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Hacking techniques include penetration testing, network security, reverse cracking, malware analysis, vulnerability exploitation, encryption cracking, social engineering, etc., used to identify and fix security flaws in systems.

HireHackking 
Qualys Security Advisory

The Return of the WIZard: RCE in Exim (CVE-2019-10149)


========================================================================
Contents
========================================================================

Summary
Local exploitation
Remote exploitation
- Non-default configurations
- Default configuration
Acknowledgments
Timeline

    Boromir: "What is this new devilry?"
    Gandalf: "A Balrog. A demon of the Ancient World."
        -- The Lord of the Rings: The Fellowship of the Ring


========================================================================
Summary
========================================================================

During a code review of the latest changes in the Exim mail server
(https://en.wikipedia.org/wiki/Exim), we discovered an RCE vulnerability
in versions 4.87 to 4.91 (inclusive). In this particular case, RCE means
Remote *Command* Execution, not Remote Code Execution: an attacker can
execute arbitrary commands with execv(), as root; no memory corruption
or ROP (Return-Oriented Programming) is involved.

This vulnerability is exploitable instantly by a local attacker (and by
a remote attacker in certain non-default configurations). To remotely
exploit this vulnerability in the default configuration, an attacker
must keep a connection to the vulnerable server open for 7 days (by
transmitting one byte every few minutes). However, because of the
extreme complexity of Exim's code, we cannot guarantee that this
exploitation method is unique; faster methods may exist.

Exim is vulnerable by default since version 4.87 (released on April 6,
2016), when #ifdef EXPERIMENTAL_EVENT became #ifndef DISABLE_EVENT; and
older versions may also be vulnerable if EXPERIMENTAL_EVENT was enabled
manually. Surprisingly, this vulnerability was fixed in version 4.92
(released on February 10, 2019):

https://github.com/Exim/exim/commit/7ea1237c783e380d7bdb8...
https://bugs.exim.org/show_bug.cgi?id=2310

but was not identified as a security vulnerability, and most operating
systems are therefore affected. For example, we exploit an up-to-date
Debian distribution (9.9) in this advisory.


========================================================================
Local exploitation
========================================================================

The vulnerable code is located in deliver_message():

6122 #ifndef DISABLE_EVENT
6123       if (process_recipients != RECIP_ACCEPT)
6124         {
6125         uschar * save_local =  deliver_localpart;
6126         const uschar * save_domain = deliver_domain;
6127
6128         deliver_localpart = expand_string(
6129                       string_sprintf("${local_part:%s}", new->address));
6130         deliver_domain =    expand_string(
6131                       string_sprintf("${domain:%s}", new->address));
6132
6133         (void) event_raise(event_action,
6134                       US"msg:fail:internal", new->message);
6135
6136         deliver_localpart = save_local;
6137         deliver_domain =    save_domain;
6138         }
6139 #endif

Because expand_string() recognizes the "${run{<command> <args>}}"
expansion item, and because new->address is the recipient of the mail
that is being delivered, a local attacker can simply send a mail to
"${run{...}}@localhost" (where "localhost" is one of Exim's
local_domains) and execute arbitrary commands, as root
(deliver_drop_privilege is false, by default):

[...]


========================================================================
Remote exploitation
========================================================================

Our local-exploitation method does not work remotely, because the
"verify = recipient" ACL (Access-Control List) in Exim's default
configuration requires the local part of the recipient's address (the
part that precedes the @ sign) to be the name of a local user:

[...]

------------------------------------------------------------------------
Non-default configurations
------------------------------------------------------------------------

We eventually devised an elaborate method for exploiting Exim remotely
in its default configuration, but we first identified various
non-default configurations that are easy to exploit remotely:

- If the "verify = recipient" ACL was removed manually by an
  administrator (maybe to prevent username enumeration via RCPT TO),
  then our local-exploitation method also works remotely.

- If Exim was configured to recognize tags in the local part of the
  recipient's address (via "local_part_suffix = +* : -*" for example),
  then a remote attacker can simply reuse our local-exploitation method
  with an RCPT TO "balrog+${run{...}}@localhost" (where "balrog" is the
  name of a local user).

- If Exim was configured to relay mail to a remote domain, as a
  secondary MX (Mail eXchange), then a remote attacker can simply reuse
  our local-exploitation method with an RCPT TO "${run{...}}@khazad.dum"
  (where "khazad.dum" is one of Exim's relay_to_domains). Indeed, the
  "verify = recipient" ACL can only check the domain part of a remote
  address (the part that follows the @ sign), not the local part.

------------------------------------------------------------------------
Default configuration
------------------------------------------------------------------------

[...]


========================================================================
Acknowledgments
========================================================================

We thank Exim's developers, Solar Designer, and the members of
distros@openwall.

"The Return of the WIZard" is a reference to Sendmail's ancient WIZ and
DEBUG vulnerabilities:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-1999-0145
https://seclists.org/bugtraq/1995/Feb/56

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-1999-0095
http://www.cheswick.com/ches/papers/berferd.pdf


========================================================================
Timeline
========================================================================

2019-05-27: Advisory sent to security@exim.

2019-05-28: Advisory sent to distros@openwall.