CVE-2021-23758: Ajax.NET Professional Deserialization of Untrusted Data Vulnerability

Immediate response plan

1. Identify exact product versions, owners, exposure paths, and business-critical dependencies.
2. Apply the latest vendor remediation or isolate the vulnerable path when immediate patching is not possible.
3. Review available telemetry for exploitation attempts before restarting, rebuilding, or rotating evidence away.
4. Validate the fixed version and control health, then record any exception with an owner and expiry date.

Official remediation direction

CISA due date: 2026-09-09. Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

Detection and validation

Confirm the affected version and reachable component, preserve useful telemetry, apply the publisher guidance, and record the evidence used to close the item.

• Confirm the installed and running version of Ajax.NET Professional against the current vendor advisory.
• Check whether the vulnerable interface is reachable from untrusted networks or lower-privileged identities.
• Look for new accounts, privilege changes, crashes, child processes, or unusual outbound traffic associated with the component.
• Run a post-remediation service and security-control health check and retain evidence with the change record.

Accuracy note

Severity, affected-version ranges, and remediation details can change as the vendor and vulnerability databases add evidence. Recheck the linked primary records before closing the incident or approving a long-lived exception.

Primary sources and references

• CISA Known Exploited Vulnerabilities: CVE-2021-23758: Ajax.NET Professional Deserialization of Untrusted Data Vulnerability