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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Qualys disclosed 21 vulnerabilities in the Exim mail transfer agent on May 4, 2021: 11 were classified as locally exploitable and 10 as remotely exploitable. Qualys said some could be chained to achieve remote code execution and root privileges, but the flaws had different prerequisites; the finding did not mean every Exim server was trivially vulnerable to unauthenticated root access. The 2021 upstream fix was Exim 4.94.2. Administrators should use their operating-system or hosting provider’s current security advisory to determine whether their installed package is fixed.
What Qualys found
Qualys’s “21Nails” disclosure covered 21 distinct Exim vulnerabilities: CVE-2020-28007 through CVE-2020-28026, plus CVE-2021-27216. Qualys and Singapore’s Cyber Security Agency (CSA) divided them into 11 local and 10 remote vulnerabilities.
The issues involved several parts of the mail server, including filesystem and spool handling, memory safety, integer overflows, SMTP message parsing, TLS, and message-header handling. This range matters because the CVE numbers describe separate flaws, not a single bug with one uniform attack path.
What the vulnerabilities could allow
Qualys reported that some of the flaws could be combined to reach remote code execution and root privileges. That is a serious potential impact, but it should not be read as a claim that all 21 vulnerabilities independently let an unauthenticated attacker take over any affected server. Conditions varied by vulnerability and could include the Exim version, TLS implementation, authentication, configuration, or available memory.
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One example of the set’s long history is CVE-2020-28017: Qualys said it affected Exim versions dating back to 2004. That statement applies to this specific vulnerability, not automatically to every CVE in the disclosure.
Which Exim versions were affected
The 2021 advisories identified Exim versions before 4.94.2 as affected and recommended upgrading to Exim 4.94.2. This is the historical upstream remediation boundary for 21Nails, not a statement that 4.94.2 is the newest Exim release today.
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Operating-system vendors sometimes backport security fixes into packages that retain an older-looking upstream version number. For that reason, compare the package installed on your server with your distribution’s security advisory and fixed package version instead of relying on the version string alone.
How administrators should respond
- Find Exim systems. Identify servers and other hosts running Exim, including systems managed by a hosting provider. If your organization uses an asset or vulnerability-management system, use it to help locate deployments and prioritize findings.
- Check the relevant vendor advisory. Consult the security notice for the operating system or hosting service on each affected host. Confirm the fixed package or update specified for that platform; package versions and remediation paths can differ between vendors.
- Install the vendor’s fixed update. Apply the update through the platform’s supported package or maintenance process. If a host is managed by a provider, confirm that the provider has applied the relevant security fix.
- Verify the result. Check the installed package against the vendor’s stated fixed version or security status. Keep the advisory and verification record with your normal patch-management documentation.
Qualys also described using its VMDR service to discover Exim assets and prioritize vulnerability findings. Asset visibility can help locate systems that need attention, but it does not replace installing and verifying the applicable security update.
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When the vulnerabilities were disclosed
Qualys says it notified the Exim team on October 20, 2020, and publicly released its coordinated disclosure on May 4, 2021. Its advisory said Qualys would not publish its own exploit code at that time. In a separate notice dated May 6, 2021, Singapore’s CSA said proof-of-concept code was publicly available for several vulnerabilities. Those dated statements describe the disclosure period; they do not establish what exploit code is available now.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the name “21Nails”?
“21Nails” refers to the 21 vulnerabilities disclosed in Exim, a mail transfer agent. It is a label for the group of findings, not the name of a single vulnerability.
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