malformed «protocol»
The malformed protocol isn’t a real protocol itself, but used by Wireshark to indicate a problem while dissecting the packet data. You could think of it as a pseudo dissector.
While Wireshark dissects the packet data, the protocol dissector in charge tried to read from the packet data at an offset simply not existing. This raised an internal Exception, leading to this malformed indication.
There are three main causes:
- protocol data is malformed
- protocol dissector is buggy
- wrong protocol dissector used
It’s difficult to say (in a general way) which is the real cause in a given scenario, without looking at the packet data and having some knowledge of the protocol (dissector) involved.
History
This feature exists in Wireshark since version 0.9.0.
Protocol dependencies
This pseudo-protocol can happen at any protocol dissector.
Example traffic
XXX — add example traffic showing malformed.
Wireshark
The malformed dissector is «fully functional»
Preference Settings
There are no preference settings affecting how malformed is dissected.
Example capture file
XXX — add a capture file example.
Display Filter
There are no display filter fields for malformed, see: display filter reference. You can simply filter on malformed to see all packets conaining malformed data:
Example: Show only malformed packets:
malformed
Capture Filter
A capture filter for the malformed pseudo protocol wouldn’t make sense, as the malformed status isn’t detected while capturing.
External links
- there are no external links
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Appendix A. Wireshark Messages
Wireshark provides you with additional information generated out of the plain packet data or it may need to indicate dissection problems. Messages generated by Wireshark are usually placed in square brackets (“[]”).
A.1. Packet List Messages
These messages might appear in the packet list.
A.1.1. [Malformed Packet]
Malformed packet means that the protocol dissector can’t dissect the contents of the packet any further. There can be various reasons:
- Wrong dissector : Wireshark erroneously has chosen the wrong protocol dissector for this packet. This will happen e.g., if you are using a protocol not on its well known TCP or UDP port. You may try Analyze|Decode As to circumvent this problem.
- Packet not reassembled : The packet is longer than a single frame and it is not reassembled, see Section 7.8, “Packet Reassembly” for further details.
- Packet is malformed : The packet is actually wrong (malformed), meaning that a part of the packet is just not as expected (not following the protocol specifications).
- Dissector is buggy : The corresponding protocol dissector is simply buggy or still incomplete.
Any of the above is possible. You’ll have to look into the specific situation to determine the reason. You could disable the dissector by disabling the protocol on the Analyze menu and check how Wireshark displays the packet then. You could (if it’s TCP) enable reassembly for TCP and the specific dissector (if possible) in the Edit|Preferences menu. You could check the packet contents yourself by reading the packet bytes and comparing it to the protocol specification. This could reveal a dissector bug. Or you could find out that the packet is indeed wrong.
A.1.2. [Packet size limited during capture]
The packet size was limited during capture, see “Limit each packet to n bytes” at the Section 4.5, “The “Capture Options” Dialog Box”. While dissecting, the current protocol dissector was simply running out of packet bytes and had to give up. There’s nothing else you can do now, except to repeat the whole capture process again with a higher (or no) packet size limitation.
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What are the criteria used by Wireshark for “Malformed” ?
I’ve searched around the internet but I could not find an answer matching exactly what I’m looking for. I’m currently working on fuzzing network protocols, with different softwares. Some are fully automated (but are not free), some are open-source but it requires I write my own scripts. Since I want to create errors with the open sources ones, I’m trying to create «Malformed packets» so the devices I’m testing have to deal with malformed data.
For my tests, I’m fuzzing ARP protocol. When I’m observing «Malformed packets» with fully automated, test version of a fuzzer, I only have full well-formed packets with my self-writted fuzzing scripts. I observed differences beetween the two kinds of packets, but I couldn’t find out what is the difference that let Wireshark say it’s «Malformed» or it’s not.
Here is an example of two similar packets which I cannot understand why there is a malformed one and a «clean» one.
What is the real difference beetween those two packets ? I can see the payload is starting 4 bytes earlier on the malformed one, but why isn’t Wireshark saying that the «opcode» is unknown ? When I generate a packet with my open source software, I can put any byte I want anywhere so the packet is the most malformed ARP packet you’ve ever seen, Wireshark will still say the code is unknown, not that the packet is malformed.
Has anyone an answer ? Thank you for your attention
arp malformed fuzzing
asked 23 May ’16, 09:19
Pygmay
11 ● 1 ● 1 ● 4
accept rate: 0%