On February 10, the Unix-based email server Exim released an update to address a heap buffer overflow vulnerability that can be used by an unauthenticated attacker to remotely execute arbitrary code. The flaw, assigned CVE-2018-6789, is noted to exist in all versions of Exim, prior to their latest release, 4.90.1, which means the attack surface potential is very wide.

Exim Buffer Overflow RCE Vulnerability (CVE-2018-6789) – What You Need to Know

An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1. By sending a handcrafted message, a buffer overflow may happen. This can be used to execute code remotely.

The remote Redhat Enterprise Linux 5 host has one or more packages installed that are affected by multiple vulnerabilities that have been acknowledged by the vendor but will not be patched.

  - exim: out-of-bounds access in string_interpret_escape() leading to buffer overflow in the SMTP delivery     process (CVE-2019-15846)

  - The open_log function in log.c in Exim 4.72 and earlier does not check the return value from (1) setuid or     (2) setgid system calls, which allows local users to append log data to arbitrary files via a symlink     attack. (CVE-2011-0017)

  - expand.c in Exim before 4.83 expands mathematical comparisons twice, which allows local users to gain     privileges and execute arbitrary commands via a crafted lookup value. (CVE-2014-2972)

Note that Nessus has not tested for these issues but has instead relied on the package manager's report that the package is installed.

This update for exim fixes the following issues :

Exim was updated to exim-4.94.2

security update (boo#1185631)

  - CVE-2020-28007: Link attack in Exim's log directory

  - CVE-2020-28008: Assorted attacks in Exim's spool     directory

  - CVE-2020-28014: Arbitrary PID file creation

  - CVE-2020-28011: Heap buffer overflow in queue_run()

  - CVE-2020-28010: Heap out-of-bounds write in main()

  - CVE-2020-28013: Heap buffer overflow in     parse_fix_phrase()

  - CVE-2020-28016: Heap out-of-bounds write in     parse_fix_phrase()

  - CVE-2020-28015: New-line injection into spool header     file (local)

  - CVE-2020-28012: Missing close-on-exec flag for     privileged pipe

  - CVE-2020-28009: Integer overflow in get_stdinput()

  - CVE-2020-28017: Integer overflow in     receive_add_recipient()

  - CVE-2020-28020: Integer overflow in receive_msg()

  - CVE-2020-28023: Out-of-bounds read in smtp_setup_msg()

  - CVE-2020-28021: New-line injection into spool header     file (remote)

  - CVE-2020-28022: Heap out-of-bounds read and write in     extract_option()

  - CVE-2020-28026: Line truncation and injection in     spool_read_header()

  - CVE-2020-28019: Failure to reset function pointer after     BDAT error

  - CVE-2020-28024: Heap buffer underflow in smtp_ungetc()

  - CVE-2020-28018: Use-after-free in tls-openssl.c

  - CVE-2020-28025: Heap out-of-bounds read in     pdkim_finish_bodyhash()

update to exim-4.94.1

  - Fix security issue in BDAT state confusion. Ensure we     reset known-good where we know we need to not be reading     BDAT data, as a general case fix, and move the places     where we switch to BDAT mode until after various     protocol state checks. Fixes CVE-2020-BDATA reported by     Qualys.

  - Fix security issue in SMTP verb option parsing     (CVE-2020-EXOPT)

  - Fix security issue with too many recipients on a message     (to remove a known security problem if someone does set     recipients_max to unlimited, or if local additions add     to the recipient list). Fixes CVE-2020-RCPTL reported by     Qualys.

  - Fix CVE-2020-28016 (PFPZA): Heap out-of-bounds write in     parse_fix_phrase()

  - Fix security issue CVE-2020-PFPSN and guard against     cmdline invoker providing a particularly obnoxious     sender full name.

  - Fix Linux security issue CVE-2020-SLCWD and guard     against PATH_MAX better.

  - bring back missing exim_db.8 manual page (fixes     boo#1173693)

  - bring in changes from current +fixes (lots of taint     check fixes)

  - Bug 1329: Fix format of Maildir-format filenames to     match other mail- related applications. Previously an     'H' was used where available info says that 'M' should     be, so change to match.

