How to Fix CVE-2026-6023: CWE-502 Deserialization of Untrusted Data in Telerik UI for ASP.NET AJAX

By Sai Kiran Pandrala

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Last verified: 2026-05-25

CVE-2026-6023 is a cwe-502 deserialization of untrusted data in Progress Software Telerik UI for ASP.NET AJAX. Fix it by upgrading to the patched build from the vendor advisory.

What is CVE-2026-6023?

CVE-2026-6023 is a cwe-502 deserialization of untrusted data flaw in Progress Software Telerik UI for ASP.NET AJAX. It carries a CVSS base score of 8.1 (high). It is not currently listed in the CISA Known Exploited Vulnerabilities catalog.

From the source record: In Progress® Telerik® UI for AJAX versions 2024.4.1114 through 2026.1.421, the RadFilter control is vulnerable to insecure deserialization when restoring filter state if the state is exposed to the client. If an attacker tampers with this state, a server-side remote code execution is possible.

Why it matters in practice: The blast radius depends on how the affected service is exposed. An internet-facing instance with no compensating controls is the highest-risk configuration.

Am I affected?

You are affected if your installation of Telerik UI for ASP.NET AJAX matches a version listed in the Affected row above.

Check the running version against the Affected row above using the product's admin console or --version flag.

How to fix CVE-2026-6023

Apply the vendor patch. Target the build named in the Fixed in row above (See vendor advisory). The runnable command set below covers the most common deployment patterns for Telerik UI for ASP.NET AJAX.

Windows update (PowerShell, run as administrator)

Install-Module -Name PSWindowsUpdate -Force -Scope CurrentUser
Import-Module PSWindowsUpdate
Get-WindowsUpdate -MicrosoftUpdate
Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -AutoReboot

After applying the patch

1. Restart the service or device so the patched binary loads.
2. Confirm the running version matches the Fixed in row using the verification command below.
3. Rotate credentials and API keys that the affected service could access if the asset was exposed during the disclosure window.

If you can't patch immediately

Until the patch lands, narrow the attack surface with these runnable controls.

Restrict network exposure

Block public access to the affected service at the perimeter. Allow only trusted source IPs.

# Linux iptables: only allow trusted admin subnet
sudo iptables -A INPUT -p tcp --dport 443 -s 10.10.10.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 443 -j DROP
sudo iptables-save | sudo tee /etc/iptables/rules.v4

# Windows firewall: only allow trusted admin subnet on management port
New-NetFirewallRule -DisplayName "Restrict-Mgmt-Allow" -Direction Inbound -Action Allow `
  -RemoteAddress 10.10.10.0/24 -Protocol TCP -LocalPort 443
New-NetFirewallRule -DisplayName "Restrict-Mgmt-Deny"  -Direction Inbound -Action Block `
  -Protocol TCP -LocalPort 443

Mitigations are temporary. Apply the vendor patch as soon as a maintenance window opens.

How to verify the fix worked

Confirm the patched build is the one actually running.

Check the running version against the Affected row above using the product's admin console or --version flag.

Expected: a version at or above the patched build named in the vendor advisory.

Also worth doing: pull recent log windows for indicators of compromise listed in the vendor advisory, and re-run an authenticated vulnerability scan with up-to-date signatures.

Frequently asked questions

Is CVE-2026-6023 being exploited in the wild?

As of 2026-05-25, CVE-2026-6023 is not listed in the CISA Known Exploited Vulnerabilities catalog. Watch the catalog and patch on a normal cadence; KEV status can change as exploitation evidence emerges.

What is the CVSS score for CVE-2026-6023?

The CVSS base score is 8.1 (High).

What version fixes this?

The vendor advisory names the patched build. See the References section.

Will a WAF or IDS rule alone close this?

No. Network filters cut down opportunistic scans but they do not remove the flaw. The vendor patch is the only durable fix.

References

• Official vendor advisory: https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/kb-security-deserialization-of-untrusted-data-cve-2026-6023
• NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-6023
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

*Assembled from the official vendor advisory, the NVD record, and the CISA KEV listing on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*