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    1 From qsa@qualys.com Wed Aug 14 01:29:25 CEST 2019
    2 Date: Tue, 13 Aug 2019 23:29:25 +0000
    3 From: Qualys Security Advisory <qsa@qualys.com>
    4 To: Heiko Schlittermann <hs@schlittermann.de>
    5 Subject: Re: Help evaluating a Bug in Exim MTA
    6 Return-Path: <qsa@qualys.com>
    7 Authentication-Results: mx.net.schlittermann.de; iprev=pass
    8  (mx0b-001ca501.pphosted.com) smtp.remote-ip=; spf=pass
    9  smtp.mailfrom=qualys.com; dkim=pass header.d=qualys.com header.s=qualyscom
   10  header.a=rsa-sha256; dkim=pass header.d=qualys.onmicrosoft.com
   11  header.s=selector2-qualys-onmicrosoft-com header.a=rsa-sha256; dmarc=none
   12  header.from=qualys.com
   13 Authentication-Results: ppops.net; spf=pass smtp.mailfrom=qsa@qualys.com
   14 Status: O
   15 Content-Length: 3899
   16 Lines: 80
   18 Hi Heiko,
   20 On Mon, Aug 12, 2019 at 11:56:12PM +0200, Heiko Schlittermann wrote:
   21 > So I'd say, you do not need to rush, but I'd like to close it sooner or
   22 > later in either manner.
   24 OK, below is our preliminary analysis. First:
   26 - From an attacker's point of view, most calls to
   27   string_interpret_escape() are uninteresting. For example, nextitem()
   28   in src/filter.c checks for buffer overflows, and string_dequote()
   29   seems to process trusted strings only (strings from configuration
   30   files).
   32 - On the other hand, string_unprinting() is very interesting:
   34   - It is used in tls_import_cert() (for peercert, for example); but
   35     certificates are in PEM format (i.e., base64) and hence unlikely to
   36     contain the problematic backslash-null-byte sequence.
   38   - It is used for peerdn and sni in src/spool_in.c; but peerdn is used
   39     only if client certificates are processed by Exim, and this is not
   40     the default (and although some sites use client-certificate
   41     authentication, this is not very common, and hence not very
   42     interesting for an attacker).
   44   - In any case, as long as Exim supports and accepts tls connections,
   45     an attacker can send an sni, and hence reach the problematic
   46     string_unprinting() and string_interpret_escape() functions.
   48 Next question: is it possible to send an sni that is written to the
   49 spool header file and that ends with the problematic backslash-null-byte
   50 sequence? The answer is yes, because of what we believe is another bug,
   51 in string_printing(): the sni is written to the spool header file via
   52 string_printing(tls_in.sni), which escapes characters with backslash,
   53 but does *not* escape the escaping character itself (backslash),
   54 although it definitely should.
   56 This bug is what makes it possible to reach and trigger the bug in
   57 string_unprinting() and string_interpret_escape(), with an sni that ends
   58 in an unescaped backslash (followed by the terminating null byte).
   60 Last question: is this exploitable? The answer is, almost certainly, yes
   61 (and, because spool_read_header() runs as root, this means remote root).
   63 The sni is read from the spool via string_unprinting(string_copy()), and
   64 both string_unprinting() and string_copy() use store_get(): as a result,
   65 the destination buffer is allocated right after the source buffer, and
   66 the characters that are read out-of-bounds after the end of the source
   67 buffer are the first characters of the destination buffer, which are
   68 fully under the attacker's control. This results in a heap overflow
   69 whose length and contents are both under the attacker's control (we
   70 verified this). This is almost certainly exploitable.
   72 Our advice is to start the security-release process as soon as possible.
   73 We know it is very painful, but we are really confident that this bug is
   74 exploitable; we will try to confirm this in the next few days. We also
   75 believe that an Exim server must support and accept tls connections to
   76 be remotely exploitable (via sni).
   78 During our analysis of this bug, we probably spotted three other bugs:
   80 - The unescaped backslash in string_printing() that we mentioned above.
   82 - A bug in spool_read_header(): before the for (;;) loop, p is set to
   83   big_buffer + 2; and inside the loop, big_buffer may be re-allocated;
   84   but p is never updated. This can lead to a use-after-free (we did not
   85   assess the security impact of this bug, though).
