CVE-2021-23758 is a critical-severity (CVSS 9.8) Deserialization of Untrusted Data vulnerability in AjaxNetProfessional. EPSS puts its 30-day exploitation probability at 82.6% (100th percentile). A fix is available for AjaxNetProfessional — see the affected versions and patch details below.
Remote Code Execution in AjaxNetProfessional
Exploitation Status
Actively exploited in the wild
- CISA’s SSVC triage classifies this CVE as under active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-23758.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-23758 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the 50–90% band (highlighted). Counts from FIRST.org, log-scaled.
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
Overview
Affected versions of this package 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.
Description
Serialization is a process of converting an object into a sequence of bytes which can be persisted to a disk or database or can be sent through streams. The reverse process of creating object from sequence of bytes is called deserialization. Serialization is commonly used for communication (sharing objects between multiple hosts) and persistence (store the object state in a file or a database). It is an integral part of popular protocols like Remote Method Invocation (RMI), Java Management Extension (JMX), Java Messaging System (JMS), Action Message Format (AMF), Java Server Faces (JSF) ViewState, etc.
Deserialization of untrusted data (CWE-502), is when the application deserializes untrusted data without sufficiently verifying that the resulting data will be valid, letting the attacker to control the state or the flow of the execution.
Java deserialization issues have been known for years. However, interest in the issue intensified greatly in 2015, when classes that could be abused to achieve remote code execution were found in a popular library (Apache Commons Collection). These classes were used in zero-days affecting IBM WebSphere, Oracle WebLogic and many other products.
An attacker just needs to identify a piece of software that has both a vulnerable class on its path, and performs deserialization on untrusted data. Then all they need to do is send the payload into the deserializer, getting the command executed.
Developers put too much trust in Java Object Serialization. Some even de-serialize objects pre-authentication. When deserializing an Object in Java you typically cast it to an expected type, and therefore Java's strict type system will ensure you only get valid object trees. Unfortunately, by the time the type checking happens, platform code has already created and executed significant logic. So, before the final type is checked a lot of code is executed from the readObject() methods of various objects, all of which is out of the developer's control. By combining the readObject() methods of various classes which are available on the classpath of the vulnerable application an attacker can execute functions (including calling Runtime.exec() to execute local OS commands).
Releases
Releases before version 21.11.29.1 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
There is no workaround available that addresses all issues except updating to latest version from GitHub.
References
Find original CVE posting here: https://security.snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971
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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | AjaxNetProfessional | all versions | 21.11.29.1dotnet add package AjaxNetProfessional --version 21.11.29.1 |
Affected Products
ajaxpro.2ajaxpro.2_projectajax.net professionalmichaelschwarzDetection & mitigation playbook
Open-source dependencyDetect
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 21.11.29.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23758 is resolved across your whole dependency graph.
Workarounds
Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.
Frequently Asked Questions
Is CVE-2021-23758 in your dependencies?
Find it across NuGet, including transitive dependencies.