  - Bug 2587: Fix pam expansion condition. Tainted values     are commonly used as arguments, so an implementation     trying to copy these into a local buffer was taking a     taint-enforcement trap. Fix by using dynamically created     buffers.

  - Bug 2586: Fix listcount expansion operator. Using     tainted arguments is reasonable, eg. to count headers.
    Fix by using dynamically created buffers rather than a     local. Do similar fixes for ACL actions 'dcc',     'log_reject_target', 'malware' and 'spam'; the arguments     are expanded so could be handling tainted values.

  - Bug 2590: Fix -bi (newaliases). A previous code     rearrangement had broken the (no-op) support for this     sendmail command. Restore it to doing nothing, silently,     and returning good status.

  - update to exim 4.94

  - some transports now refuse to use tainted data in     constructing their delivery location this WILL BREAK     configurations which are not updated accordingly. In     particular: any Transport use of $local_user which has     been relying upon check_local_user far away in the     Router to make it safe, should be updated to replace     $local_user with $local_part_data.

  - Attempting to remove, in router or transport, a header     name that ends with an asterisk (which is a     standards-legal name) will now result in all headers     named starting with the string before the asterisk being     removed.

  - switch pretrans to use lua (fixes boo#1171877)



  - bring changes from current in +fixes branch     (patch-exim-fixes-ee83de04d3087efaf808d1f2235a988275c2ee     94)

  - fixes CVE-2020-12783 (boo#1171490)

  - Regard command-line recipients as tainted.

  - Bug 2489: Fix crash in the 'pam' expansion condition.

  - Use tainted buffers for the transport smtp context.

  - Bug 2493: Harden ARC verify against Outlook, which has     been seen to mix the ordering of its ARC headers. This     caused a crash.

  - Bug 2492: Use tainted memory for retry record when     needed. Previously when a new record was being     constructed with information from the peer, a trap was     taken.

  - Bug 2494: Unset the default for dmarc_tld_file.

  - Fix an uninitialised flag in early-pipelining.
    Previously connections could, depending on the platform,     hang at the STARTTLS response.

  - Bug 2498: Reset a counter used for ARC verify before     handling another message on a connection. Previously if     one message had ARC headers and the following one did     not, a crash could result when adding an     Authentication-Results: header.

  - Bug 2500: Rewind some of the common-coding in string     handling between the Exim main code and Exim-related     utities.

  - Fix the variables set by the gsasl authenticator.

  - Bug 2507: Modules: on handling a dynamic-module     (lookups) open failure, only retrieve the errormessage     once.

  - Bug 2501: Fix init call in the heimdal authenticator.
    Previously it adjusted the size of a major service     buffer; this failed because the buffer was in use at the     time. Change to a compile-time increase in the buffer     size, when this authenticator is compiled into exim.

  - update to exim 4.93.0.4 (+fixes release)

  - Avoid costly startup code when not strictly needed. This     reduces time for some exim process initialisations. It     does mean that the logging of TLS configuration problems     is only done for the daemon startup.

  - Early-pipelining support code is now included unless     disabled in Makefile.

  - DKIM verification defaults no long accept sha1 hashes,     to conform to RFC 8301. They can still be enabled, using     the dkim_verify_hashes main option.

  - Support CHUNKING from an smtp transport using a     transport_filter, when DKIM signing is being done.
    Previously a transport_filter would always disable     CHUNKING, falling back to traditional DATA.

  - Regard command-line receipients as tainted.

  - Bug 340: Remove the daemon pid file on exit, whe due to     SIGTERM.

  - Bug 2489: Fix crash in the 'pam' expansion condition. It     seems that the PAM library frees one of the arguments     given to it, despite the documentation. Therefore a     plain malloc must be used.

  - Bug 2491: Use tainted buffers for the transport smtp     context. Previously on-stack buffers were used,     resulting in a taint trap when DSN information copied     from a received message was written into the buffer.

  - Bug 2493: Harden ARC verify against Outlook, whick has     been seen to mix the ordering of its ARC headers. This     caused a crash.

