CriticalCVE-2021-23758

Deserialization of Untrusted Data (CVE-2021-23758)

Published 03.12.2021Last modified 26.08.2026In CISA KEV since 26.08.2026Sources: OSV, CVE.org

What is affected, what to do

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 vulnerability is being actively exploited; treat it as a priority. Update ajax.net_professional to version 21.10.30.1. Immediate actions, mitigation and detection rules are in the runbook below.

Affected: AjaxPro.2 eq 0..0

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

9.8

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: NVD, CVSS v3

8.1

CVSS v3

Severity High on the scale from 0 to 10.

Source: OSV, CVE.org

82.6 %

EPSS score

High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

Source: FIRST EPSS, percentile 99.67

Show all metrics

Severity: Critical
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 220.08 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: actively exploited
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 9.8 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 8.1
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 82.6 %; High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 99.67; Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: Actively exploited
Published: 03.12.2021
In CISA KEV since The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: 26.08.2026
CISA assessment (SSVC): Exploitation: active · not automatable · Technical impact: total; Source: CISA Vulnrichment, as of 26 August 2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-502

Response and mitigation

Why act now?

Prioritisation rationale

The combination of unauthenticated network reachability, trivial exploitation via .NET deserialisation (ready-made ysoserial.net gadgets) and the end-of-life status of the legacy package explains the KEV listing — no patch for ajaxpro.2 will ever appear, so the attack surface is permanent until the component is removed. Exploitation likelihood ranks among the highest across the entire CVE corpus and the CVSS profile is worst-case (full compromise, no user interaction). Treat every internet-exposed instance as P1 and internal instances as P2 — particularly relevant for public-sector portals, customer portals and long-lived line-of-business apps common in European mid-market and KRITIS environments.

01
Runbook step 1
Immediate response (0-24 h)
- Inventory all .NET web applications (IIS, Azure App Service, self-hosted ASP.NET) for AjaxPro usage. Grep for AjaxPro.dll, ajaxpro.2.dll, AjaxPro.Utility, .ashx handlers referencing AjaxPro.AjaxHandlerFactory, and web.config entries mapping ajaxpro/*.ashx.
- Upgrade to the fixed release michaelschwarz/ajax.net_professional 21.10.30.1 (NuGet AjaxNetProfessional 21.11.29). The legacy ajaxpro.2 package is end-of-life and will not receive a fix — replacement is mandatory, no patch exists.
- If an upgrade cannot ship within hours, remove the AjaxPro.AjaxHandlerFactory mapping from web.config (<httpHandlers> / <handlers> sections) and recycle the IIS AppPool. Run functional smoke tests afterwards.
- Configure upstream reverse proxies / WAF (F5, NetScaler, Azure Front Door, nginx) to drop POST requests to *ajaxpro*.ashx and */ajaxpro/* until the upgrade is live in production.
- Incident readiness: treat affected IIS hosts as potentially compromised, capture memory snapshots, w3wp.exe dumps and 90 days of IIS logs. Exploitation likelihood for this CVE is among the highest observed.
- Rotate credentials of the AppPool identity and any downstream service accounts (SQL logins, AD service users) if any exploitation indicator surfaces.
02
Runbook step 2
Mitigation layers
- Network segmentation: move affected legacy web servers into an isolated VLAN with strict egress filtering (no outbound 80/443/445/3389 except to documented destinations) to break second-stage download and lateral movement.
- WAF rule: inspect request bodies for serialised .NET payload markers (__type, AjaxPro.AjaxRequestHelper, System.Windows.Data.ObjectDataProvider, TypeConfuseDelegate, WindowsIdentity) — these are the standard ysoserial.net gadget fingerprints. Block any hit against AjaxPro endpoints.
- Endpoint hardening: forbid w3wp.exe from spawning cmd.exe, powershell.exe, rundll32.exe, certutil.exe, bitsadmin.exe via AppLocker, WDAC or EDR ASR rule "Block all Office / web-server child processes".
- IAM least privilege: the AppPool identity must not run as LocalSystem, domain admin or local administrator. Use a dedicated gMSA with minimal database rights.
- Configuration hardening: set trust level="Medium" in web.config (reduces but does not eliminate RCE impact); apply restrictive requestValidationMode and maxRequestLength.
- Medium-term: plan migration off the legacy stack — AjaxPro has not been maintained for years and continued operation is hard to defend under NIS2 risk-management obligations.
03
Runbook step 3
Detection rules
- IIS access logs: POST requests whose URI contains ajaxpro and .ashx with unusually large cs-bytes (>5 KB) or base64-like query-string content. KQL sketch: W3CIISLog | where csUriStem has "ajaxpro" and csMethod == "POST" | where csBytes > 5000.
- Sysmon EID 1: parent w3wp.exe spawning cmd.exe, powershell.exe, csc.exe, conhost.exe or net.exe. Sigma shape: parent_image endswith '\w3wp.exe' and image endswith ('\powershell.exe','\cmd.exe','\certutil.exe').
- Sysmon EID 11 / EID 3: w3wp.exe writing files into C:\inetpub\wwwroot\* (webshell drop) or opening outbound connections to undocumented IPs on 443/4444/8080.
- Windows Security EID 4688 with command-line logging: look for base64-encoded PowerShell payloads (-enc, -EncodedCommand, FromBase64String) whose parent chain traces back through IIS.
- Suricata/Zeek: custom HTTP rule on POST to .ashx with Content-Type application/json or text/plain and payload markers __type, ObjectDataProvider, BinaryFormatter. Example: content:"ObjectDataProvider"; http_client_body;.

