What is CVE-2021-23758?

CVE-2021-23758 is Deserialization of Untrusted Data. The supplied enrichment data classifies it as Deserialization mapped to CWE-502 Deserialization of Untrusted Data.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Yes: CISA KEV signal present. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

Is there a public exploit?

13 public PoC repositories reported; 2 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS, EDR process telemetry, host service logs where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2021-23758 using CVSS 9.8, EPSS 84%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

Publicly linked actor or ransomware signals in the supplied enrichment data: TeamTNT (), Kinsing (). Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Treat as emergency remediation because CISA KEV status is present. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

CISA KEVcriticalDeserializationpublic exploit

CVE-2021-23758

CVE-2021-23758 — Deserialization of Untrusted Data

Q: Is it in CISA KEV?

Yes. The supplied enrichment data lists CVE-2021-23758 in CISA KEV with required action: 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.. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

All versions of package ajaxpro.2 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.

Published Dec 3, 2021Updated Aug 27, 2026Sources: NVD, Ajax.NET Professional (CNA), CISA, FIRST EPSS, SOCRadar CTI

Triage

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

Exploitation

Confirmed

CISA KEV, listed Aug 26, 2026

EPSS

84%

ahead of 99.7% of scored CVEs

Affects

Ajax.NET Professional

AjaxPro.2

CVSS base

9.8

critical · 3 sources

Remediation

The vendor's own words where we have them.

Upgrade AjaxPro.2 to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

CISA required action

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.

Federal deadline Sep 9, 2026 — in 10 days

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Treat as emergency remediation because CISA KEV status is present.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Ajax.NET Professional	AjaxPro.2	0	Vulnerable

Attack characteristics

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

Availability

High

Confidentiality

High

Integrity

High

Scope

Unchanged

Attack Complexity

Low

Attack Vector

Network

Privileges Req

None

User Interaction

None

Every base score collected

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

Score	Version	Severity	Expl.	Impact	Source
8.1	CVSS 3.1	high	2.2	5.9	snyk.io (CNA), Ajax.NET Professional (CNA)
9.8	CVSS 3.1	critical	3.9	5.9	NVD
7.5	CVSS 2.0	high	10	6.4	NVD

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-502 · Deserialization of Untrusted Data

Modify Application Data
Unexpected State
DoS: Resource Consumption (CPU)
Varies by Context

Mitigation: If available, use the signing/sealing features of the programming language to assure that deserialized data has not been tainted. For example, a hash-based message authentication code (HMAC) could be used to ensure that data has not been modified.

Weakness & attack patterns

CWE-502Deserialization of Untrusted Data

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

CAPEC-586 · Object Injection

CISA KEV details

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

CISAThat month →

Listed

Aug 26, 2026

Federal deadline

Sep 9, 2026

in 10 days

Ransomware use

Unknown

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 date to argue against.

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

2 of the 5 sampled are weaponized

Exploit entries

Exploit-DB and CTI exploit indexes

Social mentions

Posts naming this CVE. Attention, not evidence of exploitation — it spikes on disclosure and decays whether or not anything is exploited.

Detection

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

Prioritize edge telemetry for network-reachable AjaxPro.2.
Monitor for scanner or exploit-pattern traffic after 13 public PoC repositories were reported.

web/proxy/WAF logsapplication logsnetwork IDS/IPSEDR process telemetryhost service logs

Groups associated with this CVE

Groups reported in connection with this vulnerability. A reported association, not a confirmed observation of that group using it — and not scoped to any sector or campaign.

