CVE-2020-3811
📋 TL;DR
CVE-2020-3811 is a mail-address verification bypass vulnerability in qmail-verify used in netqmail 1.06. It allows attackers to bypass email address verification checks, potentially enabling spam or phishing emails to be accepted. Systems running netqmail with qmail-verify are affected.
💻 Affected Systems
- netqmail
- qmail-verify
📦 What is this software?
Netqmail by Netqmail
Ubuntu Linux by Canonical
⚠️ Risk & Real-World Impact
Worst Case
Attackers could bypass all email verification, allowing mass spam/phishing campaigns through vulnerable mail servers, potentially leading to credential theft or malware distribution.
Likely Case
Spammers exploit the vulnerability to bypass recipient verification, increasing unwanted email volume and potentially enabling targeted phishing attacks.
If Mitigated
With proper email filtering and security controls, impact is limited to increased spam volume that can be filtered at other layers.
🎯 Exploit Status
Exploitation requires sending specially crafted email to vulnerable server. Technical details are publicly available in security advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: qmail-verify 0.81-1.1 for Debian/Ubuntu
Vendor Advisory: https://www.debian.org/security/2020/dsa-4692
Restart Required: Yes
Instructions:
1. Update qmail-verify package: apt-get update && apt-get install qmail-verify
2. Restart qmail services: svc -t /service/qmail-smtpd
3. Verify the fix by checking version: dpkg -l qmail-verify
🔧 Temporary Workarounds
Disable qmail-verify
linuxTemporarily disable qmail-verify component to prevent exploitation
svc -d /service/qmail-smtpd
edit qmail-smtpd run script to remove qmail-verify invocation
svc -u /service/qmail-smtpd
🧯 If You Can't Patch
- Implement additional email filtering at network perimeter
- Monitor for unusual email volume or verification bypass patterns
🔍 How to Verify
Check if Vulnerable:
Check if qmail-verify version is vulnerable: dpkg -l qmail-verify | grep '0.81-1'Check Version:
dpkg -l qmail-verifyVerify Fix Applied:
Verify installed version is 0.81-1.1 or later: dpkg -l qmail-verify📡 Detection & Monitoring
Log Indicators:
- Unusual patterns in qmail logs showing verification bypass
- Increased email acceptance rates for invalid addresses
Network Indicators:
- Spike in SMTP traffic to vulnerable server
- Unusual email patterns bypassing normal verification
SIEM Query:
source="qmail.log" AND "verification bypass" OR "invalid address accepted"🔗 References
- https://bugs.debian.org/961060
- https://lists.debian.org/debian-lts-announce/2020/06/msg00002.html
- https://usn.ubuntu.com/4556-1/
- https://www.debian.org/security/2020/dsa-4692
- https://www.openwall.com/lists/oss-security/2020/05/19/8
- https://bugs.debian.org/961060
- https://lists.debian.org/debian-lts-announce/2020/06/msg00002.html
- https://usn.ubuntu.com/4556-1/
- https://www.debian.org/security/2020/dsa-4692
- https://www.openwall.com/lists/oss-security/2020/05/19/8
