#VU73857 Man-in-the-Middle (MitM) attack in Exim - CVE-2021-38371

Vulnerability identifier: #VU73857

Vulnerability risk: Medium

CVSSv4.0: 2.7 [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/E:U/U:Green]

CVE-ID: CVE-2021-38371

CWE-ID: CWE-300

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
Exim
Server applications / Mail servers

Vendor: Exim

Description

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.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

Exim: 4.00 - 4.94.2

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

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.