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CVE-2026-13182 – Decryption Oracle in RadAsyncUpload – Telerik UI for ASP.NET AJAX prior to v2026…

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Summary

CVE-2026-13182 is a high-severity cryptographic (padding) oracle vulnerability in the RadAsyncUpload component of Progress Telerik UI for ASP.NET AJAX. The flaw affects all versions from 2010.1.309 through 2026.2.519, and allows unauthenticated remote attackers to progressively recover protected metadata values by exploiting the component’s differentiated error responses during client-state processing. The vulnerability carries a CVSS v3.1 base score of 7.5 (High).

Technical details

  • Root cause: The RadAsyncUpload component’s client-state processing returns distinguishable responses depending on whether a submitted payload triggers a decryption failure or an invalid-JSON parse failure (CWE-209: Generation of Error Message Containing Sensitive Information). This behavioral difference constitutes a classic cryptographic oracle (CAPEC-463: Padding Oracle Crypto Attack).
  • Trigger condition: An attacker submits crafted client-state payloads to the RadAsyncUpload HTTP handler endpoint (accessible via the Telerik.Web.UI.WebResource.axd handler with type=rau). By iterating over malformed ciphertext blocks and observing differing server responses, the attacker can deduce plaintext content one block at a time — without needing to know the application’s encryption keys.
  • Attack vector: Network-reachable, no authentication required, no user interaction required, low attack complexity. The handler is exposed as a public HTTP endpoint on any application that uses RadAsyncUpload.
  • Impact: High confidentiality impact — protected metadata values embedded in encrypted client state are recoverable. Historically, recovering such metadata from RadAsyncUpload has enabled chaining into .NET deserialization attacks leading to remote code execution, as documented by CISA in advisory AA23-074A in the context of CVE-2019-18935 and CVE-2017-11317.

Affected software

  • Progress Telerik UI for ASP.NET AJAX versions 2010.1.309 through 2026.2.519 (all releases up to and including the 2026 Q2 base release)

Severity

CVSS v3.1 Base Score: 7.5 (High)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Mitigation and recommended actions

  • Immediate — apply the vendor patch: Upgrade to Telerik UI for ASP.NET AJAX version 2026.2.708 (2026 Q2 SP1) or later. Version 2026.2.708 introduces AES-GCM authenticated encryption for RadAsyncUpload client state, which is not vulnerable to padding oracle attacks.
  • If immediate patching is not feasible — apply vendor-recommended workarounds:
    • Remove custom Telerik.AsyncUpload.ConfigurationEncryptionKey and Telerik.Upload.ConfigurationHashKey values from web.config so the component falls back to MachineKey.Unprotect (AES + HMAC), eliminating the oracle differential.
    • Generate and configure strong machine keys in IIS using HMACSHA256 validation to harden the fallback path.
    • Set ASP.NET customErrors to "On" or "RemoteOnly" to suppress differentiated error responses visible to remote clients.
    • If RadAsyncUpload is not actively used by the application, disable the handler entirely by setting the application setting Telerik.Web.DisableAsyncUploadHandler to true in web.config.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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