Updated August 26, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Exploited CVEs / CVE-2021-23758

CVE-2021-23758

Ajax.NET Professional Deserialization of Untrusted Data Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2026-09-09 (that deadline has passed); treat that date as the outer bound for your own environment. Check the vendor security advisory for the fix.

Risk summary

CriticalCISA KEV

9.8

CVSS v3 base

84%

EPSS exploit probability

Critical

Severity

2026-08-26

Added to KEV

2026-09-09 (past due)

CISA due date

Actively exploited Ajax.NET Professional vulnerability, Critical severity, CVSS v3 9.8. Added to the CISA KEV catalog 2026-08-26.

CISA required action Official fix for CVE-2021-23758 Admin questions

Senserva AI Opinion and rich prompt for CVE-2021-23758

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2021-23758, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2021-23758.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (morning and afternoon US Central)):
- CVE-2021-23758: Ajax.NET Professional Deserialization of Untrusted Data Vulnerability
- Affected: Ajax.NET Professional
- Severity: Critical, CVSS v3 9.8; EPSS 30-day exploitation probability 84%
- Added to CISA KEV: 2026-08-26; federal remediation due 2026-09-09
- Ransomware-linked: no
- 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 dis...
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Full tracking and change history for CVE-2021-23758

Published: 2026-08-26
Last revised: 2026-08-26
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

Senserva Watchevery change we have recorded for this one

What changed, and when

The strongest signal is CISA's KEV addition on 2026-08-26, confirming active exploitation, alongside a jump from no score to Severity 9.8 and an initial EPSS of 89.1%. EPSS eased slightly to 83.6% on 2026-08-27, but with confirmed in-the-wild use and critical Severity, IT admins should treat remediation as urgent today.

Drawn by Senserva Vivid from the Senserva change record. Hover any point for its date and value.

2026-08-26 CVSS 0.0 to 9.8; EPSS first scored at 89.1%; added to the CISA KEV catalog by CISA
2026-08-27 EPSS 89.1% to 83.6%

Recorded by Senserva on the dates shown, from CISA KEV and FIRST EPSS. This is what moved and when, which neither the vendor advisory nor NVD publishes.

CISA required action

Federal remediation due date: 2026-09-09 (past due).

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.

Official fix for CVE-2021-23758

Check the vendor's security advisory channel for the patched Ajax.NET Professional versions and workarounds.

CISA KEV entry

Common questions about CVE-2021-23758

Is CVE-2021-23758 actively exploited?

Yes. CVE-2021-23758 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2026-08-26, which means exploitation in the wild has been confirmed, not predicted. Its EPSS 30-day exploitation probability is 84%.

What is the CISA deadline and required action for CVE-2021-23758?

CISA set the federal remediation due date at 2026-09-09, which has passed; treat any unremediated system as overdue. The verbatim 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.

What fixes CVE-2021-23758?

Check the vendor's security advisory for the patched Ajax.NET Professional versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2021-23758?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2021-23758's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (morning and afternoon US Central).

Scope and sources

Authoritative references: CVE record NVD detail CISA KEV catalog FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.