Exploited vulnerabilities / CVE-2021-23758
Confirmed exploited · remote code execution · Ajax.NET ProfessionalCVE-2021-23758
Ajax.NET Professional remote code execution, confirmed exploited in the wild.
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's name for it | Ajax.NET Professional Deserialization of Untrusted Data Vulnerability |
|---|---|
| Vendor and product | Ajax.NET Professional · Ajax.NET Professional |
| Type | remote code execution · CWE-502 |
| Exploited | Confirmed by CISA listed 26 August 2026 |
| Likelihood | one of the most likely to be exploited of every vulnerability tracked |
| Deadline | 9 September 2026 · already passed |
| Used in ransomware | Not reported |
| Advisories | github.com ↗ · www.cisa.gov ↗ · www.cisa.gov ↗ · nvd.nist.gov ↗ |
Three steps, in this order
Patch to the fixed version the vendor names, today if the product faces the internet.
Until it is patched, take the affected service off the internet or put it behind a VPN or allow-list.
Look for signs it was already used: new accounts, web shells, unexpected processes and outbound connections since the date it was listed.
| 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. |
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Know before someone else finds it
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CVE-2021-23758 in short
Is CVE-2021-23758 being exploited?
Yes. CISA added CVE-2021-23758 to its Known Exploited Vulnerabilities catalogue on 26 August 2026, which it does only on evidence of exploitation in the wild.
What is the CISA deadline for CVE-2021-23758?
US federal agencies must remediate it by 9 September 2026. Everyone else should treat that date as the latest safe point to patch an internet-facing system.
How likely is CVE-2021-23758 to be exploited in the next 30 days?
FIRST's Exploit Prediction Scoring System gives it 83%, the 99th percentile of every CVE it scores.
How do I fix CVE-2021-23758?
Patch to the fixed version the vendor names, today if the product faces the internet. Until it is patched, take the affected service off the internet or put it behind a VPN or allow-list. Look for signs it was already used: new accounts, web shells, unexpected processes and outbound connections since the date it was listed.
Sources: CISA Known Exploited Vulnerabilities catalogue (released 10 November 2026) and FIRST EPSS. Page compiled automatically by SAK T.L.M, the threat-intelligence tool of SAK T.E.C, on 13 November 2026.