CVE-2026-2878: Telerik UI for ASP.NET AJAX Vulnerability

CVE-2026-2878 Overview

An insufficient entropy vulnerability exists in Progress® Telerik® UI for ASP.NET AJAX versions prior to 2026.1.225. The vulnerability affects the RadAsyncUpload component, where a predictable temporary identifier based on timestamp and filename can enable collisions and file content tampering. This weakness in random value generation (CWE-331) allows attackers to potentially predict or manipulate file upload identifiers, leading to integrity violations in uploaded content.

Critical Impact

Attackers can exploit predictable temporary identifiers in RadAsyncUpload to cause file collisions and tamper with file content during the upload process, compromising data integrity.

Affected Products

• Progress Telerik UI for ASP.NET AJAX (all versions prior to 2026.1.225)

Discovery Timeline

• February 25, 2026 - CVE-2026-2878 published to NVD
• February 26, 2026 - Last updated in NVD database

Technical Details for CVE-2026-2878

Vulnerability Analysis

This vulnerability stems from insufficient entropy in the generation of temporary identifiers used by the RadAsyncUpload component. The identifier generation mechanism relies on predictable inputs—specifically timestamps and filenames—rather than cryptographically secure random values. This predictability creates a window of opportunity for attackers who can calculate or guess the temporary identifier being used for a file upload operation.

The attack requires precise timing and knowledge of the filename being uploaded, making exploitation moderately complex. However, successful exploitation allows an attacker to target and tamper with files during the upload process without requiring any authentication or user interaction. The impact is focused on integrity violations, as attackers can modify file content during transfer.

Root Cause

The root cause is the use of insufficient entropy sources (CWE-331) when generating temporary file identifiers in the RadAsyncUpload component. Instead of using cryptographically secure random number generation, the component constructs identifiers from predictable elements—the current timestamp and the original filename. This deterministic approach allows attackers who can observe or predict these values to calculate the temporary identifier and exploit the collision.

Attack Vector

The attack vector is network-based and does not require authentication or user interaction. An attacker must be able to observe or predict the timestamp and filename of an upload in progress. With this information, they can calculate the predictable temporary identifier and potentially:

1. Cause identifier collisions with legitimate uploads
2. Intercept and tamper with file content during the upload process
3. Replace legitimate uploaded files with malicious content

The attack complexity is considered high due to the timing precision required and the need to predict or observe the upload parameters. Successful exploitation results in high integrity impact but does not directly affect confidentiality or availability.

Detection Methods for CVE-2026-2878

Indicators of Compromise

• Unusual patterns of file upload collisions or overwrites in the RadAsyncUpload temporary storage
• Multiple upload requests with identical or sequential temporary identifiers from different sources
• Unexpected file content modifications detected post-upload
• Anomalous timing patterns in upload requests suggesting identifier prediction attempts

Detection Strategies

• Monitor RadAsyncUpload temporary directories for unexpected file collisions or rapid overwrites
• Implement logging for all file upload operations including temporary identifier values
• Deploy file integrity monitoring on upload directories to detect unauthorized modifications
• Analyze web server logs for patterns indicating systematic identifier prediction attempts

Monitoring Recommendations

• Enable verbose logging for the RadAsyncUpload component to capture temporary identifier generation
• Implement real-time alerting for file collision events in upload handling
• Deploy network monitoring to detect anomalous upload patterns from single sources
• Regularly audit uploaded files to verify integrity between temporary and final storage

How to Mitigate CVE-2026-2878

Immediate Actions Required

• Upgrade Progress Telerik UI for ASP.NET AJAX to version 2026.1.225 or later immediately
• Review recently uploaded files for potential tampering or unauthorized modifications
• Implement additional file integrity verification mechanisms until patching is complete
• Consider temporarily disabling RadAsyncUpload functionality if immediate patching is not possible

Patch Information

Progress has released version 2026.1.225 of Telerik UI for ASP.NET AJAX that addresses this insufficient entropy vulnerability. The patch implements cryptographically secure random number generation for temporary file identifiers, eliminating the predictability that enabled this attack. Organizations should refer to the Telerik Security Advisory CVE-2026-2878 for detailed upgrade instructions and additional security guidance.

Workarounds

• Implement server-side file integrity verification using cryptographic hashes before accepting uploads
• Add additional application-level entropy to file upload handling processes
• Restrict access to RadAsyncUpload endpoints through network segmentation or access controls
• Monitor and rate-limit file upload requests to reduce the attack window for collision attempts

Administrators should consult the Telerik Security Advisory for vendor-recommended configuration guidance and mitigation strategies specific to their deployment environment.