CVE-2021-31987 Overview
CVE-2021-31987 is an input validation vulnerability affecting Axis network devices running Axis OS. A user-controlled parameter related to SMTP test functionality is not correctly validated, making it possible for attackers to bypass blocked network recipients. This flaw could allow unauthorized communication with restricted network destinations through the device's email notification system.
Critical Impact
Attackers can exploit improper input validation in SMTP test functionality to bypass network access restrictions, potentially enabling unauthorized communication channels through Axis network devices.
Affected Products
- Axis OS (Active track) versions prior to patched release
- Axis OS 2016 (LTS track)
- Axis OS 2018 (LTS track)
- Axis OS 2020 (LTS track)
Discovery Timeline
- 2021-10-05 - CVE-2021-31987 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2021-31987
Vulnerability Analysis
This vulnerability falls under CWE-1286 (Improper Validation of Syntactic Correctness of Input), where user-supplied input to the SMTP test functionality is not adequately validated before processing. The flaw exists within the email notification configuration interface of Axis network devices.
Axis devices commonly include SMTP functionality for sending alerts and notifications. Administrators may configure network restrictions to limit which recipients can receive test emails. However, due to improper validation of user-controlled parameters, these restrictions can be circumvented, allowing test emails to be sent to blocked network destinations.
The attack requires network access to the device's management interface and user interaction, making it a targeted attack scenario rather than one suitable for mass exploitation.
Root Cause
The root cause lies in insufficient input validation of user-controlled parameters within the SMTP test functionality. The application fails to properly sanitize or validate the recipient-related input before processing, allowing specially crafted input to bypass the intended network recipient blocklist mechanism.
Attack Vector
The attack is network-based and requires an attacker to have access to the device's web management interface. The attacker must craft specific input values for the SMTP test functionality that exploit the validation weakness. While no authentication may be required (PR:N in CVSS vector), user interaction is necessary (UI:R), suggesting the attack may require social engineering or user-assisted exploitation.
The vulnerability could be leveraged to:
- Send emails to blocked recipients through the device
- Potentially exfiltrate data through unauthorized email channels
- Bypass security policies designed to restrict outbound communications
Detection Methods for CVE-2021-31987
Indicators of Compromise
- Unexpected SMTP connections from Axis devices to previously blocked network destinations
- Unusual test email activity in device logs
- Modified SMTP configuration settings without authorized changes
- Network traffic anomalies showing email communications to restricted addresses
Detection Strategies
- Monitor network traffic from Axis devices for SMTP connections to unexpected destinations
- Review device audit logs for SMTP test functionality usage patterns
- Implement network-level monitoring for outbound email traffic from IoT/camera devices
- Configure SIEM alerts for unusual email activity originating from Axis device IP ranges
Monitoring Recommendations
- Enable comprehensive logging on Axis devices, particularly for configuration changes and SMTP functionality
- Implement network segmentation to isolate Axis devices and monitor cross-segment traffic
- Deploy network detection solutions capable of inspecting SMTP traffic for anomalies
- Regularly audit device configurations for unauthorized changes to email notification settings
How to Mitigate CVE-2021-31987
Immediate Actions Required
- Review and update all affected Axis devices to the latest firmware versions
- Restrict network access to device management interfaces using firewall rules
- Implement network segmentation to limit potential impact of compromised devices
- Audit current SMTP configurations on all Axis devices for unauthorized settings
- Temporarily disable SMTP test functionality if not operationally required
Patch Information
Axis Communications has released security updates to address this vulnerability. Detailed information about affected firmware versions and remediation steps are available in the Axis Technology Note CVE-2021-31987. Organizations should consult this advisory to identify the specific patched firmware version for their device models and apply updates according to Axis's guidance.
Workarounds
- Restrict access to the device management interface to trusted networks only using firewall ACLs
- Disable SMTP functionality on devices where email notifications are not required
- Implement network-level controls to monitor and restrict outbound SMTP traffic from Axis devices
- Use a dedicated, monitored mail relay for Axis device notifications rather than direct SMTP delivery
- Apply principle of least privilege for user accounts with access to device configuration
# Example: Firewall rule to restrict management interface access
# Limit access to Axis device management port from trusted admin network only
iptables -A INPUT -p tcp --dport 443 -s 10.10.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP
# Block outbound SMTP from camera network segment
iptables -A FORWARD -s 192.168.100.0/24 -p tcp --dport 25 -j DROP
iptables -A FORWARD -s 192.168.100.0/24 -p tcp --dport 587 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
