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Qualys Security Advisory  
  
LPE and RCE in OpenSMTPD's default install (CVE-2020-8794)  
  
  
==============================================================================  
Contents  
==============================================================================  
  
Summary  
Analysis  
...  
Acknowledgments  
  
  
==============================================================================  
Summary  
==============================================================================  
  
We discovered a vulnerability in OpenSMTPD, OpenBSD's mail server. This  
vulnerability, an out-of-bounds read introduced in December 2015 (commit  
80c6a60c, "when peer outputs a multi-line response ..."), is exploitable  
remotely and leads to the execution of arbitrary shell commands: either  
as root, after May 2018 (commit a8e22235, "switch smtpd to new  
grammar"); or as any non-root user, before May 2018.  
  
Because this vulnerability resides in OpenSMTPD's client-side code  
(which delivers mail to remote SMTP servers), we must consider two  
different scenarios:  
  
- Client-side exploitation: This vulnerability is remotely exploitable  
in OpenSMTPD's (and hence OpenBSD's) default configuration. Although  
OpenSMTPD listens on localhost only, by default, it does accept mail  
from local users and delivers it to remote servers. If such a remote  
server is controlled by an attacker (either because it is malicious or  
compromised, or because of a man-in-the-middle, DNS, or BGP attack --  
SMTP is not TLS-encrypted by default), then the attacker can execute  
arbitrary shell commands on the vulnerable OpenSMTPD installation.  
  
- Server-side exploitation: First, the attacker must connect to the  
OpenSMTPD server (which accepts external mail) and send a mail that  
creates a bounce. Next, when OpenSMTPD connects back to their mail  
server to deliver this bounce, the attacker can exploit OpenSMTPD's  
client-side vulnerability. Last, for their shell commands to be  
executed, the attacker must (to the best of our knowledge) crash  
OpenSMTPD and wait until it is restarted (either manually by an  
administrator, or automatically by a system update or reboot).  
  
We developed a simple exploit for this vulnerability and successfully  
tested it against OpenBSD 6.6 (the current release), OpenBSD 5.9 (the  
first vulnerable release), Debian 10 (stable), Debian 11 (testing), and  
Fedora 31. At OpenBSD's request, and to give OpenSMTPD's users a chance  
to patch their systems, we are withholding the exploitation details and  
code until Wednesday, February 26, 2020.  
  
Last-minute note: we tested our exploit against the recent changes in  
OpenSMTPD 6.6.3p1, and our results are: if the "mbox" method is used for  
local delivery (the default in OpenBSD -current), then arbitrary command  
execution as root is still possible; otherwise (if the "maildir" method  
is used, for example), arbitrary command execution as any non-root user  
is possible.  
  
  
==============================================================================  
Analysis  
==============================================================================  
  
SMTP clients connect to SMTP servers and send commands such as EHLO,  
MAIL FROM, and RCPT TO. SMTP servers respond with either single-line or  
multiple-line replies:  
  
- the first lines begin with a three-digit code and a hyphen ('-'),  
followed by an optional text (for example, "250-ENHANCEDSTATUSCODES");  
  
- the last line begins with the same three-digit code, followed by an  
optional space (' ') and text (for example, "250 HELP").  
  
In OpenSMTPD's client-side code, these multiline replies are parsed by  
the mta_io() function:  
  
------------------------------------------------------------------------------  
1098 static void  
1099 mta_io(struct io *io, int evt, void *arg)  
1100 {  
....  
1133 case IO_DATAIN:  
1134 nextline:  
1135 line = io_getline(s->io, &len);  
....  
1146 if ((error = parse_smtp_response(line, len, &msg, &cont))) {  
------------------------------------------------------------------------------  
  
- the first lines (when line[3] == '-') are concatenated into a 2KB  
replybuf:  
  
------------------------------------------------------------------------------  
1177 if (cont) {  
1178 if (s->replybuf[0] == '\0')  
1179 (void)strlcat(s->replybuf, line, sizeof s->replybuf);  
1180 else {  
1181 line = line + 4;  
....  
1187 (void)strlcat(s->replybuf, line, sizeof s->replybuf);  
1188 }  
1189 goto nextline;  
1190 }  
------------------------------------------------------------------------------  
  
- the last line (when line[3] != '-') is also concatenated into  
replybuf:  
  
------------------------------------------------------------------------------  
1195 if (s->replybuf[0] != '\0') {  
1196 p = line + 4;  
....  
1201 if (strlcat(s->replybuf, p, sizeof s->replybuf) >= sizeof s->replybuf)  
------------------------------------------------------------------------------  
  
Unfortunately, if the last line's three-digit code is not followed by  
the optional space and text, then p (at line 1196) points to the first  
character *after* the line's '\0' terminator (which replaced the line's  
'\n' terminator in iobuf_getline()), and this out-of-bounds string is  
concatenated into replybuf (at line 1201).  
  
...  
  
  
==============================================================================  
Acknowledgments  
==============================================================================  
  
We thank OpenBSD's developers for their quick response and patches. We  
also thank Gilles for his hard work and beautiful code.  
  
  
  
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