SecuriTeam - SNMPPd Format String Vulnerability

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CanSecWest 2012

2nd Annual Cyber Security

Hack In Paris

snmppd is "an SNMP proxy daemon that is designed to work with Nagios. It loads MIBs upon startup, listens on a TCP socket for SNMP GET requests, polls the specified host, and returns the value to caller process. The caller process is usually the Nagios plugin check_snmpp".

A format string vulnerability has been discovered in SNMP Proxy daemon, allowing a remote attacker to cause the program to execute arbitrary code.

Credit:
The information has been provided by dong-hun you.
The original article can be found at: http://www.inetcop.net

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Vulnerable Systems:
* snmppd version 0.4.5 and prior

Vulnerable code:
The following code can be found in the file /snmppd-0.4.5/snmppd/snmppd_util.c:
    --
        ...
    23 void snmppd_log(int level, char *fmt, va_list args)
    24 {
    25 char buffer[2048];
    26 vsnprintf(buffer, 2048, fmt, args);
    27 if (config.debug || config.nofork) {
        ...
    38 } else {
    39 if (level >= LOG_INFO) {
    40 syslog(level, buffer);
    41 }
    42 }
    43 }
        ...
    45 void log_error(char *fmt, ...)
    46 {
        ...
    50 snmppd_log(LOG_ERR, fmt, args);
        ...
    52 }
    53
    54 void log_info(char *fmt, ...)
    55 {
        ...
    59 snmppd_log(LOG_INFO, fmt, args);
        ...
    61 }
    62
    63 void log_debug(char *fmt, ...)
    64 {
        ...
    69 snmppd_log(LOG_DEBUG, fmt, args);
        ...
    72 }
    --

As can be seen the syslog() function is called without a constant format string, further the format string is the user provided data, allowing an attacker to cause the program to execute arbitrary code.

Patch:
snmppd 0.4.5 patch:

=== snmppd-0.4.5.patch ===
--- snmppd_util.c       Thu Jul 11 18:52:34 2002
+++ patch/snmppd_util.c Sat Mar  5 23:54:54 2005
@@ -37,7 +37,7 @@
        fputc('\n', stdout);
     } else {
        if (level >= LOG_INFO) {
-           syslog(level, buffer);
+           syslog(level, "%s", buffer);
        }
     }
}

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