New Critical Exim Flaw Exposes Email Servers to Remote Attacks — Patch Released

Sep 30, 2019

A critical security vulnerability has been discovered and
fixed in the popular open-source Exim email server software,
which could allow a remote attacker to simply crash or potentially
execute malicious code on targeted servers.

Exim maintainers today released ^[1] an urgent security
update—Exim version 4.92.3—after publishing an early warning
two days ago, giving system administrators an early head-up on its
upcoming security patches that affect all versions of the email
server software from 4.92 up to and including then-latest version
4.92.2.

Exim is a widely used, open source mail transfer agent (MTA)
developed for Unix-like operating systems like Linux, Mac OSX or
Solaris, which runs almost 60 percent of the Internet’s email
servers today for routing, delivering and receiving email
messages.

This is the second time in this month when the Exim maintainers
have released an urgent security update. Earlier this month, the
team patched a critical remote code execution flaw (CVE-2019-15846)
in the software that could have allowed remote attackers to gain
root-level access to the system.

Identified as CVE-2019-16928 ^[3]
and discovered by Jeremy Harris of Exim Development Team, the
vulnerability is a heap-based buffer overflow (memory corruption)
issue in string_vformat defined in string.c file of the EHLO
Command Handler component.

The security flaw could allow remote attackers to cause a denial
of service (DoS) condition or execute arbitrary code on a targeted
Exim mail server using a specially crafted line in the EHLO command
with the rights of the targeted user.

According to the Exim advisory, a currently known PoC
exploit ^[4] for this vulnerability
allows one to only crash the Exim process by sending a long string
in the EHLO command, though other commands could also be used to
potentially execute arbitrary code.

“The currently known exploit uses an extraordinary long EHLO string
to crash the Exim process that is receiving the message,” says the
Exim developers’ team.

“While at this mode of operation, Exim already dropped its
privileges, other paths to reach the vulnerable code may exist.”

In mid-year, Exim also patched a severe remote command execution
vulnerability (CVE-2019-10149) in its email software that was
actively exploited
in the wild by various groups of hackers to compromise
vulnerable servers.

Therefore, server administrators are highly recommended to install
the latest Exim 4.92.3 version as soon as possible, since there is
no known mitigation to temporarily resolve this issue.

The team also says, “if you can’t install the above versions,
ask your package maintainer for a version containing the backported
fix. On request and depending on our resources, we will support you
in backporting the fix.”

The security update is available for Linux distributions,
including Ubuntu ^[6], Arch Linux ^[7], FreeBSD ^[8], Debian ^[9], and Fedora ^[10].

^[2]^[5]

References

^{^}
released
(www.exim.org)
^{^}
CVE-2019-15846
(thehackernews.com)
^{^}
CVE-2019-16928
(bugs.exim.org)
^{^}
known PoC exploit
(git.exim.org)
^{^}
actively exploited in the wild
(thehackernews.com)
^{^}
Ubuntu
(usn.ubuntu.com)
^{^}
Arch Linux
(www.archlinux.org)
^{^}
FreeBSD
(www.vuxml.org)
^{^}
Debian
(security-tracker.debian.org)
^{^}
Fedora
(bodhi.fedoraproject.org)

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OWASP TOP !0

Injection. Injection flaws, such as SQL, NoSQL, OS, and LDAP injection, occur when untrusted data is sent to an interpreter as part of a command or query. The attacker’s hostile data can trick the interpreter into executing unintended commands or accessing data without proper authorization. Broken Authentication. Application functions related to authentication and session management are often implemented incorrectly, allowing attackers to compromise passwords, keys, or session tokens, or to exploit other implementation flaws to assume other users’ identities temporarily or permanently. Sensitive Data Exposure. Many web applications and APIs do not properly protect sensitive data, such as financial, healthcare, and PII. Attackers may steal or modify such weakly protected data to conduct credit card fraud, identity theft, or other crimes. Sensitive data may be compromised without extra protection, such as encryption at rest or in transit, and requires special precautions when exchanged with the browser. XML External Entities (XXE). Many older or poorly configured XML processors evaluate external entity references within XML documents. External entities can be used to disclose internal files using the file URI handler, internal file shares, internal port scanning, remote code execution, and denial of service attacks. Broken Access Control. Restrictions on what authenticated users are allowed to do are often not properly enforced. Attackers can exploit these flaws to access unauthorized functionality and/or data, such as access other users’ accounts, view sensitive files, modify other users’ data, change access rights, etc. Security Misconfiguration. Security misconfiguration is the most commonly seen issue. This is commonly a result of insecure default configurations, incomplete or ad hoc configurations, open cloud storage, misconfigured HTTP headers, and verbose error messages containing sensitive information. Not only must all operating systems, frameworks, libraries, and applications be securely configured, but they must be patched/upgraded in a timely fashion. Cross-Site Scripting XSS. XSS flaws occur whenever an application includes untrusted data in a new web page without proper validation or escaping, or updates an existing web page with user-supplied data using a browser API that can create HTML or JavaScript. XSS allows attackers to execute scripts in the victim’s browser which can hijack user sessions, deface web sites, or redirect the user to malicious sites. Insecure Deserialization. Insecure deserialization often leads to remote code execution. Even if deserialization flaws do not result in remote code execution, they can be used to perform attacks, including replay attacks, injection attacks, and privilege escalation attacks. Using Components with Known Vulnerabilities. Components, such as libraries, frameworks, and other software modules, run with the same privileges as the application. If a vulnerable component is exploited, such an attack can facilitate serious data loss or server takeover. Applications and APIs using components with known vulnerabilities may undermine application defenses and enable various attacks and impacts. Insufficient Logging & Monitoring. Insufficient logging and monitoring, coupled with missing or ineffective integration with incident response, allows attackers to further attack systems, maintain persistence, pivot to more systems, and tamper, extract, or destroy data. Most breach studies show time to detect a breach is over 200 days, typically detected by external parties rather than internal processes or monitoring.

New Critical Exim Flaw Exposes Email Servers to Remote Attacks — Patch Released

References

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