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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

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

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2021-43853?

O3 pinpoints whether CVE-2021-43853 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

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 53th 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 April 2, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2021-43853: AjaxNetProfessional Cross-Site Scripting (Mediu…

EPSS Exploitation Probability

via FIRST.org ↗

0.8%probability of exploitation in next 30 days

Lower Risk53th percentile+0.59%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

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

Ajax.NET Professional (AjaxPro) is an AJAX framework available for Microsoft ASP.NET. Affected versions of this package are vulnerable to JavaScript object injection which may result in cross site scripting when leveraged by a malicious user. The affected core relates to JavaScript object creation when parsing json input. Releases before version 21.12.22.1 are affected. A workaround exists that replaces one of the core JavaScript files embedded in the library. See the GHSA-5q7q-qqw2-hjq7 for workaround details.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`AjaxNetProfessional`	all versions	21.12.22.1

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for AjaxNetProfessional. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-43853 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2021-43853. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

Ajax.NET Professional (AjaxPro) is an AJAX framework available for Microsoft ASP.NET. Affected versions of this package are vulnerable to JavaScript object injection which may result in cross site scripting when leveraged by a malicious user. The affected core relates to JavaScript object creation when parsing json input. Releases before version 21.12.22.1 are affected. A workaround exists that replaces one of the core JavaScript files embedded in the library. See the GHSA-5q7q-qqw2-hjq7 for workaround details.

O3 Security · Impact-Aware SCA

Is CVE-2021-43853 in your dependencies?

O3 detects CVE-2021-43853 across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my dependencies How O3 SCA works

CVE-2021-43853