CVE-2021-23758: in the CISA KEV catalogue
Ajax.NET Professional Deserialization of Untrusted Data Vulnerability. CISA added CVE-2021-23758 to the Known Exploited Vulnerabilities catalogue on 2026-08-26 with a default due date of 2026-09-09. Known ransomware campaign use: Unknown. Forensic triage flag: No.
Ajax.NET Professional (AjaxPro) contains a deserialization of untrusted data vulnerability that could allow for remote code execution via arbitrary .NET classes. The impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version. (CISA description)
- Vendor / product
- Ajax.NET Professional Ajax.NET Professional
- CWE
- CWE-502
- CISA required action
- 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.
- EPSS
- 83%, percentile 99 (scored 2026-11-03)
EPSS puts about 83 in 100 odds (83%) on exploitation activity being observed in the next 30 days. Its score is higher than or equal to that of 99% of scored CVEs. This is a model estimate about exploitation anywhere, not about your environment.
Vendor and reference links (from CISA)
- https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57
- https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk
- 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
Your own deadline under CISA BOD 26-04 depends on exposure, automatability and technical impact: work it out with the BOD 26-04 calculator.
Frequently asked questions
Is CVE-2021-23758 in the CISA KEV catalogue?
Yes. CISA added CVE-2021-23758 (Ajax.NET Professional Deserialization of Untrusted Data Vulnerability) to the Known Exploited Vulnerabilities catalogue on 2026-08-26. CISA's default due date for it is 2026-09-09.
Is CVE-2021-23758 used in ransomware attacks?
CISA marks known ransomware campaign use as Unknown for CVE-2021-23758. That means CISA lacks confirmation, not that ransomware groups do not use it.
What should I do about CVE-2021-23758?
CISA's required action: 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.
Sources
- CISA, Known Exploited Vulnerabilities Catalog (accessed 2026-11-03)
- FIRST, Exploit Prediction Scoring System (EPSS) (EPSS by FIRST; scores generated by Empirical Security)
- NVD record for CVE-2021-23758