diff --git a/raven/security/network.py b/raven/security/network.py
index 42bf28a9..a4e7406d 100644
--- a/raven/security/network.py
+++ b/raven/security/network.py
@@ -23,8 +23,32 @@
     ipaddress.ip_network("fe80::/10"),
 ]
 
+_NAT64_WELL_KNOWN = ipaddress.ip_network("64:ff9b::/96")
+
+
+def _embedded_ipv4(addr: ipaddress.IPv6Address) -> ipaddress.IPv4Address | None:
+    """The embedded IPv4 destination of an IPv6-to-IPv4 transition address.
+
+    IPv4-mapped IPv6 (::ffff:a.b.c.d), 6to4 (2002::/16), and NAT64
+    (64:ff9b::/96) all route to an IPv4 destination, so the guard must
+    judge that destination rather than the IPv6 literal spelling.
+    """
+    mapped = getattr(addr, "ipv4_mapped", None)
+    if mapped is not None:
+        return mapped
+    sixtofour = getattr(addr, "sixtofour", None)
+    if sixtofour is not None:
+        return sixtofour
+    if addr in _NAT64_WELL_KNOWN:
+        return ipaddress.IPv4Address(int(addr) & 0xFFFFFFFF)
+    return None
+
 
 def _is_private(addr: ipaddress.IPv4Address | ipaddress.IPv6Address) -> bool:
+    if isinstance(addr, ipaddress.IPv6Address):
+        embedded = _embedded_ipv4(addr)
+        if embedded is not None:
+            addr = embedded
     return any(addr in net for net in _BLOCKED_NETWORKS)
 
 
diff --git a/tests/test_security_network.py b/tests/test_security_network.py
index 02f35eec..f3d7cc35 100644
--- a/tests/test_security_network.py
+++ b/tests/test_security_network.py
@@ -55,6 +55,60 @@ def test_blocks_unique_local_v6() -> None:
     assert "private/internal" in err
 
 
+def test_blocks_v4_mapped_loopback() -> None:
+    ok, err = net.validate_url_target("http://[::ffff:127.0.0.1]/x")
+    assert not ok
+    assert "private/internal" in err
+
+
+@pytest.mark.parametrize(
+    "url",
+    [
+        "http://[::ffff:10.0.0.1]/x",
+        "http://[::ffff:7f00:1]/x",  # hex form of ::ffff:127.0.0.1
+        "http://[::ffff:169.254.169.254]/latest/meta-data/",
+    ],
+)
+def test_blocks_v4_mapped_private(url: str) -> None:
+    ok, err = net.validate_url_target(url)
+    assert not ok, f"should block {url}"
+    assert "private/internal" in err
+
+
+def test_allows_v4_mapped_public() -> None:
+    ok, err = net.validate_url_target("http://[::ffff:93.184.216.34]/x")
+    assert ok, f"unexpectedly blocked: {err}"
+
+
+@pytest.mark.parametrize(
+    "url",
+    [
+        "http://[2002:7f00:1::]/x",  # 6to4 embedding 127.0.0.1
+        "http://[64:ff9b::7f00:1]/x",  # NAT64 well-known prefix -> 127.0.0.1
+        "http://[2002:a00:1::]/x",  # 6to4 embedding 10.0.0.1
+        "http://[64:ff9b::a9fe:a9fe]/x",  # NAT64 -> 169.254.169.254 (metadata)
+    ],
+)
+def test_blocks_transition_prefixes_with_private_embedded(url: str) -> None:
+    ok, err = net.validate_url_target(url)
+    assert not ok, f"should block {url}"
+    assert "private/internal" in err
+
+
+@pytest.mark.parametrize(
+    "url",
+    [
+        "http://[2002:5db8:d822::]/x",  # 6to4 embedding public 93.184.216.34
+        "http://[64:ff9b::5db8:d822]/x",  # NAT64 -> public 93.184.216.34
+    ],
+)
+def test_allows_transition_prefixes_with_public_embedded(url: str) -> None:
+    """DNS64 networks synthesize NAT64/6to4 addresses for legitimate public
+    IPv4 services; those must not be blocked wholesale."""
+    ok, err = net.validate_url_target(url)
+    assert ok, f"unexpectedly blocked: {err}"
+
+
 @pytest.mark.parametrize(
     "url",
     [
@@ -115,6 +169,12 @@ def test_resolved_blocks_private_ip_literal() -> None:
     assert "private" in err
 
 
+def test_resolved_blocks_v4_mapped_private() -> None:
+    ok, err = net.validate_resolved_url("http://[::ffff:10.0.0.1]/x")
+    assert not ok
+    assert "private" in err
+
+
 def test_resolved_blocks_private_via_dns(monkeypatch: pytest.MonkeyPatch) -> None:
     """Redirect target is a hostname that resolves to a private IP → block."""
     _mock_resolve(monkeypatch, "10.0.0.1")