  - Bug 2492: Use tainted memory for retry record when     needed. Previously when a new record was being     constructed with information from the peer, a trap was     taken.

  - Bug 2494: Unset the default for dmarc_tld_file.
    Previously a naiive installation would get error     messages from DMARC verify, when it hit the nonexistent     file indicated by the default. Distros wanting DMARC     enabled should both provide the file and set the option.
    Also enforce no DMARC verification for command-line     sourced messages.

  - Fix an uninitialised flag in early-pipelining.
    Previously connections could, depending on the platform,     hang at the STARTTLS response.

  - Bug 2498: Reset a counter used for ARC verify before     handling another message on a connection. Previously if     one message had ARC headers and the following one did     not, a crash could result when adding an     Authentication-Results: header.

  - Bug 2500: Rewind some of the common-coding in string     handling between the Exim main code and Exim-related     utities. The introduction of taint tracking also did     many adjustments to string handling. Since then, eximon     frequently terminated with an assert failure.

  - When PIPELINING, synch after every hundred or so RCPT     commands sent and check for 452 responses. This slightly     helps the inefficieny of doing a large alias-expansion     into a recipient-limited target. The max_rcpt transport     option still applies (and at the current default, will     override the new feature). The check is done for either     cause of synch, and forces a fast-retry of all 452'd     recipients using a new MAIL FROM on the same connection.
    The new facility is not tunable at this time.

  - Fix the variables set by the gsasl authenticator.
    Previously a pointer to library live data was being     used, so the results became garbage. Make copies while     it is still usable.

  - Logging: when the deliver_time selector ise set, include     the DT= field on delivery deferred (==) and failed (**)     lines (if a delivery was attemtped). Previously it was     only on completion (=>) lines.

  - Authentication: the gsasl driver not provides the $authN     variables in time for the expansion of the     server_scram_iter and server_scram_salt options.

spec file cleanup to make update work

  - add docdir to spec

  - update to exim 4.93

  - SUPPORT_DMARC replaces EXPERIMENTAL_DMARC

  - DISABLE_TLS replaces SUPPORT_TLS

  - Bump the version for the local_scan API.

  - smtp transport option hosts_try_fastopen defaults to     '*'.

  - DNSSec is requested (not required) for all queries.
    (This seemes to ask for trouble if your resolver is a     systemd-resolved.)

  - Generic router option retry_use_local_part defaults to     'true' under specific pre-conditions.

  - Introduce a tainting mechanism for values read from     untrusted sources.

  - Use longer file names for temporary spool files (this     avoids name conflicts with spool on a shared file     system).

  - Use dsn_from main config option (was ignored     previously).

  - update to exim 4.92.3

  - CVE-2019-16928: fix against Heap-based buffer overflow     in string_vformat, remote code execution seems to be     possible

  - update to exim 4.92.2

  - CVE-2019-15846: fix against remote attackers executing     arbitrary code as root via a trailing backslash

  - update to exim 4.92.1

  - CVE-2019-13917: Fixed an issue with $(sort) expansion     which could allow remote attackers to execute other     programs with root privileges (boo#1142207)

  - spec file cleanup

  - fix DANE inclusion guard condition

  - re-enable i18n and remove misleading comment

  - EXPERIMENTAL_SPF is now SUPPORT_SPF

  - DANE is now SUPPORT_DANE

  - update to exim 4.92

  - $(l_header:<name>) expansion

  - $(readsocket) now supports TLS

  - 'utf8_downconvert' option (if built with SUPPORT_I18N)

  - 'pipelining' log_selector

  - JSON variants for $(extract ) expansion

  - 'noutf8' debug option

  - TCP Fast Open support on MacOS

  - CVE-2019-10149: Fixed a Remote Command Execution     (boo#1136587)

  - add workaround patch for compile time error on missing     printf format annotation (gnu_printf.patch)

  - update to 4.91

  - DEFER rather than ERROR on redis cluster MOVED response.

  - Catch and remove uninitialized value warning in exiqsumm

  - Disallow '/' characters in queue names specified for the     'queue=' ACL modifier. This matches the restriction on     the commandline.

