Exim Mail Server string_vformat Heap-based Buffer Overflow Remote Code Execution (RCE) Vulnerability (CVE-2010-4344)
Executive & SecOps Summary
SECOPS ASSESSMENTExim Mail Transfer Agent (MTA) versions prior to 4.70
Unauthenticated remote attackers can execute arbitrary code with root privileges and fully compromise the mail server via a crafted SMTP session.
Immediately upgrade Exim to the latest stable version (4.70 or higher) and restrict SMTP port access to trusted networks until patched.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
The primary remediation is upgrading Exim to version 4.70 or later.
For Debian/Ubuntu-based systems:
sudo apt-get update
sudo apt-get install --only-upgrade exim4
# Verify the installed version
exim4 -bVFor RedHat/CentOS-based systems:
sudo yum update exim
# Or using dnf
sudo dnf upgrade exim
# Restart the service
sudo systemctl restart exim2. Temporary Workaround & Hardening Configuration
If patching cannot be applied immediately, restrict access to SMTP ports (25, 465, 587) using firewall rules to allow only trusted IP ranges.
# Example: Restrict SMTP access to a trusted subnet (e.g., 192.168.1.0/24)
sudo iptables -A INPUT -p tcp -s 192.168.1.0/24 --dport 25 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 25 -j DROP3. Log Audit & IoC Verification Commands
Use the following commands to check the running Exim version and inspect mail logs for potential exploitation attempts or anomalous rejection patterns.
# 1. Check the running Exim version
echo '=== Exim Version Check ==='
exim -bV | grep -i 'version'
# 2. Scan mail logs for abnormal rejection patterns
echo '=== Scanning Mail Logs for Anomalies ==='
sudo grep -i 'rejected' /var/log/exim4/mainlog | head -n 20
sudo zgrep -i 'string_vformat' /var/log/exim4/mainlog* 2>/dev/null || echo 'No immediate string_vformat crash logs found.'Stop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2010-4344) is a heap-based buffer overflow in the string_vformat function within string.c of the Exim Mail Transfer Agent (MTA). The root cause lies in the improper validation of input lengths when Exim formats error messages or rejection logs, allowing input data to exceed the boundaries of the allocated heap buffer.
2. Exploitation Mechanics & Attack Scenario
An unauthenticated remote attacker can exploit this vulnerability by establishing an SMTP connection (typically port 25) and executing the following sequence:
- The attacker transmits two consecutive
MAILcommands within a single SMTP session. - Concurrently, the attacker sends a large, specially crafted email message containing oversized headers.
- This triggers Exim's improper rejection logging mechanism.
- When
string_vformatattempts to process and log the rejection, the heap buffer overflows, overwriting adjacent memory structures or function pointers to hijack the execution flow and run arbitrary shellcode.
3. Infrastructure & Business Impact
Exim is a critical infrastructure component widely deployed as the default MTA on numerous Linux and Unix servers. With a CVSS score of 9.8 (Critical) and its inclusion in the CISA KEV (Known Exploited Vulnerabilities) catalog, this vulnerability poses an extreme risk. Successful exploitation allows complete system takeover, enabling attackers to exfiltrate sensitive emails, pivot into internal networks, or utilize the server as a spam distribution node.
Official Vendor Advisories & References
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.
Frequently Asked SecOps Questions (CVE-2010-4344)
FAQQ1. How critical is CVE-2010-4344 and is it actively exploited in the wild?
CVE-2010-4344 is rated CVSS 9.8 (CRITICAL) with a 30-day EPSS exploitation probability of 71.7%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2010-4344?
Primary impacted vendors include exim across exim. Scope summary: All Exim versions prior to 4.70
Q3. What temporary mitigations or workarounds exist for CVE-2010-4344 before patching?
[Mitigation] Immediately upgrade Exim to the latest stable version (4.70 or higher) and restrict SMTP port access to trusted networks until patched. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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