---
id: "CVE-2010-4344"
kind: "vulnerability"
aliases: ["CVE-2010-4344"]
title: "Exim Exim: Exim Heap-Based Buffer Overflow Vulnerability"
severity: "critical"
cvss: 9.8
epss: 0.71706
exploited: true
status: "open"
category: "runtime"
products: [{"name": "Exim Exim", "fixed": [], "affected": "", "ecosystem": "vendor"}, {"name": "Exim", "fixed": ["4.70", "4.73"], "affected": "< 4.70", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exim", "exploited", "mail-server", "runtime", "self-hosted", "smtp"]
not_affected_if: ["exim-4.70-or-later", "managed-hosting"]
published_at: "2010-12-14T16:00:04+00:00"
first_seen_at: "2026-10-23T19:11:52+00:00"
updated_at: "2026-10-23T21:57:35+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2010-4344", "first_seen_at": "2026-10-23T19:11:52+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2010-4344", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "analyst", "url": null, "first_seen_at": "2026-10-23T21:22:38+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2010-4344", "first_seen_at": "2026-10-23T21:57:35+00:00"}]
---

# Exim Exim: Exim Heap-Based Buffer Overflow Vulnerability

Heap overflow in Exim string_vformat(), fixed in 4.70 (Nov 2009) but not flagged as security at the time. Remote unauthenticated attacker sends an SMTP session with crafted headers on a rejected message; the default config logs rejected headers, which overflows the log buffer and corrupts the MAIL FROM ACL. A second MAIL FROM on the same connection then runs attacker ${run} commands as the Exim user. Chained with CVE-2010-4345 this gives root. Exploited in the wild since Dec 2010.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Exim Exim affected ? fixed ?
- vendor:Exim affected < 4.70 fixed 4.70, 4.73

## How to check

Run 'exim -bV' (or 'exim4 -bV') and read the version line: anything below 4.70 is vulnerable. On Debian/Ubuntu also check the package build: exim4 4.69-9 on Lenny is vulnerable, 4.69-11ubuntu4.1 (karmic) and 4.69-2ubuntu0.2 (hardy) are backported fixes. Mitigated if log_selector contains -rejected_header.

## Fix

Upgrade to Exim 4.73 or later (Exim recommendation; 4.70 contains the fix for this CVE alone). Distro packages with backported patches are fine, for example Debian Lenny security update or Ubuntu 4.69-11ubuntu4.1. Stopgap only: set 'log_selector = -rejected_header', which blocks the known exploit path but not the underlying bug. Also build with ALT_CONFIG_ROOT_ONLY to block the CVE-2010-4345 root escalation.

## Timeline

- 2026-10-23T19:11:52+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2010-4344
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2010-4344
- 2026-10-23T21:22:38+00:00 analyst — Exim team post (exim-dev, 2010-12-10) confirms bug fixed in 4.70 (bug 787, commit 24c929a2), exploited in the wild vs Debian 4.69; 4.73 notes recommend 4.73 (also fixes CVE-2010-4345 root escalation). Ubuntu notes public exploit 2010-12-10. Affected range read as < 4.70 (no lower bound stated).
- 2026-10-23T21:57:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2010-4344

## Documents

- [advisory] [exim-dev] Remote root vulnerability in Exim (Woodhouse, Exim team) https://alioth-lists.debian.net/pipermail/pkg-exim4-maintainers/2010-December/009993.html
- [describes] VU#682457 Exim string_format() buffer overflow https://kb.cert.org/vuls/id/682457
- [describes] Ubuntu CVE-2010-4344 https://ubuntu.com/security/CVE-2010-4344
- [fix] Exim 4.73 release notes https://code.exim.org/exim/wiki/wiki/Exim473ReleaseNotes

## From the sources

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_