CVE-2020-28009
📋 TL;DR
CVE-2020-28009 is an integer overflow vulnerability in Exim mail transfer agent versions before 4.94.2. It allows remote attackers to cause buffer overflow via unbounded reads in get_stdinput function, potentially leading to remote code execution. All systems running vulnerable Exim versions are affected, particularly mail servers.
💻 Affected Systems
- Exim
📦 What is this software?
Exim by Exim
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with root privileges, complete system compromise, and attacker persistence.
Likely Case
Denial of service through service crashes, with potential for RCE requiring extended exploitation time.
If Mitigated
Limited impact due to exploitation requiring multiple days of sustained attack traffic.
🎯 Exploit Status
Exploitation requires days of sustained traffic to trigger overflow; practical attacks may be limited by this requirement.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.94.2
Vendor Advisory: https://www.exim.org/static/doc/security/CVE-2020-qualys/CVE-2020-28009-STDIN.txt
Restart Required: Yes
Instructions:
1. Download Exim 4.94.2 or later from exim.org. 2. Compile and install following official documentation. 3. Restart Exim service.
🔧 Temporary Workarounds
Rate limit incoming connections
linuxLimit connection rates to reduce attack surface for sustained exploitation attempts
iptables -A INPUT -p tcp --dport 25 -m limit --limit 60/min --limit-burst 100 -j ACCEPT
iptables -A INPUT -p tcp --dport 25 -j DROP
🧯 If You Can't Patch
- Implement network segmentation to isolate Exim servers from critical systems
- Deploy intrusion detection systems to monitor for sustained connection attempts
🔍 How to Verify
Check if Vulnerable:
Check Exim version with: exim -bV | grep versionCheck Version:
exim -bV | grep versionVerify Fix Applied:
Verify version is 4.94.2 or higher: exim -bV | grep version📡 Detection & Monitoring
Log Indicators:
- Unusually high volume of incoming SMTP connections
- Exim process crashes or restarts
Network Indicators:
- Sustained high-volume SMTP traffic from single sources
- Connection attempts exceeding normal baselines
SIEM Query:
source="exim.log" ("crash" OR "segfault" OR "restart") OR source="firewall.log" dest_port=25 rate>1000/min