  - Fix pgsql lookup for multiple result-tuples with a     single column. Previously only the last row was     returned.

  - Bug 2217: Tighten up the parsing of DKIM signature     headers.

  - Bug 2215: Fix crash associated with dnsdb lookup done     from DKIM ACL.

  - Fix issue with continued-connections when the DNS shifts     unreliably.

  - Bug 2214: Fix SMTP responses resulting from non-accept     result of MIME ACL.

  - The 'support for' informational output now, which built     with Content Scanning support, has a line for the     malware scanner interfaces compiled in. Interface can be     individually included or not at build time.

  - The 'aveserver', 'kavdaemon' and 'mksd' interfaces are     now not included by the template makefile 'src/EDITME'.
    The 'STREAM' support for an older ClamAV interface     method is removed.

  - Bug 2223: Fix mysql lookup returns for the no-data case     (when the number of rows affected is given instead).

  - The runtime Berkeley DB library version is now     additionally output by 'exim -d -bV'. Previously only     the compile-time version was shown.

  - Bug 2230: Fix cutthrough routing for nonfirst messages     in an initiating SMTP connection.

  - Bug 2229: Fix cutthrough routing for nonstandard port     numbers defined by routers.

  - Bug 2174: A timeout on connect for a callout was also     erroneously seen as a timeout on read on a GnuTLS     initiating connection, resulting in the initiating     connection being dropped.

  - Relax results from ACL control request to enable     cutthrough, in unsupported situations, from error to     silently (except under debug) ignoring.

  - Fix Buffer overflow in base64d() (CVE-2018-6789)

  - Fix bug in DKIM verify: a buffer overflow could corrupt     the malloc metadata, resulting in a crash in free().

  - Fix broken Heimdal GSSAPI authenticator integration.

  - Bug 2113: Fix conversation closedown with the Avast     malware scanner.

  - Bug 2239: Enforce non-usability of     control=utf8_downconvert in the mail ACL.

  - Speed up macro lookups during configuration file read,     by skipping non- macro text after a replacement     (previously it was only once per line) and by skipping     builtin macros when searching for an uppercase lead     character.

  - DANE support moved from Experimental to mainline. The     Makefile control for the build is renamed.

  - Fix memory leak during multi-message connections using     STARTTLS.

  - Bug 2236: When a DKIM verification result is overridden     by ACL, DMARC reported the original. Fix to report (as     far as possible) the ACL result replacing the original.

  - Fix memory leak during multi-message connections using     STARTTLS under OpenSSL

  - Bug 2242: Fix exim_dbmbuild to permit directoryless     filenames.

  - Fix utf8_downconvert propagation through a redirect     router.

  - Bug 2253: For logging delivery lines under PRDR, append     the overall DATA response info to the (existing)     per-recipient response info for the 'C=' log element.

  - Bug 2251: Fix ldap lookups that return a single     attribute having zero- length value.

  - Support Avast multiline protocol, this allows passing     flags to newer versions of the scanner.

  - Ensure that variables possibly set during message     acceptance are marked dead before release of memory in     the daemon loop.

  - Bug 2250: Fix a longstanding bug in heavily-pipelined     SMTP input (such as a multi-recipient message from a     mailinglist manager).

  - The (EXPERIMENTAL_DMARC) variable $dmarc_ar_header is     withdrawn, being replaced by the $(authresults )     expansion.

  - Bug 2257: Fix pipe transport to not use a socket-only     syscall.

  - Set a handler for SIGTERM and call exit(3) if running as     PID 1. This allows proper process termination in     container environments.

  - Bug 2258: Fix spool_wireformat in combination with LMTP     transport. Previously the 'final dot' had a newline     after it; ensure it is CR,LF.

  - SPF: remove support for the 'spf' ACL condition outcome     values 'err_temp' and 'err_perm', deprecated since 4.83     when the RFC-defined words ' temperror' and 'permerror'     were introduced.

  - Re-introduce enforcement of no cutthrough delivery on     transports having transport-filters or DKIM-signing.

