Man-in-the-Middle (MitM) attack in Exim - CVE-2021-38371

Vulnerability identifier: #VU73857

CSH Severity: Medium

CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N]

CVE-ID: CVE-2021-38371

CWE-ID: CWE-300

Exploitation vector: Remote access

Exploit availability: No public exploit available

The vulnerability allows a remote attacker to perform MitM attack.

The vulnerability exists due to the way Exim handles concurrent STARTTLS sessions when sending out emails. A malicious server can send a response to the *next* command within the existing STARTTLS sessions and force the Exim to treat this session as trusted.

Exim
Amazon Linux AMI
Fedora
Ubuntu
SmartFabric Storage Software
exim4 (Ubuntu package)
exim4-base (Ubuntu package)
eximon4 (Ubuntu package)
exim

Install updates from vendor's website.

Exim - update to 4.95
SmartFabric Storage Software - update to 1.4.3
exim4 (Ubuntu package) - addressed in versions Ubuntu Pro, 4.93-13ubuntu1.11
exim4-base (Ubuntu package) - addressed in versions Ubuntu Pro, 4.93-13ubuntu1.11
eximon4 (Ubuntu package) - addressed in versions Ubuntu Pro, 4.93-13ubuntu1.11
exim - update to 4.92-1.36
exim - update to 4.95-1.el8

• https://www.exim.org
• https://nostarttls.secvuln.info
• https://www.exim.org/static/doc/security/CVE-2021-38371.txt

• Multiple vulnerabilities in Exim
• Amazon Linux AMI update for exim
• Ubuntu update for exim4
• Fedora EPEL 8 update for exim
• Dell SmartFabric Storage Software update for third-party components