# CVE-2021-23758: Ajax.NET RCE via Deserialization of Untrusted Data

> CVE-2021-23758 in Ajax.NET Professional allows remote code execution via untrusted data deserialization, with CISA confirming active exploitation.

- Published: 2026-09-01T02:58:41.000Z
- Severity: critical
- Category: Vulnerabilities
- Tags: CVE-2021-23758, Ajax NET Professional, Deserialization, Remote Code Execution, CISA KEV
- CVEs: CVE-2021-23758
- Author: Runtime Rebel Intel
- Primary source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-23758
- Canonical: https://runtimerebel.com/blog/cve-2021-23758-ajax-net-rce-via-deserialization-of-untrusted-data

## Key points

- Remote Code Execution (RCE) is possible via CVE-2021-23758, with confirmed active exploitation in the wild.
- Ajax.NET Professional (AjaxPro) is vulnerable to deserialization of untrusted data.
- Discontinue use, transition to a supported version, or apply vendor mitigations if available.

## [CVE](/glossary#cve)-2021-23758: Remote Code Execution in Ajax.NET Professional Confirmed Exploited

The [Cybersecurity and Infrastructure Security Agency (CISA)](/glossary#cybersecurity-and-infrastructure-security-agency-cisa) has added [CVE-2021-23758](https://nvd.nist.gov/vuln/detail/CVE-2021-23758) to its Known Exploited Vulnerabilities ([KEV](/glossary#kev)) Catalog, signaling confirmed active exploitation of a critical deserialization flaw in Ajax.NET Professional (AjaxPro). This [vulnerability](/glossary#vulnerability), categorized as a deserialization of untrusted data issue, can lead to remote code execution ([RCE](/glossary#rce)) via arbitrary .NET classes. Given the severe implications of RCE and the confirmation of in-the-wild exploitation by CISA, organizations utilizing Ajax.NET Professional must prioritize immediate remediation to safeguard their systems.

### Technical Details: Ajax.NET Professional Deserialization Vulnerability Analysis

The vulnerability, identified as [CVE-2021-23758](https://nvd.nist.gov/vuln/detail/CVE-2021-23758), specifically impacts Ajax.NET Professional (AjaxPro), a framework used for integrating AJAX functionality into ASP.NET applications. At its core, this flaw is a classic deserialization of untrusted data vulnerability, mapping to [CWE-502](https://cwe.mitre.org/data/definitions/502.html). Deserialization vulnerabilities arise when an application deserializes data without proper validation or sanitization. Attackers can craft malicious serialized objects that, when deserialized by the vulnerable application, execute arbitrary code or commands on the host system. In the context of Ajax.NET Professional, this means an attacker could potentially achieve remote code execution by submitting specially crafted input.

A critical aspect highlighted by CISA is that the impacted product, Ajax.NET Professional, may be end-of-life (EoL) and/or end-of-service (EoS). This status significantly complicates remediation efforts, as official vendor support and patching may no longer be available. The presence of such a severe, actively exploited vulnerability in an unsupported product creates a substantial security risk for any organization still deploying it. The CISA KEV listing for [CVE-2021-23758](https://nvd.nist.gov/vuln/detail/CVE-2021-23758) emphasizes that federal agencies are required to address this vulnerability, with a federal remediation due date of 2026-09-09. This mandate underscores the perceived risk and the necessity for a structured approach to remediation, even for EoL software.

### Impact and [Risk Assessment](/glossary#risk-assessment) of CISA KEV CVE-2021-23758 Exploitation

The active exploitation of [CVE-2021-23758](https://nvd.nist.gov/vuln/detail/CVE-2021-23758) presents a direct and severe threat to any system running vulnerable versions of Ajax.NET Professional. Remote Code Execution (RCE) is one of the most critical vulnerability types, as it typically grants attackers complete control over the compromised system. This can lead to:

*   **[Data Breach](/glossary#data-breach)**: Unauthorized access, exfiltration, or destruction of sensitive data.
*   **System Takeover**: Installation of backdoors, creation of new user accounts, or full compromise of the underlying server infrastructure.
*   **[Lateral Movement](/glossary#lateral-movement)**: Using the compromised system as a pivot point to gain access to other systems within the network.
*   **[Ransomware](/glossary#ransomware) Deployment**: Installation of ransomware or other malicious payloads.

The inclusion in CISA's KEV catalog explicitly means CISA has confirmed active exploitation in the wild, indicating that threat actors are successfully leveraging this vulnerability. The long remediation due date for federal agencies (2026-09-09) for a component potentially already EoL/EoS suggests a significant challenge in fully phasing out or mitigating all instances of this software across large enterprises. Organizations need to understand their exposure, particularly focusing on internet-facing assets that might utilize Ajax.NET Professional.

### Mitigation and Remediation Strategies: How to Mitigate CVE-2021-23758 in Ajax.NET Professional

For organizations still using Ajax.NET Professional, immediate action is required to address the confirmed exploitation of [CVE-2021-23758](https://nvd.nist.gov/vuln/detail/CVE-2021-23758). The primary recommendations from CISA, as outlined in their [Known Exploited Vulnerabilities Catalog](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-23758), are clear:

*   **Discontinue Use**: The most secure recommendation is to discontinue the use of Ajax.NET Professional entirely, especially given its potential end-of-life or end-of-service status.
*   **Transition to a Supported Version/Alternative**: Migrate to a modern, supported framework or component that offers similar functionality without the known critical vulnerabilities.
*   **Apply Vendor Mitigations**: If discontinuing use immediately is not feasible, apply any available mitigations in accordance with vendor instructions. However, for EoL/EoS products, such instructions may be non-existent or insufficient.
*   **Compliance with CISA BOD 26-04**: Adhere to CISA's Binding Operational Directive (BOD) 26-04, "Prioritizing Security Updates Based on Risk," and CISA’s “Forensics Triage Requirements.” This guidance assists federal agencies in managing and prioritizing security updates, but its principles are broadly applicable to all organizations.
*   **Internet Exposure Evaluation**: Stakeholders are responsible for evaluating each asset's internet exposure. Prioritize identifying and securing internet-facing systems that could be vulnerable to this RCE flaw. If mitigations are unavailable for cloud services, discontinuing use is advised.

Given the confirmed active exploitation and the severity of remote code execution, a thorough inventory of assets to identify all instances of Ajax.NET Professional is a critical first step. Organizations must then develop and execute a plan to either decommission, migrate, or apply stringent compensating controls to systems where immediate removal is not possible, ensuring compliance with established security policies and best practices for vulnerable software.

**Related:** [CVE-2026-63077: JetBrains TeamCity RCE via Deserialization](/blog/cve-2026-63077-jetbrains-teamcity-rce-via-deserialization), [CVE-2026-45659: SharePoint RCE Exploitation - Mitigation Guide](/blog/cve-2026-45659-sharepoint-rce-exploitation-mitigation-guide)

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