TeamTNTKinsing

hash37cabc04da36e710dd4aee8609ab7553c039a54dd085460854e9ddb49b0e703286
Public Malware Feeds · Aug 20, 2026
hashd190b349d791267a9583ba9f4a1ab0e4199d1a3abfd4dae514ed5def0754ba9471
Cisco Talos · Aug 20, 2026
hash8280502c2c6902e61fc4c02a9a81b4720688449a5bca3d89dbd1e2edd507c69a71
Cisco Talos · Aug 20, 2026
hash23a83c6bbdd7d6c09a5187338065d15f2a90a252772813cba83b9818aa56cef771
Cisco Talos · Aug 20, 2026
hash00892f276299a13721642e8a9bcbcb949a658547c6c8271866a1997b79f1e5c571
Cisco Talos · Aug 20, 2026

5 more indicators on this CVEShowing 5 of 10 · Hashes, addresses and domains from a live threat exchangeUnlock free

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 9.8 (AV:N)
Aug 27, 2026 · NVD
Added to CISA KEV catalog
Aug 26, 2026 · CISA
2024
CVE is exploited. "k0mi-tg/CVE-POC"
Feb 1, 2024 · SOCRadar CTI
2023
CVE is exploited. "ARPSyndicate/cvemon"
Dec 4, 2023 · SOCRadar CTI
CVE is exploited. "manas3c/CVE-POC"
Dec 4, 2023 · SOCRadar CTI
CVE is exploited. "nomi-sec/PoC-in-GitHub"
Dec 4, 2023 · SOCRadar CTI
CVE is exploited. "numanturle/CVE-2021-23758-POC"
Dec 4, 2023 · SOCRadar CTI
CVE is exploited. "soosmile/POC"
Dec 4, 2023 · SOCRadar CTI
Bulletin released on NIST.
Nov 14, 2023 · SOCRadar CTI
2022
Bulletin released on NIST.
Apr 17, 2022 · SOCRadar CTI
2021
Bulletin released on MITRE_REFERENCES.
Dec 4, 2021 · SOCRadar CTI
Initial · CVE published
Dec 3, 2021 · NVD
CVE published by NIST.
Dec 3, 2021 · SOCRadar CTI

Source activity

Readings a source repeats on a schedule, counted rather than listed.

8×Github ReposDec 8, 2019 – Dec 4, 2023 · SOCRadar CTI
7×ModifiedDec 6, 2021 – Aug 27, 2026 · SOCRadar CTI
4×TweetsNov 3, 2023 – Aug 29, 2026 · SOCRadar CTI
4×SvrsApr 18, 2022 – Aug 29, 2026 · SOCRadar CTI
1×CVE appeared in the Cve Trends.Aug 27, 2026 · SOCRadar CTI

References

8 on the record

packetstormsecurity.com/files/175677/AjaxPro-Deserialization-Remote-Code-Execution.html
Exploit, VDB Entry, NIST, GITHUB
github.com/michaelschwarz/Ajax.NET-Professional/commit/b0e63be5f0bb20dfce507cb8a1a9568f6e73de57
Patch, Third Party Advisory, NIST, MITRE_REFERENCES, Vendor Advisory
snyk.io/vuln/SNYK-DOTNET-AJAXPRO2-1925971
Third Party Advisory, NIST, GITHUB, MITRE_REFERENCES
blog.talosintelligence.com/uat-10147-chinese-speaking-adversary-integrates-agentic-ai-into-post-compromise-operations/
Exploit, Third Party Advisory, NIST, GITHUB
www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-23758
US Government Resource, NIST
www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk
US Government Resource
www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk
US Government Resource
nvd.nist.gov/vuln/detail/CVE-2021-23758
Vendor Advisory

Coverage

6 stories from the feeds we poll

1 more reports on this CVEShowing 5 of 6 · Vendor research and security press coverageUnlock free

Elsewhere on this site

Ajax.NET Professionalevery CVE for this vendor
August 2026what else CISA listed
Deserializationsame class
CWE-502other pages naming this weakness

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.

Only the 5 indicator(s) reported for this CVE are available; no packet-level or host-forensic 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 record1

What should defenders know first?

CVE-2021-23758 is Deserialization of Untrusted Data, a critical vulnerability affecting AjaxPro.2 from Ajax.NET Professional. The current evidence lists it in CISA KEV, and the exploit status is: Yes: CISA KEV signal present. Public exploit evidence is: 13 public PoC repositories reported; 2 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 9.8 describes technical severity, while EPSS 84% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.