⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 5.3

How to Fix CVE-2026-2878: Cwe-331 insufficient entropy in Telerik UI for ASP.NET AJAX

By Sai Kiran Pandrala. Last verified: 2026-05-25.

CVE-2026-2878 is a cwe-331 insufficient entropy in Progress Software Telerik UI for ASP.NET AJAX. The fix is to apply the vendor patch noted below.

⚡ At a glance
Severity5.3 (Medium)
Actively exploited?No public listing in CISA KEV
AffectedTelerik UI for ASP.NET AJAX 2011.2.712 to <2026.1.225
Fixed inSee vendor advisory
Type (CWE)CWE-331 Insufficient Entropy

What is CVE-2026-2878?

In Progress® Telerik® UI for AJAX, versions prior to 2026.1.225, an insufficient entropy vulnerability exists in RadAsyncUpload, where a predictable temporary identifier, based on timestamp and filename, can enable collisions and file content tampering. The CVSS base score is 5.3 (Medium). The official advisory is at https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/kb-security-insufficient-entropy-cve-2026-2878.

Am I affected?

Check the version of Telerik UI for ASP.NET AJAX you are running and compare it against the Affected row above (Telerik UI for ASP.NET AJAX 2011.2.712 to <2026.1.225). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


# Linux package check
dpkg -s telerik-ui-for-asp-net-ajax 2>/dev/null | grep -i ^Version
rpm -q telerik-ui-for-asp-net-ajax 2>/dev/null
command -v telerik-ui-for-asp-net-ajax >/dev/null && telerik-ui-for-asp-net-ajax --version 2>/dev/null


# Windows (PowerShell)
Get-Package -Name "*Telerik UI for ASP.NET AJAX*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Telerik UI for ASP.NET AJAX" 2>$null

How to fix CVE-2026-2878

Upgrade Telerik UI for ASP.NET AJAX to a patched build: See vendor advisory. The vendor advisory is the source of truth for the exact fixed version.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade telerik-ui-for-asp-net-ajax
dpkg -s telerik-ui-for-asp-net-ajax | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh telerik-ui-for-asp-net-ajax -y
# or for older releases:
sudo yum update telerik-ui-for-asp-net-ajax -y
rpm -q telerik-ui-for-asp-net-ajax

SUSE / openSUSE


sudo zypper refresh
sudo zypper update telerik-ui-for-asp-net-ajax
rpm -q telerik-ui-for-asp-net-ajax

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-2878 affecting Telerik UI for ASP.NET AJAX
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-2878-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detect installed version"
    $pkg = Get-Package -Name "*Telerik UI for ASP.NET AJAX*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\telerik-ui-for-asp-net-ajax-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Telerik UI for ASP.NET AJAX" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "Telerik UI for ASP.NET AJAX" --silent --accept-source-agreements --accept-package-agreements
    if ($LASTEXITCODE -ne 0) {
        # Fallback: pull latest via OS update channel
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verify the patched build"
    winget list --name "Telerik UI for ASP.NET AJAX"
    Write-Host "Patch applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)


#!/usr/bin/env bash
# Fix script for CVE-2026-2878 affecting Telerik UI for ASP.NET AJAX
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-2878-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detect installed version"
if command -v dpkg >/dev/null; then
    dpkg -s telerik-ui-for-asp-net-ajax 2>/dev/null | grep -i ^Version || echo "telerik-ui-for-asp-net-ajax not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q telerik-ui-for-asp-net-ajax || echo "telerik-ui-for-asp-net-ajax not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-2878-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/telerik-ui-for-asp-net-ajax /etc/telerik-ui-for-asp-net-ajax.d /etc/telerik-ui-for-asp-net-ajax.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: See vendor advisory)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y telerik-ui-for-asp-net-ajax
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y telerik-ui-for-asp-net-ajax
elif command -v yum >/dev/null; then
    yum update -y telerik-ui-for-asp-net-ajax
elif command -v zypper >/dev/null; then
    zypper --non-interactive update telerik-ui-for-asp-net-ajax
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s telerik-ui-for-asp-net-ajax 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q telerik-ui-for-asp-net-ajax
fi
echo "Done. Restart any service that loaded the old library."

If you can't patch immediately

Apply at least one of the following inline controls until you can deploy the patched build. None replace the upgrade.

Restrict exposure with nftables (Linux)


# Allow only trusted CIDR to reach the affected service ports
sudo nft add table inet filter 2>/dev/null || true
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }' 2>/dev/null || true
sudo nft 'add rule inet filter input tcp dport {80, 443} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Block at the host firewall (Windows)


New-NetFirewallRule -DisplayName "Block-CVE-2026-2878" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-2878"

Disable the affected service (Linux)


sudo systemctl stop telerik-ui-for-asp-net-ajax 2>/dev/null || true
sudo systemctl disable telerik-ui-for-asp-net-ajax 2>/dev/null || true

If the vendor advisory lists an official workaround, prefer that wording verbatim. If no workaround is published, the only safe remediation is the patch.

How to verify the fix worked

After upgrading, confirm the installed version matches the patched build and that no old library is still loaded by a long-running process.


# Linux
dpkg -s telerik-ui-for-asp-net-ajax 2>/dev/null | grep -i ^Version
rpm -q telerik-ui-for-asp-net-ajax 2>/dev/null || true
# Pid map check for old library handles
sudo lsof +c0 2>/dev/null | grep -i "DEL.*lib" || true


# Windows
Get-Package -Name "*Telerik UI for ASP.NET AJAX*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above See vendor advisory. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

Is CVE-2026-2878 actively exploited?

There is no public confirmation of exploitation in the wild listed in CISA KEV at the time of this writing. Patch anyway. Public exploits commonly follow disclosure within weeks.

Do I need to reboot after patching CVE-2026-2878?

For kernel and OS-level updates, yes. For most userland packages a systemctl restart <service> is enough on Linux, and restarting the Windows service or app on Windows. Any process that loaded the old library keeps using it until restarted.

What is the CVSS score for CVE-2026-2878?

5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N.

Where is the official advisory for CVE-2026-2878?

The vendor advisory is at https://www.telerik.com/products/aspnet-ajax/documentation/knowledge-base/kb-security-insufficient-entropy-cve-2026-2878. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-2878.

References

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