# CVE-2021-23758 — AjaxPro.2

**CISA KEV**

> As of 2026-08-27, CVE-2021-23758 is a HIGH-severity vulnerability in AjaxPro.2, CVSS v3.1 8.1, EPSS 82.5% (99.6th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-08-26), with a US federal remediation deadline of 2026-09-09. Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2021-23758, most recently “Chinese-speaking threat group UAT-10147 uses agentic AI to automate exploitation of internet-facing web servers”.

**Last updated:** 2026-08-27

## What is CVE-2021-23758?

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.

The record classifies CVE-2021-23758 under weakness class [CWE-502](https://intel.threadlinqs.com/cwe/CWE-502). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity, availability. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 1767 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 8.1 — HIGH (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **EPSS (FIRST):** 82.5% probability of exploitation in the next 30 days, higher than 99.6% of all scored CVEs
- **CISA KEV:** Listed since 2026-08-26, federal remediation deadline 2026-09-09
- **Threadlinqs priority:** 9/10 — CISA KEV-listed, which Threadlinqs floors at 9
- **Published:** 2021-12-03, last modified 2026-08-27

## Is CVE-2021-23758 being exploited?

CISA added CVE-2021-23758 to the Known Exploited Vulnerabilities catalog on 2026-08-26, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2026-09-09 under BOD 22-01. Weaponised exploit code for CVE-2021-23758 is publicly available. 2 public proof-of-concept repositories are tracked for this identifier. It currently carries a trending score of 43 in the Threadlinqs vulnerability feed.

- [numanturle/CVE-2021-23758-POC](https://github.com/numanturle/CVE-2021-23758-POC) (github)
- [trickest/cve](https://github.com/trickest/cve/blob/main/2021/CVE-2021-23758.md) (trickest)

## Affected products and versions

- **(vendor unspecified)**: AjaxPro.2

Affected software packages:

- `nuget:AjaxNetProfessional` — patched in 21.11.29.1

## How to fix CVE-2021-23758

The record marks a vendor fix as available for CVE-2021-23758. Fixed versions are recorded for 1 package: nuget:AjaxNetProfessional 21.11.29.1. Patch reference: [https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57](https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57). Because CVE-2021-23758 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2026-09-09. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.

## Threat activity tracking CVE-2021-23758

1 tracked threat in the Threadlinqs corpus references CVE-2021-23758, either in the campaign’s CVE list or as an indicator on the campaign record.

- [Chinese-speaking threat group UAT-10147 uses agentic AI to automate exploitation of internet-facing web servers](https://intel.threadlinqs.com/threat/TL-2026-2096) — HIGH · 2026-08-21

## Sources

Enriched from CVE.org, NVD, FIRST EPSS, CISA KEV, GitHub Security Advisories, public proof-of-concept repositories. Last verified by Threadlinqs on 2026-10-05. This product uses the NVD API but is not endorsed or certified by the NVD.

**Other references**

- [snyk.io](https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971)
- [github.com](https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57)
- [packetstormsecurity.com](http://packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html)

Canonical: https://intel.threadlinqs.com/cve/CVE-2021-23758
Full detection coverage and IOCs for threats exploiting CVE-2021-23758 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp