How severe is CVE-2021-43853?

CVE-2021-43853 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-43853?

CVE-2021-43853 affects the following packages: AjaxNetProfessional (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2021-43853?

Update AjaxNetProfessional to 21.12.22.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43853 is resolved across your whole dependency graph.

How do I detect CVE-2021-43853 in my NuGet dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for AjaxNetProfessional, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-43853 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-43853 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-43853 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-43853?

CVE-2021-43853 has an EPSS (Exploit Prediction Scoring System) score of 0.8%, placing it in the 56th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-43853?

CVE-2021-43853 is classified as Cross-site Scripting (XSS) (CWE-79), Deserialization of Untrusted Data (CWE-502). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-43853 published, and has it been updated?

CVE-2021-43853 was published on December 22, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

.NET

.NET NuGet

Not in CISA KEV

MEDIUM severity

CVE-2021-43853 — AjaxNetProfessional

MEDIUMFix: michaelschwarz/Ajax.NET-Professional@c89e39b

CVE-2021-43853 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in AjaxNetProfessional. A fix is available for AjaxNetProfessional — see the affected versions and patch details below.

AjaxNetProfessional deserializes arbitrary JavaScript objects

Also known asGHSA-5q7q-qqw2-hjq7

Published

Dec 22, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs56th percentile — riskier than 56% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-43853 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

.NETAjaxNetProfessional

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

Overview

Affected versions of this package are vulnerable to Deserialization of Untrusted Data due to the possibility of deserialization of arbitrary JavaScript objects.

Description

Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user within the output it generates without validating or encoding it.

XSS effects vary in range from petty nuisance to significant security risk, depending on the sensitivity of the data handled by the vulnerable site and the nature of any security mitigation implemented by the site's owner network.

Releases

Releases before version 21.12.22.1 are affected. Please be careful to download any binary DLL from other web sites, especially we found NuGet packages not owned by us that contain vulnerable versions.

Workarounds

A workaround exists that replaces one of the core JavaScript files embedded in the library. Using a XML configuration allows to replace the default JavaScript code to be replaced with the version on GitHub.

<configuration>
	<configSections>
		<sectionGroup name="ajaxNet">
			<section name="ajaxSettings" type="AjaxPro.AjaxSettingsSectionHandler,AjaxPro.2" requirePermission="false" restartOnExternalChanges="true"/>
		</sectionGroup>
	</configSections>
	<ajaxNet>
		<ajaxSettings>
			<coreScript>~/ajaxpro-core-fixed.js</coreScript>
		</ajaxSettings>
	</ajaxNet>
</configuration>

Copy the file core.js from the main project folder to your web server root folder and rename that ajaxpro-core-fixed.js.

Clients need to refresh the web page to download the changed JavaScript code.

References

Commit fixing the issue: c89e39b9679fcb8ab6644fe21cc7e652cb615e2b

Note: the official Ajax.NET Professional (AjaxPro) NuGet package is available here: https://www.nuget.org/packages/AjaxNetProfessional/

For more information

If you have any questions or comments about this advisory:

Open an issue on this GitHub repository

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`AjaxNetProfessional`	all versions	21.12.22.1`dotnet add package AjaxNetProfessional --version 21.12.22.1`

Affected Products

1 product · 1 configurations

Application

ajax.net professionalajax.net_professional_project

< 21.12.22.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for AjaxNetProfessional, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update AjaxNetProfessional to 21.12.22.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43853 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Overview Affected versions of this package are vulnerable to Deserialization of Untrusted Data due to the possibility of deserialization of arbitrary JavaScript objects. ### Description Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses inp