Handle invalid buffers at protocol level

See #4042.

telethon/network/connection/connection.py

@@ -13,7 +13,7 @@ try:
 except ImportError:
     python_socks = None
 
-from ...errors import InvalidChecksumError
+from ...errors import InvalidChecksumError, InvalidBufferError
 from ... import helpers
 
 
@@ -336,6 +336,9 @@ class Connection(abc.ABC):
                 elif isinstance(e, InvalidChecksumError):
                     msg = 'The server response had an invalid checksum'
                     self._log.info(msg)
+                elif isinstance(e, InvalidBufferError):
+                    msg = 'The server response had an invalid buffer'
+                    self._log.error(msg)
                 else:
                     msg = 'Unexpected exception in the receive loop'
                     self._log.exception(msg)

telethon/network/connection/tcpfull.py

@@ -2,7 +2,7 @@ import struct
 from zlib import crc32
 
 from .connection import Connection, PacketCodec
-from ...errors import InvalidChecksumError
+from ...errors import InvalidChecksumError, InvalidBufferError
 
 
 class FullPacketCodec(PacketCodec):
@@ -24,6 +24,13 @@ class FullPacketCodec(PacketCodec):
     async def read_packet(self, reader):
         packet_len_seq = await reader.readexactly(8)  # 4 and 4
         packet_len, seq = struct.unpack('<ii', packet_len_seq)
+        if packet_len < 0 and seq < 0:
+            # It has been observed that the length and seq can be -429,
+            # followed by the body of 4 bytes also being -429.
+            # See https://github.com/LonamiWebs/Telethon/issues/4042.
+            body = await reader.readexactly(4)
+            raise InvalidBufferError(body)
+
         body = await reader.readexactly(packet_len - 8)
         checksum = struct.unpack('<I', body[-4:])[0]
         body = body[:-4]