CVE-2018-6789: Exim Buffer Overflow Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 3 Nov 2021, 1,364 days after the CVE record was published.

How likely is exploitation in the next 30 days?

82.1%

EPSS score, percentile 99.7 of all scored records

How severe?

9.8 Critical

CVSS v3.1 base score, from CISA's enrichment of the record

What should I do?

Required action (CISA), due 3 May 2022: Apply updates per vendor instructions.

Advisories and patches (2)

Which versions are affected?

Not stated. The CVE record gives vendor and product as “n/a” and the version as “unspecified”.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

CISA due date for this action: 3 May 2022.

Advisories and patches

• USN-3565-1 (Vendor advisory)
• DSA-4110 (Vendor advisory)

Linked from CISA's catalog entry

• https://nvd.nist.gov/vuln/detail/CVE-2018-6789

Affected products and versions

The CVE record does not name affected products or versions: vendor and product are “n/a” and the version is “unspecified”.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

Exim contains a buffer overflow vulnerability in the base64d function part of the SMTP listener that may allow for remote code execution.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

• CISA added CVE-2018-6789 to its Known Exploited Vulnerabilities (KEV) catalog on 3 Nov 2021.
• CISA marks it as known to be used in ransomware campaigns.
• The due date CISA sets for the required action is 3 May 2022.
• The CVE record was published on 8 Feb 2018, 1,364 days before it was added to KEV.
• The CVE record carries a CVSS v3.1 base score of 9.8 (Critical), taken from CISA's enrichment of the record.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 82.1% (percentile 99.7), as of 28 Oct 2026.
• CISA's SSVC assessment in the CVE record: exploitation active, automatable yes, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1. By sending a handcrafted message, a buffer overflow may happen. This can be used to execute code remotely.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

• CWE-120: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
• CWE-119

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

References

• 44571 (Exploit)
• 45671 (Exploit)
• 1040461 (Vulnerability database)
• 103049 (Vulnerability database)
• exim.org
• [oss-security] 20180207 CVE-2018-6789 Exim 4.90 and earlier: buffer overflow
• openwall.com
• [debian-lts-announce] 20180210 [SECURITY] [DLA 1274-1] exim4 security update
• git.exim.org
• devco.re

1 more in the CVE record.

More exploited Exim vulnerabilities

• CVE-2010-4344: Exim Heap-Based Buffer Overflow Vulnerability (added 25 Mar 2022)
• CVE-2010-4345: Exim Privilege Escalation Vulnerability (added 25 Mar 2022)
• CVE-2019-16928: Exim Out-of-bounds Write Vulnerability (added 3 Mar 2022)
• CVE-2019-10149: Exim Mail Transfer Agent (MTA) Improper Input Validation (added 10 Jan 2022)