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Source: OSV (Open Source Vulnerabilities), CISA KEV Catalog, NVD (NIST), cvelistv5

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
—	AjaxPro.2	`0`	—
ajaxpro.2_project	ajaxpro.2	`—`	—
michaelschwarz	ajax.net_professional	`< 21.10.30.1`	`21.10.30.1`
nuget	ajaxnetprofessional	`21.11.29` `21.10.30` `21.11.22`	— — —

Vendors and products

Michaelschwarz Ajax.NET-Professional

Weakness classes (CWE)

CWE-502Deserialization of Untrusted Data
Base
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.
Entry at MITRE CWE-502

Public exploit references

Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

Public exploit references

CISA KEV Catalog	CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov	actively exploited	26.08.2026
Packet Storm	AjaxPro Deserialization Remote Code Execution (opens in a new tab)packetstormsecurity.com	PoC (publicly reported)reported, not verified	—

References & sources

github.commichaelschwarz/Ajax.NET-Professional/security/advisories/GHSA-6…web
NVD (NIST)vuln/detail/CVE-2021-23758advisory
github.commichaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce5…web
github.commichaelschwarz/Ajax.NET-Professionalpackage
security.snyk.iovuln/SNYK-DOTNET-AJAXPRO2-1925971web
packetstormsecurity.comfiles/175677/AjaxPro-Deserialization-Remote-Code-Execution.htmlweb
snyk.iovuln/SNYK-DOTNET-AJAXPRO2-1925971
blog.talosintelligence.comuat-10147-chinese-speaking-adversary-integrates-agentic-ai-into…Third-party advisory
cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-23758government-resource

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

26.08.2026NVD analysis modifiedNVD

Product mapping changed:

Ajaxpro.2
Michaelschwarz Ajax.NET-Professional (before 21.10.30.1)

New references:

Show 5 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Product configuration	changed	OR cpe:2.3:a:ajaxpro.2_project:ajaxpro.2::::::.net:: versions up to (excluding) 21.10.30.1	OR cpe:2.3:a:ajaxpro.2_project:ajaxpro.2::::::.net:: cpe:2.3:a:michaelschwarz:ajax.net_professional::::::.net:: versions up to (excluding) 21.10.30.1
Reference type	added	—	Snyk: http://packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html Types: Exploit, VDB Entry
Reference type	added	—	CISA-ADP: https://blog.talosintelligence.com/uat-10147-chinese-speaking-adversary-integrates-agentic-ai-into-post-compromise-operations/ Types: Exploit, Third Party Advisory
Reference type	added	—	CISA-ADP: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-23758 Types: US Government Resource
Reference type	added	—	CVE: http://packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html Types: Exploit, VDB Entry

Source identifier: nvd@nist.gov

26.08.2026CISA KEV entry updated9119a7d8…
4 individual changes: Date Added, Due Date, Required Action, Vulnerability Name
Show 4 original changes
Individual changes of this event as published by NVD
Type Action Before After
Date Added added
—
2026-08-26
Due Date added
—
2026-08-26
Required Action added
—
2026-08-26
Vulnerability Name added
—
2026-08-26
Source identifier: 9119a7d8-5eab-497f-8521-727c672e3725

Individual changes of this event as published by NVD
Type	Action	Before	After
Date Added	added	—	2026-08-26
Due Date	added	—	2026-08-26
Required Action	added	—	2026-08-26
Vulnerability Name	added	—	2026-08-26

26.08.2026Record modifiedsnyk.io

New references:

Show 8 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	http://packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html
Reference	added	—	https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57
Reference	added	—	https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971
Reference	removed	http://packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html	—
Reference	removed	https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57	—
Reference	removed	https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971	—
Reference type	removed	https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57 Types: Patch, Third Party Advisory	—
Reference type	removed	https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971 Types: Third Party Advisory	—

Source identifier: report@snyk.io

26.08.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: active, automatable: no

Weakness type: CWE-502

New references:

Show 4 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Weakness type (CWE)	added	—	CWE-502
Reference	added	—	https://blog.talosintelligence.com/uat-10147-chinese-speaking-adversary-integrates-agentic-ai-into-post-compromise-operations/
Reference	added	—	https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-23758
SSVC assessment	added	—	{"id":"CVE-2021-23758","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-08-26T17:44:04.350622Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

26.08.2026Record modifiedCVE Program

New references:

Show 8 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	http://packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html
Reference	added	—	https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57
Reference	added	—	https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971
Reference	removed	http://packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html	—
Reference	removed	https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57	—
Reference	removed	https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971	—
Reference type	removed	https://github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57 Types: Patch, Third Party Advisory	—
Reference type	removed	https://snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971 Types: Third Party Advisory	—

Source identifier: af854a3a-2127-422b-91ae-364da2661108

Linked advisories

CISA Adds 10 CVEs to Known Exploited Vulnerabilities Catalog, Five with Three-day Mitigation DeadlinesSANS NewsBites28.08.2026

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<a href="https://intel.neosec.eu/en/cve/CVE-2021-23758"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-23758.svg?lang=en" alt="CVE-2021-23758 – NEOSEC Intel" height="20"></a>

What is affected, what to do

Metrics

Response and mitigation

Prioritisation rationale

Immediate response (0-24 h)

Mitigation layers

Detection rules

Detection rules

Classification under NIS2 and CRA

Description

Affected products

Vendors and products

Weakness classes (CWE)

Public exploit references

References & sources

Changes to this CVE

Linked advisories