CVE-2021-3657
📋 TL;DR
This vulnerability in mbsync allows remote attackers to execute arbitrary code by exploiting buffer overflows when processing extremely large IMAP literals. Affected users are those running mbsync versions prior to 1.4.4 to synchronize email with IMAP servers. The vulnerability can be triggered by malicious IMAP servers or potentially by external email senders.
💻 Affected Systems
- mbsync (isync)
📦 What is this software?
Fedora by Fedoraproject
Isync by Isync Project
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with full system compromise, allowing attackers to install malware, steal data, or pivot to other systems.
Likely Case
Denial of service through application crashes, with potential for remote code execution in targeted attacks.
If Mitigated
Limited impact if mbsync is not used or if network segmentation restricts IMAP access to trusted servers only.
🎯 Exploit Status
Exploitation requires interaction with a malicious IMAP server or receiving specially crafted emails. The buffer overflow mechanism is straightforward but requires specific conditions.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.4.4
Vendor Advisory: https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html
Restart Required: No
Instructions:
1. Update mbsync to version 1.4.4 or later using your distribution's package manager. 2. For Debian/Ubuntu: sudo apt update && sudo apt install mbsync. 3. For RHEL/CentOS: Use EPEL repository or compile from source. 4. For source compilation: Download from https://isync.sourceforge.io/ and follow build instructions.
🔧 Temporary Workarounds
Disable mbsync usage
linuxTemporarily stop using mbsync for email synchronization until patched.
sudo systemctl stop mbsync-timer
sudo systemctl disable mbsync-timer
killall mbsync
Restrict IMAP server access
linuxConfigure firewall rules to only allow mbsync to connect to trusted IMAP servers.
sudo iptables -A OUTPUT -p tcp --dport 993 -d trusted.server.ip -j ACCEPT
sudo iptables -A OUTPUT -p tcp --dport 993 -j DROP
🧯 If You Can't Patch
- Disable mbsync entirely and use alternative email synchronization methods
- Implement network segmentation to isolate systems running vulnerable mbsync versions
🔍 How to Verify
Check if Vulnerable:
Check mbsync version with: mbsync --version | head -1Check Version:
mbsync --version | head -1Verify Fix Applied:
Verify version is 1.4.4 or higher: mbsync --version | grep -q '1\.4\.[4-9]\|1\.[5-9]\|2\.' && echo 'Patched'📡 Detection & Monitoring
Log Indicators:
- Segmentation fault crashes in mbsync logs
- Unexpected termination of mbsync processes
- Large IMAP literal transfer attempts in mail logs
Network Indicators:
- Unusual large data transfers on IMAP ports (143, 993)
- Connections to unknown IMAP servers from mbsync
SIEM Query:
process_name='mbsync' AND (event_id='SIGSEGV' OR exit_code=139)🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=2028932
- https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html
- https://security.gentoo.org/glsa/202208-15
- https://www.openwall.com/lists/oss-security/2021/12/03/1
- https://bugzilla.redhat.com/show_bug.cgi?id=2028932
- https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html
- https://security.gentoo.org/glsa/202208-15
- https://www.openwall.com/lists/oss-security/2021/12/03/1
