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

In short

Ajax.NET Professional Ajax.NET Professional has a weakness classified as deserialization of untrusted data (CWE-502). CISA added it to its Known Exploited Vulnerabilities catalog on 2026-08-26. NVD lists CVSS 3.1 at 9.8 CRITICAL.

What it is

“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 KEV, retrieved 2026-11-01

“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.” — NVD, retrieved 2026-11-01

Who is affected

Ajax.NET Professional Ajax.NET Professional. NVD lists the following versions as affected:

• ajaxpro.2 Project ajaxpro.2: all versions listed (no range given)
• Michaelschwarz ajax.net Professional: before 21.10.30.1

Exploitation status

CISA KEV added: 2026-08-26. Known ransomware campaign use: Not reported by CISA. Due date CISA set for US federal civilian agencies: 2026-09-09.

Scores

NVD CVSS 3.1: 9.8 CRITICAL; vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H; source [email protected].

EPSS by FIRST: 82.58%; percentile 99.67; score date 2026-10-31. EPSS estimates the probability of exploitation in the next 30 days.

CWE: CWE-502.

Timeline

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.” — CISA KEV

What to do

• Confirm whether you run Ajax.NET Professional Ajax.NET Professional and which version.
• Apply the vendor update or mitigation in the advisory.
• Follow CISA’s required action.
• Review the vendor guidance on signs of compromise.
• Track the fix through completion.

References

• Patch, Third Party Advisory: https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57
• Third Party Advisory: https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971
• NVD detail
• CVE.org record
• CISA KEV catalog

Sources and dates

CISA Known Exploited Vulnerabilities Catalog, version 2026.10.30; retrieved 2026-11-01. CISA KEV data: CC0 1.0. No endorsement by CISA.

NVD retrieved: 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.

EPSS scores by FIRST; score date 2026-10-31.