CVE-2021-23758 KEVPublic exploit Critical · 9.8 EPSS 82.6%

Ajax.NET Professional Deserialization of Untrusted Data Vulnerability

Ajax.NET Professional · Ajax.NET Professional

Summary

Ajax.NET Professional (the ajaxpro.2 package) contains a deserialization of untrusted data flaw, which permits deserialization of arbitrary .NET classes. An attacker with network access can exploit this without authentication or user interaction, using a crafted request to gain remote code execution.

Official description · NVD

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.

Breakdown

79.8 / 100

Exploited: Yes, in CISA KEV
Reports: 0
Tactics: 2/15

How an attack unfolds

Based on mappings. Select a step to see the evidence.

How attackers get in
- Break in through an internet-facing appT1190 · Exploit Public-Facing ApplicationAI-written (claude-haiku-5-5)Medium confidence149 general detection rules
What they do
- Run commands on the systemT1059 · Command and Scripting InterpreterAI-written (claude-haiku-5-5)Medium confidence95 general detection rules

First seen exploited 5 years after disclosure. Public exploit code (Metasploit) was available the same day.

DisclosedPublic exploit codeMetasploit3 Dec 2021day 0
First seen exploitedVulnCheck KEV evidence20 Aug 2026+5 yrs
Added to CISA KEVEU database agrees26 Aug 2026+5 yrs

CISA KEV listing is when exploitation was confirmed, not necessarily the first attack.

See the family tree

3 related vulnerabilities in the same code over 2 years. The latest fix listed for AjaxNetProfessional is 21.12.22.1.

Same bug familyLinked by evidence that a fix failed or the same code was patched again. Oldest first.

This CVECVE-2021-23758Ajax.NET Professional Deserialization of Untrusted Data3 Dec 2021KEV
relatedHigh
Why these are linked:
- Fixed one release later21.11.29.1 → 21.12.22.1 (AjaxNetProfessional)
- Same code patched: Constant.cs, core.js (machine-matched)Functions: Constant, extendcommit b0e63be commit c89e39b
- Similar weakness in the same product (machine-matched)same package + description similarity
CVE-2021-43853Cross-Site Scripting in AjaxNetProfessional22 Dec 2021
same family
CVE-2023-49289Cross-site Scripting in Ajax.NET Professional4 Dec 2023

Fixes in AjaxNetProfessionalNuGet package, by fixed version. Earlier and later fixes in the same package, not necessarily the same bug.

21.11.29.1CVE-2021-237583 Dec 2021KEV
21.12.22.1CVE-2021-4385322 Dec 2021
21.12.22.1CVE-2023-492894 Dec 2023Latest listed

techniques

2 , grouped by in kill-chain order, 2 cited passages 2

Initial AccessTA0001

Exploit Public-Facing Application T1190 Medium

AI mapping from the description · how it is exploited

Remote unauthenticated deserialization exploit against public-facing app

Deserialization of Untrusted Data due to the possibility of deserialization of arbitrary .NET classes

From the official description (NVD) Original ↗

ExecutionTA0002

Command and Scripting Interpreter T1059 Medium

AI mapping from the description · primary impact

Deserialization yields arbitrary code execution

can be abused to gain remote code execution

From the official description (NVD) Original ↗

Detection

0 rules for this CVE · 244 general rules for its techniques

Rules for this CVE

0 that reference CVE-2021-23758 directly.

CVE-specific rules are not available in this build. Search SigmaHQ ↗

General rules for this CVE’s techniques

These detect the technique in general, not this specific vulnerability.

Exploit Public-Facing ApplicationT1190149 general rules

Arcadyan Router Exploitationscriticaltest
Citrix ADS Exploitation CVE-2020-8193 CVE-2020-8195criticaltest
Citrix Netscaler Attack CVE-2019-19781criticaltest
Confluence Exploitation CVE-2019-3398criticaltest
CVE-2010-5278 Exploitation Attemptcriticaltest
CVE-2020-0688 Exchange Exploitation via Web Logcriticaltest
CVE-2020-10148 SolarWinds Orion API Auth Bypasscriticaltest
CVE-2020-5902 F5 BIG-IP Exploitation Attemptcriticaltest

All 149 on SigmaHQ ↗

Command and Scripting InterpreterT105995 general rules

HackTool - Sliver C2 Implant Activity Patterncriticaltest
Lazarus Group Activitycriticaltest
REvil Kaseya Incident Malware Patternscriticaltest
Turla Group Lateral Movementcriticaltest
Windows Defender AMSI Trigger Detectedhighstable
Windows Defender Threat Detectedhighstable
Abusable DLL Potential Sideloading From Suspicious Locationhightest
Add Insecure Download Source To Wingethightest

All 95 on SigmaHQ ↗

Sigma rules © SigmaHQ, Detection Rule License 1.1; linked, not reproduced.

Am I affected? What do I upgrade to?

From GitHub Security Advisories and OSV

1 package in 1 ecosystem. Main package: AjaxNetProfessional.

NuGetAjaxNetProfessionalGHSA-6r7c-6w96-8pvw

Upgrade to 21.11.29.1 or later

Set the version to 21.11.29.1 or later in your .csproj.

The first fix for CVE-2021-23758. No other CVE tracked by Patchfirst affects this package.

Affected versions

If you run	Upgrade to
All versions before 21.11.29.1	21.11.29.1

“Up to” means the second version is the fix and is not affected.

Check your lockfile against all CVEs

What security researchers reported

Not named in any threat report yet

Also mentioned in 34 community threat pulses (AlienVault OTX)

Weaker evidence

Pulses are indicator collections that anyone can publish. A mention is not confirmation of exploitation.

Actively_Exploited_Kev | Oct 29, 202628 Oct 2026
Actively_Exploited_Kev | Oct 28, 202627 Oct 2026
Actively_Exploited_Kev | Oct 27, 202626 Oct 2026
Actively_Exploited_Kev | Oct 24, 202623 Oct 2026
Actively_Exploited_Kev | Oct 23, 202622 Oct 2026
Actively_Exploited_Kev | Oct 22, 202621 Oct 2026
28 more on OTX

References

5 links, grouped by NVD tag (3 duplicates merged)

Exploitation evidence

Exploited in the wild

Listed since 26 Aug 2026. Federal due date 9 Sep 2026. No recorded ransomware use.

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.

ENISA

Marked exploited since 26 Aug 2026. EUVD-2021-2466

Public exploitAdvisories link exploit code.

, 30 Oct 2026

82.6%Top 0.33%

Chance of exploitation in the next 30 days. of scored CVEs (riskier than 99.67%).

3.1

9.8Critical · 9.8

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Priority factorsFormula

In a KEV cataloglisted in CISA KEV and VulnCheck KEV 35.0/35

Recent KEV listingadded 72 days ago 10.0/10

Known ransomware usenot known 0.0/10

EPSS percentilepercentile 0.9967 19.9/20

Threat reports0 reports 0.0/10

Public exploit code2 NVD references tagged Exploit; 1 PoC-in-GitHub repo; 1 Metasploit module 5.0/5

Mentioned in OTX pulses34 pulses 3.0/3

CVSS base scorebase score 9.8 6.9/7

End-of-life productsupported or unknown 0.0

Total79.8

Record

Published: 3 Dec 2021
Modified: 27 Aug 2026
: CWE-502 Deserialization of Untrusted Data
Elsewhere: NVD CVE.org KEV JSON

How an attack unfolds

1How attackers get in

2What they do

Exploitation timelineExploitation timelineDated milestones for a CVE: when it was disclosed, when exploit code became public, when exploitation was confirmed, and when threat reports named it.

Patch historyPatch historyEarlier and later CVEs in the same code, in date order. It shows when a fix was bypassed or incomplete and which version finally fixed it.

Same bug familyLinked by evidence that a fix failed or the same code was patched again. Oldest first.

Fixes in AjaxNetProfessionalNuGet package, by fixed version. Earlier and later fixes in the same package, not necessarily the same bug.

ATT&CKMITRE ATT&CKA public catalogue of the techniques attackers use, maintained by MITRE. Patchfirst uses it to describe what attackers do with a vulnerability. techniques

Initial AccessTA0001

ExecutionTA0002

Detection

Rules for this CVE

General rules for this CVE’s techniques

Am I affected? What do I upgrade to?

Affected versions

What security researchers reported

Also mentioned in 34 community threat pulses (AlienVault OTX)

References

Patch 1

US Government Resource 1

Third Party Advisory 2

Exploit 1

How attackers get in

What they do

techniques