Exploited. On CISA's list since 2021-11-03, and used by ransomware.

Progress Telerik UI for ASP.NET AJAX Deserialization of Untrusted Data Vulnerability

CVE-2019-18935 · Progress Telerik UI for ASP.NET AJAX · Published 2019-12-11

How bad would it be?

9.8 out of 10

CVSS rates it critical: about as damaging as flaws get.

How likely is it to be used?

over 99%

Estimated chance of exploitation in the next 30 days, among the top 1% of all scored flaws (EPSS, 2026-10-29).

Is it already being used?

Yes

CISA has seen it exploited in real attacks. For US government agencies the fix is deadline passed 1641 days ago (2022-05-03).

What CISA says to do

Apply updates per vendor instructions.

Progress Telerik UI for ASP.NET AJAX contains a deserialization of untrusted data vulnerability through RadAsyncUpload which leads to code execution on the server in the context of the w3wp.exe process.

nvd.nist.gov

What the flaw is

Progress Telerik UI for ASP.NET AJAX through 2019.3.1023 contains a .NET deserialization vulnerability in the RadAsyncUpload function. This is exploitable when the encryption keys are known due to the presence of CVE-2017-11317 or CVE-2017-11357, or other means. Exploitation can result in remote code execution. (As of 2020.1.114, a default setting prevents the exploit. In 2019.3.1023, but not earlier versions, a non-default setting can prevent exploitation.)

From the NVD.

Fixes and advisories

www.telerik.comPatch, Vendor Advisory
www.telerik.comVendor Advisory
www.telerik.comPatch, Vendor Advisory
www.telerik.comVendor Advisory

Learn this

Explore this week’s weaknesses and all learning topics

Understand the kind of weakness behind this flaw. These resources teach the weakness, rather than reproduce this specific CVE. Follow the vendor's advice above to fix the affected product.

Insecure deserialization

Rebuilding objects from untrusted saved data can trigger unintended behaviour.

CWE-502 · CISA KEV; NVD record: 134c704f-9b21-4f2e-91b3-4a467353bcc0; NVD record: nvd@nist.gov

Reading and practice labs · Free account for labs. Basic HTTP knowledge; some labs use Burp Suite.

Explore the labs at PortSwigger Web Security Academy How it is prevented

CWE is a catalogue of weakness types. Labels come from the sources shown and can differ or change. Resource matches reviewed 16 October 2026.

On SentryFeed

No story in the last 14 days has mentioned it.

Exploited flaws from CISA's KEV catalogue. Chance of exploitation from EPSS by FIRST. Descriptions and scores from the NVD. This product uses the NVD API but is not endorsed or certified by the NVD.