CVE-2026-31798 Overview
JumpServer, an open source bastion host and operation and maintenance security audit system developed by Fit2cloud, contains an improper certificate validation vulnerability in its Custom SMS API Client. Prior to version v4.10.16-lts, JumpServer fails to properly validate certificates when sending MFA/OTP codes via the Custom SMS API. This vulnerability allows an attacker positioned in the network path to intercept SMS API requests and capture verification codes before they reach the intended user's phone.
Critical Impact
Attackers can intercept MFA/OTP verification codes through man-in-the-middle attacks, potentially bypassing multi-factor authentication protections and gaining unauthorized access to JumpServer-managed infrastructure.
Affected Products
- Fit2cloud JumpServer versions prior to v4.10.16-lts
Discovery Timeline
- March 13, 2026 - CVE CVE-2026-31798 published to NVD
- March 18, 2026 - Last updated in NVD database
Technical Details for CVE-2026-31798
Vulnerability Analysis
This vulnerability falls under CWE-295 (Improper Certificate Validation), a cryptographic weakness that occurs when an application fails to properly verify the authenticity of SSL/TLS certificates during secure communications. In JumpServer's implementation, the Custom SMS API Client does not adequately validate server certificates when establishing connections to external SMS gateway providers.
When JumpServer sends MFA or OTP codes to users via the Custom SMS API, the connection to the SMS provider should be secured with proper TLS certificate validation. However, due to improper validation, an attacker can perform a man-in-the-middle (MITM) attack by presenting a fraudulent certificate. The JumpServer client accepts this certificate without proper verification, allowing the attacker to decrypt, inspect, and potentially modify the SMS API traffic.
The attack requires network positioning between JumpServer and the SMS API endpoint, along with user interaction, making exploitation more complex but still viable in targeted attack scenarios.
Root Cause
The root cause of this vulnerability is improper certificate validation in the Custom SMS API Client component. The application either disables certificate verification entirely, fails to check the certificate chain properly, or does not validate the certificate's Common Name (CN) or Subject Alternative Name (SAN) against the expected hostname. This implementation flaw allows TLS connections to be established with untrusted or malicious endpoints.
Attack Vector
The attack requires network-level access between the JumpServer instance and the external SMS API provider. An attacker must position themselves to intercept network traffic, typically through ARP spoofing on a local network, DNS hijacking, BGP hijacking, or by compromising network infrastructure. Once positioned, the attacker can:
- Intercept the HTTPS request from JumpServer to the SMS API endpoint
- Present a self-signed or malicious certificate to JumpServer
- JumpServer accepts the certificate due to improper validation
- The attacker captures the SMS message contents, including MFA/OTP codes
- Forward the request to the legitimate SMS provider (optional, to avoid detection)
- Use the captured verification code before the legitimate user receives it
The vulnerability allows capture of time-sensitive authentication codes, which can then be used to complete authentication flows before the legitimate user.
Detection Methods for CVE-2026-31798
Indicators of Compromise
- Unexpected certificate errors or warnings in JumpServer logs related to SMS API connections
- Authentication failures from users reporting they never received MFA codes
- Multiple failed MFA attempts followed by successful authentication from different IP addresses
- Unusual network traffic patterns between JumpServer and SMS API endpoints
Detection Strategies
- Monitor network traffic for TLS connections to SMS API endpoints that use unexpected or self-signed certificates
- Implement alerting for authentication anomalies where MFA codes are used before SMS delivery confirmation
- Deploy network intrusion detection systems (IDS) to identify potential MITM attack patterns
- Review JumpServer access logs for authentication patterns indicating code interception
Monitoring Recommendations
- Enable detailed logging for all Custom SMS API transactions in JumpServer
- Implement certificate transparency monitoring for domains used by your SMS providers
- Monitor for DNS resolution anomalies affecting SMS API endpoint hostnames
- Set up alerts for concurrent authentication attempts from geographically disparate locations
How to Mitigate CVE-2026-31798
Immediate Actions Required
- Upgrade JumpServer to version v4.10.16-lts or later immediately
- Review authentication logs for signs of MFA code interception or suspicious login patterns
- Consider temporarily switching to alternative MFA methods (TOTP authenticator apps) until patching is complete
- Audit network segmentation between JumpServer and external API endpoints
Patch Information
The vulnerability is fixed in JumpServer version v4.10.16-lts. Organizations should upgrade to this version or later to address the improper certificate validation issue. For detailed information about the security fix, refer to the JumpServer Security Advisory.
Workarounds
- Consider using TOTP-based authenticator applications instead of SMS-based MFA until the patch can be applied
- Implement network-level controls to restrict JumpServer's outbound connections to known, trusted SMS API endpoints only
- Deploy certificate pinning at the network level if possible to prevent MITM attacks against SMS API connections
- Use VPN or dedicated secure channels between JumpServer and SMS provider infrastructure
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
