CVE-2023-38686
📋 TL;DR
Sydent, an identity server for Matrix, fails to verify SMTP server certificates when sending emails via TLS, making email communications vulnerable to interception. Attackers with network access can intercept room invitations and address confirmation emails. This affects all Sydent deployments using TLS email configuration prior to version 2.5.6.
💻 Affected Systems
- Sydent
📦 What is this software?
Sydent by Matrix
⚠️ Risk & Real-World Impact
Worst Case
Attackers intercept sensitive Matrix communications including room invitations and account verification emails, enabling account takeover, unauthorized room access, and data exfiltration.
Likely Case
Privileged network attackers intercept email-based communications, potentially compromising user privacy and enabling social engineering attacks.
If Mitigated
With proper certificate verification, email communications remain secure even if network traffic is intercepted.
🎯 Exploit Status
Requires privileged network access to perform MITM attacks against SMTP traffic.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.5.6
Vendor Advisory: https://github.com/matrix-org/sydent/security/advisories/GHSA-p6hw-wm59-3g5g
Restart Required: Yes
Instructions:
1. Upgrade Sydent to version 2.5.6 or later. 2. Ensure Sydent trusts the SMTP server's certificate. 3. For self-signed certificates, add the CA certificate to the system's trust store. 4. Restart Sydent service.
🔧 Temporary Workarounds
Disable Email Functionality
allConfigure SMTP server to a non-routable address to prevent email sending
Edit Sydent configuration to set SMTP server to loopback address (e.g., 127.0.0.1:25)
🧯 If You Can't Patch
- Implement network segmentation to restrict access to SMTP traffic
- Use VPN or encrypted tunnels for all SMTP communications
🔍 How to Verify
Check if Vulnerable:
Check Sydent version and verify if using TLS email configuration in versions prior to 2.5.6Check Version:
Check Sydent logs or configuration for version informationVerify Fix Applied:
Confirm Sydent version is 2.5.6+ and test email sending with TLS to verify certificate validation📡 Detection & Monitoring
Log Indicators:
- Failed email deliveries
- SMTP connection errors
- Certificate validation warnings
Network Indicators:
- Unencrypted SMTP traffic despite TLS configuration
- Unexpected MITM activity on SMTP ports
SIEM Query:
Search for SMTP connection attempts without certificate validation or TLS handshake failures🔗 References
- https://docs.python.org/3/library/ssl.html?highlight=ssl#security-considerations
- https://github.com/matrix-org/sydent/commit/1cd748307c6b168b66154e6c4db715d4b9551261
- https://github.com/matrix-org/sydent/pull/574
- https://github.com/matrix-org/sydent/releases/tag/v2.5.6
- https://github.com/matrix-org/sydent/security/advisories/GHSA-p6hw-wm59-3g5g
- https://github.com/python/cpython/issues/91826
- https://peps.python.org/pep-0476/
- https://docs.python.org/3/library/ssl.html?highlight=ssl#security-considerations
- https://github.com/matrix-org/sydent/commit/1cd748307c6b168b66154e6c4db715d4b9551261
- https://github.com/matrix-org/sydent/pull/574
- https://github.com/matrix-org/sydent/releases/tag/v2.5.6
- https://github.com/matrix-org/sydent/security/advisories/GHSA-p6hw-wm59-3g5g
- https://github.com/python/cpython/issues/91826
- https://peps.python.org/pep-0476/
