
+    a false CRITICAL / grade F on almost every dual-stack TLS 1.0/1.1 host.
+    """
+    if "SSLv2" not in deprecated:
+        return None
+    return TLSFinding(
+        severity="CRITICAL", cvss=9.8, cve="CVE-2016-0800",
+        title="DROWN — Decrypting RSA with Obsolete and Weakened eNcryption",
+        detail="SSLv2 is enabled. DROWN can decrypt RSA-encrypted TLS sessions "
+               "that share this key, including modern TLS connections."
+    )
 
 
 # ─── Grading ─────────────────────────────────────────────────────────────────────

test_tls.py

@@ -240,7 +240,7 @@ def test_analyze_cipher_empty():
 def test_probe_deprecated_excludes_placeholder(monkeypatch):
     # Force the "missing constant" path for TLSv1 and ensure it does NOT leak
     # an untestable placeholder into the deprecated list (which would inflate
-    # the DROWN heuristic and grading).
+    # grading).
     monkeypatch.setitem(tls._PROTO_MAP, "TLSv1", None)
     # Pretend nothing else negotiates either, to isolate the placeholder path.
     monkeypatch.setattr(tls, "_try_connect", lambda *a, **k: None)
@@ -260,6 +260,35 @@ def fake_connect(host, port, min_v, max_v, timeout=5.0, expected_version=None):
     assert "TLSv1.2" in supported and "TLSv1.3" in supported
 
 
+# ─── DROWN must not fire on TLS 1.0+1.1 (CVE-2016-0800 requires SSLv2) ─────────
+
+def test_drown_not_inferred_from_tls10_and_tls11():
+    # The common legacy stack that still offers TLS 1.0 and 1.1. probe_protocols
+    # puts both in `deprecated`; that must not become a confirmed DROWN CRITICAL.
+    assert tls.check_drown(["TLSv1.0", "TLSv1.1"]) is None
+    assert tls.check_drown(["TLSv1.0"]) is None
+    assert tls.check_drown([]) is None
+
+
+def test_drown_only_when_sslv2_confirmed():
+    finding = tls.check_drown(["SSLv2", "TLSv1.0"])
+    assert finding is not None
+    assert finding.severity == "CRITICAL"
+    assert finding.cve == "CVE-2016-0800"
+
+
+def test_probe_tls10_tls11_does_not_emit_drown(monkeypatch):
+    """Full analyzer path: a host that speaks TLS 1.0+1.1 (and 1.2) is not DROWN."""
+    def fake_connect(host, port, min_v, max_v, timeout=5.0, expected_version=None):
+        return expected_version
+    monkeypatch.setattr(tls, "_try_connect", fake_connect)
+    _supported, deprecated = tls.probe_protocols("198.51.100.1", 443)
+    finding = tls.check_drown(deprecated)
+    assert finding is None
+    # Grade without the false CRITICAL is D (two deprecated protocols), not F.
+    assert tls.calculate_grade([], deprecated, None) == "D"
+
+
 # ─── grading ────────────────────────────────────────────────────────────────────
 
 def test_grade_clean_is_a():