   87 - A bug in spool_write_header(): the return value of tls_export_cert()
   88   is not checked (for ourcert, but more importantly, for peercert). If
   89   this function fails (maybe because big_buffer is not big enough), then
   90   big_buffer may be uninitialized or unterminated, and garbage may be
   91   written to the spool file (we did not assess the security impact of
   92   this bug, either).
   94 We are at your disposal for questions, comments, and further
   95 discussions. Thank you very much for reaching out! With best regards,
   97 --
   98 the Qualys Security Advisory team
   99 From qsa@qualys.com Mon Aug 19 00:23:03 CEST 2019
  100 Date: Sun, 18 Aug 2019 22:23:03 +0000
  101 From: Qualys Security Advisory <qsa@qualys.com>
  102 To: Heiko Schlittermann <hs@schlittermann.de>
  103 Subject: Re: Help evaluating a Bug in Exim MTA
  104 Return-Path: <qsa@qualys.com>
  105 Authentication-Results: mx.net.schlittermann.de; iprev=pass
  106  (mx0a-001ca501.pphosted.com) smtp.remote-ip=; spf=pass
  107  smtp.mailfrom=qualys.com; dkim=pass header.d=qualys.com header.s=qualyscom
  108  header.a=rsa-sha256; dkim=pass header.d=qualys.onmicrosoft.com
  109  header.s=selector2-qualys-onmicrosoft-com header.a=rsa-sha256; dmarc=none
  110  header.from=qualys.com
  111 Authentication-Results: ppops.net; spf=pass smtp.mailfrom=qsa@qualys.com
  112 Status: RO
  113 Content-Length: 2484
  114 Lines: 59
  116 Hi Heiko,
  118 On Tue, Aug 13, 2019 at 11:29:25PM +0000, Qualys Security Advisory wrote:
  119 > we are really confident that this bug is exploitable
  121 We can confirm that this bug is indeed exploitable: we wrote a
  122 rudimentary exploit that remotely obtains root privileges (because
  123 deliver_message() runs as root).
  125 Some general notes on this exploit:
  127 - To the best of our knowledge, the string_interpret_escape() bug
  128   (backslash-null) is remotely exploitable if and only if Exim supports
  129   and accepts TLS connections (because the only attack vector that we
  130   know of is the string_unprinting() of SNI).
  132 - Both OpenSSL and GnuTLS installations are exploitable.
  134 - Our exploit is Linux-specific (because our heap-overflow exploitation
  135   is specific to glibc's malloc implementation), but works on both i386
  136   and amd64.
  138 Some detailed notes on this exploit:
  140 - First, we connect to Exim with TLS and send an SNI that ends with
  141   backslash-null (this SNI is written unmodified to the spool because of
  142   the unescaped-backslash bug in string_printing2()).
  144 - Second, we exploit the backslash-null bug in string_interpret_escape()
  145   (our SNI is read from the spool and unescaped by string_unprinting()),
  146   and we transform this out-of-bounds read into an out-of-bounds write
  147   (a heap overflow).
  149 - Next, we use this heap overflow to overwrite the header of a free
  150   malloc chunk, and increase its size to make it overlap with other,
  151   already-allocated malloc chunks.
  153 - Last, we allocate this enlarged malloc chunk, and use it to overwrite
  154   large parts of the heap (the already-allocated malloc chunks) with
  155   arbitrary data:
  157   . we overwrite the "id" string: it is used to build the message-log
  158     file name, and therefore allows us to write to "/etc/passwd" (by
  159     overwriting "id" with "/../../../../../../../../etc/passwd");
  161   . we overwrite the "sender_address" string: it is written to the
  162     message-log file, and therefore allows us to add a new user to
  163     "/etc/passwd".
  165 Other exploitation methods may exist. We will not publish our exploit:
  166 it is a quick and dirty proof of concept, and we will not have the time
  167 to clean it anytime soon. However, please feel free to quote us on the
  168 exploitability of this bug (we do have a working exploit), and please
  169 feel free to quote all or parts of this email in your announcements.
  171 We are at your disposal for questions, comments, and further
  172 discussions. Thank you very much! With best regards,
  174 --
  175 the Qualys Security Advisory team