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snmp: Validate input data #431

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wants to merge 2 commits into from
Closed

snmp: Validate input data #431

wants to merge 2 commits into from

Commits on Jul 19, 2022

  1. snmp: Validate length fields before use 

    The snmp code does not prevent overflows during pointer arithmetics on
32 bit architectures. Check length before performing additions.

This can lead to out of boundary accesses or endless loops if
debug output is enabled (proof of concept exists for OOB access).

Proof of Concept:

1. Create and run server with crashing payload

```
cat > poc.py << EOF
from socket import AF_INET, SOCK_DGRAM, socket
s=socket(AF_INET, SOCK_DGRAM)
s.bind(('',161))
print('Waiting for incoming SNMP data')
m, addr=s.recvfrom(1024)
print('Replying with crash payload')
s.sendto(b'\x30\x07\x02\x84\xDE\xAD\xBA\xBE\x00', addr)
EOF
sudo python poc.py
```

2. Run snmp backend (or try to "Add Printer")

`/usr/lib/cups/backend/snmp localhost`
    @ferivoz
    ferivoz committed Jul 19, 2022
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  2. snmp: Prevent signed integer overflows 

    Shifting bits into highest bit of integers is undefined behavior
according to C standards.

It can be easily prevented by using an unsigned type during shifts,
casting to signed at the end.
    @ferivoz
    ferivoz committed Jul 19, 2022
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