CVE-2021-20247 – This vulnerability in mbsync allows a malicious... (How to Fix)

Q: What is the severity of CVE-2021-20247?

CVE-2021-20247 has a CVSS score of 7.4 (HIGH). This vulnerability in mbsync allows a malicious or compromised IMAP server to use specially crafted mailbox names containing '..' path components to access data outside designated mailboxes during synchronization. This could lead to unauthorized data access or modification. Users of mbsync versions before 1.3.5 or 1.4.1 who synchronize with untrusted or potentially compromised IMAP servers are affected.

📋 TL;DR

This vulnerability in mbsync allows a malicious or compromised IMAP server to use specially crafted mailbox names containing '..' path components to access data outside designated mailboxes during synchronization. This could lead to unauthorized data access or modification. Users of mbsync versions before 1.3.5 or 1.4.1 who synchronize with untrusted or potentially compromised IMAP servers are affected.

💻 Affected Systems

Products:

mbsync (isync)

Versions: All versions before 1.3.5 and 1.4.1

Operating Systems: Linux, Unix-like systems

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects users who synchronize with IMAP servers. The vulnerability is triggered by malicious server responses, not client configuration.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

A compromised IMAP server could access and exfiltrate or modify sensitive email data from other mailboxes on the client side, potentially including confidential communications or authentication credentials.

🟠

Likely Case

Malicious IMAP servers could access adjacent mailboxes during synchronization, potentially reading or modifying emails that should be restricted.

🟢

If Mitigated

If using only trusted IMAP servers with proper authentication and monitoring, the risk is significantly reduced as the vulnerability requires server-side exploitation.

🌐 Internet-Facing: MEDIUM

🏢 Internal Only: LOW

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploitation requires control of the IMAP server or ability to inject malicious responses. The client must initiate synchronization with the malicious server.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 1.3.5 or 1.4.1

Vendor Advisory: https://bugzilla.redhat.com/show_bug.cgi?id=1928963

Restart Required: No

Instructions:

1. Check current mbsync version with 'mbsync --version'. 2. Update using your distribution's package manager: For Debian/Ubuntu: 'sudo apt update && sudo apt upgrade mbsync'. For Fedora/RHEL: 'sudo dnf update mbsync'. 3. Verify update with 'mbsync --version'.

🔧 Temporary Workarounds

Use only trusted IMAP servers

all

Restrict mbsync synchronization to known, trusted IMAP servers with proper security controls.

Monitor synchronization logs

linux

Enable and monitor mbsync logs for unusual mailbox names or synchronization patterns.

mbsync -V 3 > sync.log

🧯 If You Can't Patch

Discontinue use of mbsync with untrusted IMAP servers until patched.
Implement network segmentation to isolate mbsync traffic to trusted servers only.

🔍 How to Verify

Check if Vulnerable:

Run 'mbsync --version' and check if version is below 1.3.5 or 1.4.1.

Check Version:

mbsync --version

Verify Fix Applied:

After updating, run 'mbsync --version' to confirm version is 1.3.5 or higher, or 1.4.1 or higher.

📡 Detection & Monitoring

Log Indicators:

Unusual mailbox names containing '..' patterns in synchronization logs
Unexpected file access patterns during sync operations

Network Indicators:

Synchronization with untrusted or unknown IMAP servers
Unusual data transfer volumes during sync

SIEM Query:

process.name:"mbsync" AND network.destination.ip:(list_of_untrusted_servers)

📊 Metadata

CVE ID: CVE-2021-20247

CVSS v3 Score: 7.4 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

CWE: CWE-20

Published: February 23, 2021

Last Updated: November 21, 2024

🔗 References

https://bugzilla.redhat.com/show_bug.cgi?id=1928963 Issue Tracking,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CAXQLCK35QGRCRENRTGKJO4VVZGUXUJJ/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/GVDEBZQJMWDW5JFK4NTHH6DAFNAZTESW/
https://security.gentoo.org/glsa/202208-15 Third Party Advisory
https://www.openwall.com/lists/oss-security/2021/02/22/1 Exploit,Mailing List,Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1928963 Issue Tracking,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CAXQLCK35QGRCRENRTGKJO4VVZGUXUJJ/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/GVDEBZQJMWDW5JFK4NTHH6DAFNAZTESW/
https://security.gentoo.org/glsa/202208-15 Third Party Advisory
https://www.openwall.com/lists/oss-security/2021/02/22/1 Exploit,Mailing List,Third Party Advisory

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