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CVE-2021-23758

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in Ajax.NET Professional. For every flaw being used in attacks, see Exploited flaws.

CVE-2021-23758 is a security flaw in Ajax.NET Professional. Attackers can run their own code on Ajax.NET Professional. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 26 Aug 2026.[1]

Ajax.NET Professional is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version unspecified of AjaxPro.2.

Description

CISA calls it a deserialization of untrusted data in Ajax.NET Professional Ajax.NET Professional. In its words: “Ajax.NET Professional (AjaxPro) contains a deserialization of untrusted data vulnerability that could allow for remote code execution via arbitrary .NET classes. The impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.”[1]

The underlying weakness is deserialization of untrusted data (CWE-502), in MITRE’s list of common software weaknesses.[2]

It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]

Affected versions

NVD’s record lists one fixed version: everything before it is affected.[2]

ProductFixed in
AjaxPro.2unspecified

Most families won’t have Ajax.NET Professional; it’s for whoever looks after IT at work.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 26 Aug 2026, 1,727 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 9 Sep 2026 to fix it.[1]

FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 83%. That’s in the top 1% of the flaws it scores.[3]

CISA assesses it as not automatable: each attack takes some work.

CISA doesn’t say who was attacked, or how.

Fix

CISA’s 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).”[1]

  1. Check whether you use it.Ajax.NET Professional is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
  2. Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version unspecified of AjaxPro.2), or not in use. CISA gave US government agencies until 9 Sep 2026 to fix it.
  3. If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.

You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.

Timeline

DateEvent
The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2]
CISA adds it to its catalog of flaws known to be used in attacks.[1]
CISA’s deadline for US government agencies to fix it.[1]
FIRST puts the odds of it being used in attacks in the next 30 days at 83%.[3]

See also

Other deserialization of untrusted data flaws

  • CVE-2026-63077, JetBrains TeamCity: deserialization of untrusted data ·
  • CVE-2026-50522, Microsoft SharePoint: deserialization of untrusted data ·
  • CVE-2026-58644, Microsoft SharePoint: deserialization of untrusted data ·
  • CVE-2026-45659, Microsoft SharePoint Server: deserialization of untrusted data ·
  • CVE-2026-45247, Mirasvit Full Page Cache Warmer: full page cache warmer deserialization of untrusted data ·

Added the same day

  • CVE-2026-8452, Citrix NetScaler ADC and NetScaler Gateway: improper restriction of operations within the bounds of a memory buffer ·
  • CVE-2022-0995, Linux Kernel: out-of-bounds write ·
  • CVE-2019-1068, Microsoft SQL Server: remote code execution ·
  • CVE-2015-5287, Red Hat Automatic Bug Reporting Tool: privilege escalation ·
  • CVE-2015-3246, Red Hat Libuser: race condition ·

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Ajax.NET Professional Deserialization of Untrusted Data Vulnerability”. Known Exploited Vulnerabilities Catalog, added 26 Aug 2026.
  2. ^ National Institute of Standards and Technology. “CVE-2021-23758 Detail”. National Vulnerability Database, published 3 Dec 2021.
  3. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.826, percentile 99.7, 31 Oct 2026.
  4. CVE Program. “CVE-2021-23758”. The official CVE record.

This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.