  - Cutthrough: for a final-dot response timeout (and     nonunderstood responses) in defer=pass mode supply a 450     to the initiator. Previously the message would be     spooled.

  - DANE: add dane_require_tls_ciphers SMTP Transport     option; if unset, tls_require_ciphers is used as before.

  - Malware Avast: Better match the Avast multiline     protocol.

  - Fix reinitialisation of DKIM logging variable between     messages.

  - Bug 2255: Revert the disable of the OpenSSL session     caching.

  - Add util/renew-opendmarc-tlds.sh script for safe renewal     of public suffix list.

  - DKIM: accept Ed25519 pubkeys in     SubjectPublicKeyInfo-wrapped form, since the IETF WG has     not yet settled on that versus the original 'bare'     representation.

  - Fix syslog logging for syslog_timestamp=no and     log_selector +millisec. Previously the millisecond value     corrupted the output. Fix also for syslog_pid=no and     log_selector +pid, for which the pid corrupted the     output.

  - Replace xorg-x11-devel by individual pkgconfig()     buildrequires. 

  - update to 4.90.1

  - Allow PKG_CONFIG_PATH to be set in Local/Makefile and     use it correctly during configuration. Wildcards are     allowed and expanded.

  - Shorten the log line for daemon startup by collapsing     adjacent sets of identical IP addresses on different     listening ports. Will also affect 'exiwhat' output.

  - Tighten up the checking in isip4 (et al): dotted-quad     components larger than 255 are no longer allowed.

  - Default openssl_options to include +no_ticket, to reduce     load on peers. Disable the session-cache too, which     might reduce our load. Since we currrectly use a new     context for every connection, both as server and client,     there is no benefit for these.

  - Add $SOURCE_DATE_EPOCH support for reproducible builds,     per spec at     <https://reproducible-builds.org/specs/source-date-epoch     />.

  - Fix smtp transport use of limited max_rcpt under     mua_wrapper. Previously the check for any unsuccessful     recipients did not notice the limit, and erroneously     found still-pending ones.

  - Pipeline CHUNKING command and data together, on kernels     that support MSG_MORE. Only in-clear (not on TLS     connections).

  - Avoid using a temporary file during transport using     dkim. Unless a transport-filter is involved we can     buffer the headers in memory for creating the signature,     and read the spool data file once for the signature and     again for transmission.

  - Enable use of sendfile in Linux builds as default. It     was disabled in 4.77 as the kernel support then wasn't     solid, having issues in 64bit mode. Now, it's been long     enough. Add support for FreeBSD also.

  - Add commandline_checks_require_admin option.

  - Do pipelining under TLS.

  - For the 'sock' variant of the malware scanner interface,     accept an empty cmdline element to get the documented     default one. Previously it was inaccessible.

  - Prevent repeated use of -p/-oMr

  - DKIM: enforce the DNS pubkey record 'h' permitted-hashes     optional field, if present.

  - DKIM: when a message has multiple signatures matching an     identity given in dkim_verify_signers, run the dkim acl     once for each.

  - Support IDNA2008.

  - The path option on a pipe transport is now expanded     before use

  - Have the EHLO response advertise VRFY, if there is a     vrfy ACL defined.

  - Several bug fixes

  - Fix for buffer overflow in base64decode() (boo#1079832     CVE-2018-6789)

Buffer overflow in b64decode() function, possibly leading to remote code execution :

An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1. By sending a handcrafted message, a buffer overflow may happen. This can be used to execute code remotely.
(CVE-2018-6789)

The remote host is affected by the vulnerability described in GLSA-201803-01 (Exim: Multiple vulnerabilities)

    Multiple vulnerabilities have been discovered in Exim. Please review the       CVE identifiers referenced below for details.
  Impact :

    A remote attacker, by connecting to the SMTP listener daemon, could       possibly execute arbitrary code with the privileges of the process or       cause a Denial of Service condition.
  Workaround :

    There is no known workaround at this time.

The remote host is running Exim, a message transfer agent.

Versions of Exim earlier than 4.90.1 are affected by an overflow condition in an unspecified utility function that is triggered as certain input is not properly validated when parsing messages. This may allow a remote attacker to cause a buffer overflow and potentially execute arbitrary code.

