CVE-2021-43853 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in AjaxNetProfessional. O3 Security confirms whether CVE-2021-43853 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
AjaxNetProfessional deserializes arbitrary JavaScript objects
EPSS Exploitation Probability
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.
How urgent is this, really
CVE-2021-43853 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 0 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
AjaxNetProfessionalReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | AjaxNetProfessional | all versions | 21.12.22.1 |
Detection & mitigation playbook
Open-source dependencyDetect
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
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.