What is CVE-2018-6789?

CVE-2018-6789 is a critical-severity vulnerability in Exim, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2018-02-08.

How severe is CVE-2018-6789?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Is CVE-2018-6789 known to be exploited?

Yes. CVE-2018-6789 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 24 public proof-of-concept repositories are indexed.

Buffer overflow in Exim

CVE-2018-6789

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.

Vulnerability class: Buffer Overflow

Published 2018-02-08 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.821 (99.7th percentile), scored 2026-10-17.

High probability of exploitation in the next 30 days — patch promptly. 100th percentile — 99.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.821 · newest: 0.821 · change: 0.000

Affected products

Exim
Canonical Ubuntu Linux — versions 14.04, 16.04, 17.10
Debian Linux — versions 7.0, 8.0, 9.0

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2021-11-03. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2022-05-03.

Required action: Apply updates per vendor instructions.

Known ransomware campaign use: yes.

Public proof-of-concept exploits

References

exim.org/static/doc/security/CVE-2018-6789.txt (Vendor Advisory)
www.openwall.com/lists/oss-security/2018/02/07/2 (mailing-list, Mailing List, Third Party Advisory)
usn.ubuntu.com/3565-1/ (vendor-advisory, Third Party Advisory)
openwall.com/lists/oss-security/2018/02/10/2 (Mailing List, Third Party Advisory)
www.securitytracker.com/id/1040461 (VDB Entry, Third Party Advisory, vdb-entry, Broken Link)
www.debian.org/security/2018/dsa-4110 (vendor-advisory, Mailing List, Third Party Advisory)
www.exploit-db.com/exploits/44571/ (Exploit, VDB Entry, Third Party Advisory, exploit)
lists.debian.org/debian-lts-announce/2018/02/msg00009.html (mailing-list, Mailing List, Third Party Advisory)
git.exim.org/exim.git/commit/cf3cd306062a08969c41a1cdd32c6855f1abecf1 (Patch)
devco.re/blog/2018/03/06/exim-off-by-one-RCE-exploiting-CVE-2018-6789-en/ (Exploit, Third Party Advisory)

Frequently asked questions

What is CVE-2018-6789?: CVE-2018-6789 is a critical-severity vulnerability in Exim, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2018-02-08.
How severe is CVE-2018-6789?: Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2018-6789 known to be exploited?: Yes. CVE-2018-6789 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 24 public proof-of-concept repositories are indexed.