According to its banner, the version of Exim running on the remote host is prior to 4.90.1. It is, therefore, potentially affected by a buffer overflow vulnerability. A flaw exists base64d() function due to improper validation of parsed messages. A remote attacker could potentially cause a buffer overflow condition and execute arbitrary code.

This is an update fixing mysql module.

----

This is new version fixing CVE-2018-6789.

Note that Tenable Network Security has extracted the preceding description block directly from the Fedora update system website.
Tenable has attempted to automatically clean and format it as much as possible without introducing additional issues.

This update for exim fixes the following issues :

  - CVE-2018-6789: Fixed a buffer overflow in the     base64decode function, which could be used to execute     code remotely. (boo#1079832)

The remote Ubuntu 14.04 LTS / 16.04 LTS host has packages installed that are affected by a vulnerability as referenced in the USN-3565-1 advisory.

    Meh Chang discovered that Exim incorrectly handled memory in certain decoding operations. A remote     attacker could use this issue to cause Exim to crash, resulting in a denial of service, or possibly     execute arbitrary code.

Tenable has extracted the preceding description block directly from the Ubuntu security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

Exim developers report :

There is a buffer overflow in base64d(), if some pre-conditions are met. Using a handcrafted message, remote code execution seems to be possible.

Meh Chang discovered a buffer overflow flaw in a utility function used in the SMTP listener of Exim, a mail transport agent. A remote attacker can take advantage of this flaw to cause a denial of service, or potentially the execution of arbitrary code via a specially crafted message.

Meh Chang discovered a buffer overflow flaw in a utility function used in the SMTP listener of Exim, a mail transport agent. A remote attacker can take advantage of this flaw to cause a denial of service, or potentially the execution of arbitrary code via a specially crafted message.

For Debian 7 'Wheezy', this problem has been fixed in version 4.80-7+deb7u6.

We recommend that you upgrade your exim4 packages.

NOTE: Tenable Network Security has extracted the preceding description block directly from the DLA security advisory. Tenable has attempted to automatically clean and format it as much as possible without introducing additional issues.

ID	Name	Product	Family	Severity
198550	RHEL 5 : exim (Unpatched Vulnerability)	Nessus	Red Hat Local Security Checks	critical
149614	openSUSE Security Update : exim (openSUSE-2021-677) (Stack Clash)	Nessus	SuSE Local Security Checks	critical
107241	Amazon Linux AMI : exim (ALAS-2018-970)	Nessus	Amazon Linux Local Security Checks	critical
107178	GLSA-201803-01 : Exim: Multiple vulnerabilities	Nessus	Gentoo Local Security Checks	critical
700223	Exim < 4.90.1 Remote Code Execution	Nessus Network Monitor	SMTP Servers	high
107149	Exim < 4.90.1 Buffer Overflow RCE Vulnerability	Nessus	SMTP problems	critical
107009	Fedora 27 : exim (2018-5aec14e125)	Nessus	Fedora Local Security Checks	critical
107007	Fedora 26 : exim (2018-25a7ba3cb6)	Nessus	Fedora Local Security Checks	critical
106888	openSUSE Security Update : exim (openSUSE-2018-170)	Nessus	SuSE Local Security Checks	critical
106791	Ubuntu 14.04 LTS / 16.04 LTS : Exim vulnerability (USN-3565-1)	Nessus	Ubuntu Local Security Checks	critical
106733	FreeBSD : exim -- a buffer overflow vulnerability, remote code execution (316b3c3e-0e98-11e8-8d41-97657151f8c2)	Nessus	FreeBSD Local Security Checks	critical
106728	Debian DSA-4110-1 : exim4 - security update	Nessus	Debian Local Security Checks	critical
106722	Debian DLA-1274-1 : exim4 security update	Nessus	Debian Local Security Checks	critical

Detections

Analytics

CVE-2018-6789

critical

Tenable Plugins

critical

critical

critical

critical

high

critical

critical

critical

critical

critical

critical

critical

critical