CVE-2021-23758 — Ajax.NET Professional Ajax.NET Professional: Insecure Deserialization | CVSS 8.1 HIGH

HIGH

CVSS 8.1 HIGH  ·  EPSS 89%  ·  Ajax.NET Professional Ajax.NET Professional

Severity Overview

Summary

All versions of package ajaxpro.2 are vulnerable to Deserialization of Untrusted Data due to the possibility of deserialization of arbitrary .NET classes, which can be abused to gain remote code execution.

What the Attack Looks Like

How it works: The application deserializes untrusted data without validation. A crafted payload triggers arbitrary code execution during the deserialization process itself.

If successfully exploited: Full confidentiality and integrity impact means an attacker can both read and modify sensitive data — useful for credential harvesting, data theft, or manipulating application state. High availability impact means the vulnerability can also cause a denial of service, which may be used for disruption or as a diversion during a broader attack.

Analyst Takeaway

The attack is launched over the network (remotely exploitable without physical access) and no authentication is required. Exploitation requires specific conditions to be met (high attack complexity), which may reduce reliability in opportunistic campaigns. This vulnerability is already in CISA KEV, which means exploitation has been confirmed in the wild — treat this as active risk, not theoretical exposure. The CVSS score is high enough to justify expedited remediation, especially for exposed systems or assets that handle sensitive data. In parallel with patching, defenders should review external exposure, hunt for signs of exploitation, and validate whether compensating controls are in place for vulnerable assets.

MITRE ATT&CK Mapping

• T1195.002 – Compromise Software Supply Chain
  Rationale: The vulnerability context suggests compromise of software or its delivery/update path.

Detection Guidance

• CWE-502 — Deserialization of Untrusted Data
  • Serialized data in requests: Search request bodies and cookies for known serialization format signatures: Java (AC ED 00 05 hex / rO0 base64), PHP (O: object notation), Python pickle (\x80\x02 or cos), .NET ViewState.
  • Process execution post-deserialization: Deserialization exploits frequently trigger process execution during the deserialization step itself. Hunt for unexpected child processes spawned by the application process — especially Java apps spawning bash or PowerShell.
  • Network callbacks (out-of-band detection): Many deserialization exploits use DNS or HTTP callbacks to confirm exploitation. Monitor for unexpected outbound DNS lookups or HTTP requests from the application server, particularly to services like Burp Collaborator or Interactsh infrastructure.
• T1195.002 — Software Supply Chain Compromise
  • Validate integrity of software updates, packages, and repositories associated with the affected product against known-good vendor hashes.
  • Review CI/CD pipeline activity, build logs, and package download history for anomalies around the vulnerability disclosure window.
  • Confirm software was sourced from official vendor channels and that update infrastructure communications match expected vendor endpoints.

Hunting Considerations

These are proactive hunts mapped to the ATT&CK techniques identified for this CVE. Run them now — do not wait for an alert to fire.

• T1195.002 — Software Supply Chain Compromise
  • Binary integrity: Compare hashes of recently updated software packages and executables against known-good values from the vendor’s official release. Unexpected hash changes in software update paths are the primary supply chain indicator.
  • Build system activity: Review CI/CD pipeline logs for unexpected jobs, unauthorized commits, changes to build scripts, or outbound network connections from build servers to external IPs not in your allowlist.
  • Update server communications: Flag connections to update endpoints that do not resolve to known vendor infrastructure. Supply chain attacks commonly involve redirecting update traffic to attacker-controlled servers.
  • Prioritize recently updated systems: Identify all hosts running the affected product version and focus investigation on those that applied updates during the window of potential compromise — an update received during an active supply chain attack is the highest-risk scenario.

Recommended Actions

Immediate (0–24 Hours)

• Inventory: Identify all systems running Ajax.NET Professional Ajax.NET Professional. Include production, staging, dev, and cloud environments — untracked instances are the most likely to remain unpatched.
• Validate internet-facing exposure: Determine which of the affected systems are reachable from the public internet. Prioritize these for immediate remediation or compensating controls.
• Verify software integrity: Before applying updates, confirm that software packages and update mechanisms have not been tampered with. Compare hashes against vendor-published values.

Remediation

• Apply the vendor patch: 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.
• CISA directive deadline: 2026-09-09 — this is the mandatory deadline for US federal civilian agencies under BOD 22-01. All organizations should treat this date as a strong target regardless of federal mandate.
• Verify remediation: After patching, confirm the correct version is installed on all affected hosts. Run a vulnerability scan or use your asset management tooling to verify — do not rely solely on change tickets.

Detection Coverage

• Verify ATT&CK coverage: Confirm your SIEM and EDR have detection logic in place for T1195.002. Review the Detection Guidance and Hunting Considerations sections of this alert for the specific log sources and behavioral patterns to monitor.
• Unauthenticated exploitation monitoring: Because this vulnerability requires no authentication, internet-facing scanning and exploitation attempts may begin within hours of public disclosure. Ensure alerting is in place before the end of the day.
• Threat intelligence feeds: Monitor your TI feeds and vendor advisory channels for published indicators of compromise (IOCs), proof-of-concept exploit releases, or active campaign reporting associated with this CVE — these should trigger an immediate hunt even if no internal alerts have fired.

Detection Playbooks

Vulnerability Details

Additional Notes

This vulnerability affects an open-source component, third-party library, protocol, or proprietary implementation that could be used by different products. For more information, please see: https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57 ; BOD 26-04: https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk ; Forensics Triage Requirements: https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk ; https://nvd.nist.gov/vuln/detail/CVE-2021-23758