Handle malformed identify varint prefixes

libp2p/identity/identify/identify.py

@@ -12,6 +12,9 @@
     StreamHandlerFn,
     TProtocol,
 )
+from libp2p.exceptions import (
+    ParseError,
+)
 from libp2p.network.stream.exceptions import (
     StreamClosed,
 )
@@ -87,17 +90,21 @@ def parse_identify_response(response: bytes) -> Identify:
     """
     # Try new format first: length-prefixed protobuf
     if len(response) >= 1:
-        length, varint_size = decode_varint_with_size(response)
-        if varint_size > 0 and length > 0 and varint_size + length <= len(response):
-            protobuf_data = response[varint_size : varint_size + length]
-            try:
-                identify_response = Identify()
-                identify_response.ParseFromString(protobuf_data)
-                # Sanity check: must have agent_version (protocol_version is optional)
-                if identify_response.agent_version:
-                    return identify_response
-            except Exception:
-                pass  # Fall through to old format
+        try:
+            length, varint_size = decode_varint_with_size(response)
+        except ParseError:
+            pass  # Fall through to old format
+        else:
+            if varint_size > 0 and length > 0 and varint_size + length <= len(response):
+                protobuf_data = response[varint_size : varint_size + length]
+                try:
+                    identify_response = Identify()
+                    identify_response.ParseFromString(protobuf_data)
+                    # Sanity check: must have agent_version.
+                    if identify_response.agent_version:
+                        return identify_response
+                except Exception:
+                    pass  # Fall through to old format
 
     # Fall back to old format: raw protobuf
     try:

newsfragments/1328.bugfix.rst

@@ -0,0 +1 @@
+Allowed identify response parsing to fall back to raw protobuf messages when the length-prefix probe raises ``ParseError``.

tests/core/identity/identify/test_identify.py

@@ -5,6 +5,10 @@
     Multiaddr,
 )
 
+from libp2p.exceptions import (
+    ParseError,
+)
+from libp2p.identity.identify import identify as identify_module
 from libp2p.identity.identify.identify import (
     AGENT_VERSION,
     ID,
@@ -22,6 +26,27 @@
 logger = logging.getLogger("libp2p.identity.identify-test")
 
 
+def test_parse_identify_response_falls_back_after_prefix_parse_error(monkeypatch):
+    raw_response = identify_module.Identify(
+        agent_version=AGENT_VERSION,
+        protocol_version=PROTOCOL_VERSION,
+    ).SerializeToString()
+
+    def raise_parse_error(data: bytes) -> tuple[int, int]:
+        raise ParseError("Unexpected end of data while decoding varint")
+
+    monkeypatch.setattr(
+        identify_module,
+        "decode_varint_with_size",
+        raise_parse_error,
+    )
+
+    identify_response = parse_identify_response(raw_response)
+
+    assert identify_response.agent_version == AGENT_VERSION
+    assert identify_response.protocol_version == PROTOCOL_VERSION
+
+
 @pytest.mark.trio
 async def test_identify_protocol(security_protocol):
     async with host_pair_factory(security_protocol=security_protocol) as (