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MQTT: do not crash when MQTTPublish has no underlayer #5089

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MQTT: do not crash when MQTTPublish has no underlayer #5089

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11 changes: 6 additions & 5 deletions scapy/contrib/mqtt.py
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Original file line number Diff line number Diff line change
Expand Up @@ -198,12 +198,13 @@ class MQTTPublish(Packet):
StrLenField("topic", "",
length_from=lambda pkt: pkt.length),
ConditionalField(ShortField("msgid", None),
lambda pkt: (pkt.underlayer.QOS == 1 or
pkt.underlayer.QOS == 2)),
lambda pkt: pkt.underlayer is not None and
pkt.underlayer.QOS in (1, 2)),
StrLenField("value", "",
length_from=lambda pkt: pkt.underlayer.len - pkt.length - 2
if pkt.underlayer.QOS == 0 else
pkt.underlayer.len - pkt.length - 4)
length_from=lambda pkt: 0 if pkt.underlayer is None else
(pkt.underlayer.len - pkt.length - 2
if pkt.underlayer.QOS == 0 else
pkt.underlayer.len - pkt.length - 4))
]


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7 changes: 7 additions & 0 deletions test/contrib/mqtt.uts
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Original file line number Diff line number Diff line change
Expand Up @@ -187,3 +187,10 @@ assert MQTTUnsubscribe in u and len(u.topics) == 2 and u.topics[1].topic == b"c/
= MQTTSubscribe
u = MQTT(b'\x82\x10\x00\x01\x00\x03\x61\x2F\x62\x02\x00\x03\x63\x2F\x64\x00')
assert MQTTSubscribe in u and len(u.topics) == 2 and u.topics[1].topic == b"c/d"

= MQTTPublish without underlayer does not crash
p = MQTTPublish()
p.show()
assert p.topic == b""
assert p.value == b""
assert p.msgid is None
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