CVE-2021-23758 — Ajax.NET Professional Deserialization of Untrusted Data Vulnerability

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

• Documented patch — github.com (NIST National Vulnerability Database)
• Nuget AjaxNetProfessional: fixed in version 21.11.29.1 — github.com (GitHub Security Advisories)

CISA required action CISA

Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

US federal deadline: Sep 9, 2026 — passed 36 day(s) ago

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

1. Identify exposed assets running the affected vendor/product/version combinations (Ajaxpro.2 Project Ajaxpro.2, Michaelschwarz Ajax.net Professional, Nuget AjaxNetProfessional).
2. Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
3. The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
4. Apply the fixed version or the patch documented under Remediation.
5. EPSS ahead of 100% of scored CVEs: prioritize it in the patch queue.
6. Search available logs for exploit probes matching the vulnerability class (insecure deserialization), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-502 · Deserialization of Untrusted Data

Other CVEs with this weakness →

CISA KEV details CISA

CISA lists a CVE once it has evidence of exploitation against real targets.

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.

Public exploits, PoCs and detection templates

Public code that demonstrates or tests the flaw raises the odds of mass exploitation — and Nuclei templates help find vulnerable assets in your environment.

Warning: public PoCs are sometimes disguised malware. Use them only for authorized testing in an isolated environment. This portal does not host exploit code — it only points to the sources.

EPSS over time FIRST

Oct 12, 2026 → Oct 14, 2026 · 3 reading(s) · max 82.58%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

• Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
• Monitor serialized objects in requests and unexpected processes spawned by the runtime.
• Hunt retroactively in logs from before Aug 26, 2026 (KEV listing): exploitation may have started earlier.

Timeline

What happened to this CVE, newest first.

• 2026
• Fix documented
  Oct 13, 2026 · GitHub Security Advisories
• Affected products added · nuget/ajaxnetprofessional
  Oct 13, 2026 · GitHub Security Advisories
• SSVC assessment updated
  Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
• Record published
  Oct 13, 2026
• Fix documented
  Oct 13, 2026 · NIST National Vulnerability Database
• Affected products added · ajaxpro.2 project/ajaxpro.2, michaelschwarz/ajax.net professional
  Oct 13, 2026 · NIST National Vulnerability Database
• CVSS changed · 9.8
  Oct 13, 2026 · NIST National Vulnerability Database
• Source status changed · → Analyzed
  Oct 13, 2026 · NIST National Vulnerability Database
• Added to CISA KEV (detected)
  Oct 13, 2026 · CISA Known Exploited Vulnerabilities
• Record created in this database
  Oct 13, 2026 · CISA Known Exploited Vulnerabilities
• Added to CISA KEV catalog
  Aug 25, 2026 · CISA
• 2021
• CVE published
  Dec 3, 2021 · snyk

Source activity

Repeated readings are counted rather than listed.

• 2× References added · Oct 13, 2026 – Oct 13, 2026
• 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

9 on the record

• github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a
  Patch, Third Party Advisory, NIST National Vulnerability Database
• www.cisa.gov/known-exploited-vulnerabilities-catalog
  US Government Resource, NIST National Vulnerability Database
• github.com/advisories/GHSA-6r7c-6w96-8pvw
  GitHub Security Advisories
• nvd.nist.gov/vuln/detail/CVE-2021-23758
  GitHub Security Advisories
• packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html
  Exploit, VDB Entry, NIST National Vulnerability Database
• security.snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971
  GitHub Security Advisories
• snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971
  Third Party Advisory, NIST National Vulnerability Database
• blog.talosintelligence.com/uat-10147-chinese-speaking-adversary-integrates-agentic-ai-into-post-
  Exploit, Third Party Advisory, NIST National Vulnerability Database
• github.com/michaelschwarz/Ajax.NET-Professional/security/advisories/GHSA-6r7c-6w
  GitHub Security Advisories

Elsewhere on this site

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

• No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
• No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
• No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2021-23758 is "Ajax.NET Professional Deserialization of Untrusted Data Vulnerability", rated Critical. It affects Ajaxpro.2 (Ajaxpro.2 Project); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 9.8 describes technical severity. EPSS 82.58% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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