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

CVE-2021-23758 is a deserialization of untrusted data weakness in Ajax.NET Professional (also known as AjaxPro). In plain terms, the component can be made to process attacker-controlled serialized input in a way that may lead to remote code execution through arbitrary .NET classes. That matters because AjaxPro has historically been embedded in ASP.NET web applications to support AJAX-style calls; a successful exploit against an exposed instance can give an attacker a foothold on the application server. Public detail is limited to the class of issue and CISA’s summary; confirm exact scope, fixed builds, and configuration notes against the vendor advisory. The product may be end-of-life or end-of-service, which raises the priority of finding and removing or replacing it.

CISA notes that known ransomware use is not documented for this CVE, but remote code execution flaws in web-facing .NET components remain high-value targets. Teams should treat internet-exposed instances as urgent inventory and remediation work, consistent with BOD 26-04-style prioritization of risk and exposure.

How it works

The underlying weakness is CWE-502: deserialization of untrusted data. Deserialization turns a byte stream or structured payload back into live objects. When an application accepts serialized input from a client or other untrusted source and reconstructs objects without strict type allowlisting, an attacker can supply data that instantiates dangerous types or triggers gadget chains in the .NET runtime and loaded assemblies.

For Ajax.NET Professional specifically, CISA states the vulnerability could allow remote code execution via arbitrary .NET classes. At a defender level, that means a request path that hands user-controlled data to the AjaxPro deserialization path may be abused to run code in the context of the web application pool identity. Exact request shapes, endpoints, and exploit mechanics are not provided here; do not assume a particular payload format. Treat any unauthenticated or broadly reachable AjaxPro handler as potentially abusable until you have confirmed hardening or removal against the vendor advisory.

Am I affected? How to find it in your systems

Ajax.NET Professional typically appears inside custom or legacy ASP.NET applications—often as referenced assemblies, script handlers, or AJAX bridge endpoints rather than as a standalone product install. It may be present in older line-of-business sites, internal tools published through reverse proxies, or forgotten virtual directories still reachable from the internet.

• Inventory binaries and configs: Search application servers, deployment packages, and source repos for AjaxPro-related assembly names, config sections, and HTTP handlers/modules that register Ajax.NET Professional. Check both production and secondary environments (staging, DR, old IIS sites).
• Map exposure: Identify which sites bind these handlers to externally reachable URLs. Note authentication requirements; lack of auth increases risk but authenticated-only paths can still be abused after credential theft or CSRF-style abuse depending on design.
• Versions and support status: Record the assembly versions you find and compare them only to what the vendor advisory lists as affected or fixed. CISA warns the product could be EoL/EoS—if no supported fixed line exists for your build, plan to discontinue use.
• Telemetry and logs: Review IIS/HTTP logs for unusual traffic to AjaxPro-related paths, spikes in errors from serialization/type-load failures, unexpected process spawns under the app pool identity, and outbound connections from web servers that do not match baseline behavior. Correlate with EDR alerts for .NET process injection or suspicious child processes. Absence of known public exploit signatures does not mean absence of abuse.

How to remediate

Patch or replace first. Apply the vendor-provided update or mitigation exactly as described in the vendor advisory, and ensure your process aligns with CISA’s direction to apply mitigations per vendor instructions and BOD 26-04 prioritization based on risk and internet exposure. If mitigations are unavailable because the product is unsupported, discontinue use and transition to a supported alternative that does not require unsafe deserialization of client input.

• Remove or replace AjaxPro where the feature is no longer required; delete unused handlers and assemblies so the attack surface cannot be re-enabled by config drift.
• Eliminate untrusted deserialization as a pattern: do not accept serialized .NET object graphs from clients; prefer simple data formats (for example JSON with strict schema validation) and explicit DTOs without polymorphic type discriminators controlled by the client.
• Harden the app pool: run with least privilege, disable unused protocols and modules, and keep the broader .NET and IIS stack patched so secondary gadgets are harder to leverage.
• Re-scan and verify: after change, confirm handlers are gone, assemblies are updated or removed, and external scanners no longer see the old endpoints.

If you can't patch immediately

If you cannot immediately apply a vendor fix or complete a migration, reduce exposure until you can.

• Segment and restrict access: Pull affected applications off the public internet; require VPN or zero-trust access; limit source IPs to known admin or partner ranges at the WAF or network layer.
• Disable the feature: Turn off AjaxPro handlers and related routes in IIS/web.config if business function allows; prefer a temporary feature flag or static fallback over leaving deserialization reachable.
• Virtual patching: Use a WAF or reverse proxy to block or strictly allowlist traffic to AjaxPro endpoints. Rules should be tuned to your observed legitimate traffic; confirm they do not break required flows. Virtual patching is a bridge, not a substitute for removal or a vendor fix.
• Intensify monitoring: Alert on any hit to remaining AjaxPro paths, anomalous POST bodies, app-pool crashes, and new processes or scheduled tasks spawned from the web identity. Preserve forensic-quality logs in line with your incident response and any applicable CISA forensics triage expectations.
• Plan discontinuation: For EoL/EoS deployments where no mitigation path exists, schedule forced cutover; CISA advises discontinuing use when mitigations are unavailable.

If your data may have been exposed

Actively exploited remote code execution vulnerabilities often lead to web shells, credential theft, lateral movement, and data theft even when ransomware use is not documented for the specific CVE. If logs or EDR suggest exploitation—or if an internet-facing instance ran unpatched for a meaningful window—initiate incident response: isolate hosts, capture volatile evidence, rotate secrets the app pool could reach, and assess what data stores the application identity could access. As a routine hygiene step for individuals on the team, you can run a free exposure scan of your work email addresses against known breach datasets to see whether those identities already appear in public compilations, then force password resets and stronger MFA where needed. Confirm all product-specific remediation details against the vendor advisory before closing the ticket.