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Actively exploited

CVE-2010-4344 Exim Heap-Based Buffer Overflow Vulnerability

Heap-based buffer overflow in the string_vformat function in string.c in Exim before 4.70 allows remote attackers to execute arbitrary code via an SMTP session.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 25 Mar 2022.
  • Rated Critical severity (CVSS 9.8, scored by CISA).
  • It has been in the catalogue for 1,687 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

Heap-based buffer overflow in the string_vformat function in string.c in Exim before 4.70 allows remote attackers to execute arbitrary code via an SMTP session that includes two MAIL commands in conjunction with a large message containing crafted headers, leading to improper rejection logging.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Exim. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 15 Apr 2022 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Exim Flaws

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CVE-2010-4345Exim Privilege Escalation VulnerabilityAdded 25 Mar 2022CVE-2019-16928Exim Out-of-bounds Write VulnerabilityAdded 3 Mar 2022CVE-2019-10149Exim Mail Transfer Agent (MTA) Improper Input ValidationAdded 10 Jan 2022CVE-2018-6789 · RansomwareExim Buffer Overflow VulnerabilityAdded 3 Nov 2021

All 5 exploited Exim vulnerabilities

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2010-4344
  3. CVE record: CVE-2010-4344
  4. FIRST EPSS
  5. lists.opensuse.org/opensuse-security-announce/2010-12/msg00003.html
  6. exim.org/lurker/message/20101207.215955.bb32d4f2.en.html
  7. bugs.exim.org/show_bug.cgi?id=787
  8. securitytracker.com/id?1024858
  9. redhat.com/support/errata/RHSA-2010-0970.html
  10. vupen.com/english/advisories/2010/3186
  11. securityfocus.com/bid/45308
  12. metasploit.com/modules/exploit/unix/smtp/exim4_string_format