sqlmapproject/sqlmap · error · InterfaceError

malformed backend message: %s

Error message

malformed backend message: %s

What it means

Error "malformed backend message: %s" thrown in sqlmapproject/sqlmap.

Source

Thrown at extra/dbwire/postgres.py:221

                                try:
                                    row.append(raw.decode("utf-8"))
                                except UnicodeDecodeError:
                                    row.append(raw)  # non-UTF-8 (e.g. a SQL_ASCII db): keep bytes (hex-encoded), not lossy U+FFFD
                    rows.append(tuple(row))
                elif mtype == b"C":  # CommandComplete ("SELECT 3", "INSERT 0 1", ...)
                    tag = payload[:-1].decode("utf-8", "replace").split()
                    if tag and tag[-1].isdigit():
                        rowcount = int(tag[-1])
                elif mtype == b"G":  # CopyInResponse - server now waits for client CopyData; refuse to avoid a deadlock
                    _send(self._sock, b"f", b"COPY FROM STDIN is not supported\x00")  # CopyFail
                elif mtype == b"E":  # ErrorResponse
                    error = _error_message(payload)
                elif mtype == b"Z":  # ReadyForQuery (end of response); payload byte = transaction status
                    self._txn_status = payload[:1] or b"I"
                    break
                # ParameterStatus(S)/NoticeResponse(N)/EmptyQueryResponse(I)/CopyData(d)/CopyDone(c)/... ignored
            except (struct.error, IndexError, ValueError) as ex:
                raise InterfaceError("malformed backend message: %s" % ex)
        if error is not None:
            _raise_server_error(*error)
        return description, rows, 0, rowcount

def _authenticate(sock, user, password):
    client_nonce = cfirst_bare = salted = auth_message = None
    while True:
        mtype, payload = _read_message(sock)
        if mtype in (b"N", b"S"):  # NoticeResponse / ParameterStatus may legally precede AuthenticationOk
            continue
        if mtype == b"E":
            _raise_server_error_as_operational(payload)
        if mtype != b"R":
            raise InterfaceError("unexpected message %r during authentication" % mtype)
        (code,) = struct.unpack("!I", payload[:4])
        if code == 0:  # AuthenticationOk (also the trust case)
            return
        elif code == 3:  # cleartext password

View on GitHub (pinned to 0a35b20e39)

When it happens

Trigger: Thrown at extra/dbwire/postgres.py:221 when the library encounters an invalid state.

Common situations: See trigger scenarios.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

AI-assisted analysis of sqlmapproject/sqlmap@0a35b20e39 (2026-08-26). Data as JSON: /api/errors/253291c0b8456657. Report an issue: GitHub.