CVE-2021-32749: Possible RCE vulnerability in mailing action using mailutils (mail-whois)

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 3.4% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2021-07-16 · updated 2026-06-17
Weakness: CWE-78

Description

fail2ban is a daemon to ban hosts that cause multiple authentication errors. In versions 0.9.7 and prior, 0.10.0 through 0.10.6, and 0.11.0 through 0.11.2, there is a vulnerability that leads to possible remote code execution in the mailing action mail-whois. Command `mail` from mailutils package used in mail actions like `mail-whois` can execute command if unescaped sequences (`\n~`) are available in "foreign" input (for instance in whois output). To exploit the vulnerability, an attacker would need to insert malicious characters into the response sent by the whois server, either via a MITM attack or by taking over a whois server. The issue is patched in versions 0.10.7 and 0.11.3. As a workaround, one may avoid the usage of action `mail-whois` or patch the vulnerability manually.

Affected products

fail2ban: through 0.9.7; from 0.10.0 before 0.10.7; from 0.11.0 before 0.11.3 — fixed in 0.10.7, 0.11.3
Fedoraproject Fedora: 34, 35

How to fix

Upgrade to a fixed version: fail2ban 0.10.7, 0.11.3. Check the vendor advisory for mitigations.

References

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