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Format strings: bug #2: LPRng
From: Chris Evans <chris () SCARY BEASTS ORG>
Date: Tue, 26 Sep 2000 00:57:43 +0100

Hi,

SUMMARY
-------

LPRng is almost certainly vulnerable to remote-root compromise on account
of a format string bug. The flaw is almost identical to the rpc.statd one
I found; namely a faulty syslog() wrapper. This is becoming a very common
flaw.

Details
-------

Here is a code excerpt from:

/LPRng-3.6.22/src/common/errormsg.c, use_syslog()
---
static void use_syslog(int kind, char *msg)
...
# ifdef HAVE_OPENLOG
        /* use the openlog facility */
        openlog(Name, LOG_PID | LOG_NOWAIT, SYSLOG_FACILITY );
        syslog(kind, msg);
        closelog();

# else
    (void) syslog(SYSLOG_FACILITY | kind, msg);
# endif                                                 /* HAVE_OPENLOG */
...
---

Here we see two classic format string bugs. Both syslog() calls are
missing a "%s" format string argument.

But how exploitable is this? Does the daemon log any use supplied
data? Let's do a test on the extremely recently released RedHat7.0, which
has switched to LPRng.

[chris () localhost chris]$ telnet localhost printer
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
Mary had a little lamb.
Connection closed by foreign host.

...

grep lamb /var/log/messages
Sep 24 07:38:36 localhost SERVER[1282]: Dispatch_input: bad request line
'Mary had a little lamb.^M'

...

Well, how obliging of (my particular version) of LPRng to log any input
line I give it.


Just to confirm %'s cause trouble:

Client:
[chris () localhost chris]$ telnet localhost printer
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
%s%s%s%s%s%s%s%s%s%s

Server:
Program received signal SIGSEGV, Segmentation fault.
0x400f7c66 in _IO_vfprintf (s=0x80c53a0,
    format=0xbffff190 "Dispatch_input: bad request line
'%s%s%s%s%s%s%s%s%s%s^M'", ap=0xbfffed0c) at
../sysdeps/i386/i486/bits/string.h:529
(gdb) bt
#0  0x400f7c66 in _IO_vfprintf (s=0x80c53a0,
    format=0xbffff190 "Dispatch_input: bad request line
'%s%s%s%s%s%s%s%s%s%s^M'", ap=0xbfffed0c) at
../sysdeps/i386/i486/bits/string.h:529
#1  0x4017d60b in vsyslog (pri=6,
    fmt=0xbffff190 "Dispatch_input: bad request line
'%s%s%s%s%s%s%s%s%s%s^M'", ap=0xbfffed08) at syslog.c:193
#2  0x4017d447 in syslog (pri=6,
    fmt=0xbffff190 "Dispatch_input: bad request line
'%s%s%s%s%s%s%s%s%s%s^M'")
    at syslog.c:102
...

The program has root privs at this time;

(gdb) print geteuid()
$1 = 4                   <-- initially encouraging
(gdb) print getuid()
$2 = 0                   <-- depressing


FIX
---

- Add "%s" arguments to the syslog() calls
- Firewall the printer port

Exploit
-------

Not my scene. I'm sure some friendly black hat will come out with one
soon, though. Perhaps remembering to credit me this time? ;-)


Cheers
Chris


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  • Format strings: bug #2: LPRng Chris Evans (Sep 26)
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