⚠ 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 6.9

How to Fix CVE-2026-29052: Cross-site scripting in calendar

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

CVE-2026-29052 is a cross-site scripting in humhub calendar. The fix is to apply the vendor patch noted below.

⚡ At a glance
Severity6.9 (Medium)
Actively exploited?No public listing in CISA KEV
Affectedcalendar < 1.8.11
Fixed inSee vendor advisory
Type (CWE)CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

What is CVE-2026-29052?

The Calendar module for HumHub enables users to create one-time or recurring events, manage attendee invitations, and efficiently track all scheduled activities. Prior to version 1.8.11, a Stored Cross-Site Scripting (XSS) vulnerability in the Event Types of the HumHub Calendar module impacts users viewing events created by an administrative account. This issue has been patched in version 1.8.11. The CVSS base score is 6.9 (Medium). The official advisory is at https://github.com/humhub/calendar/security/advisories/GHSA-gqj3-pmp2-mrx8.

Am I affected?

Check the version of calendar you are running and compare it against the Affected row above (calendar < 1.8.11). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


# Linux package check
dpkg -s calendar 2>/dev/null | grep -i ^Version
rpm -q calendar 2>/dev/null
command -v calendar >/dev/null && calendar --version 2>/dev/null


# Windows (PowerShell)
Get-Package -Name "*calendar*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "calendar" 2>$null

How to fix CVE-2026-29052

Upgrade calendar 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 calendar
dpkg -s calendar | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh calendar -y
# or for older releases:
sudo yum update calendar -y
rpm -q calendar

SUSE / openSUSE


sudo zypper refresh
sudo zypper update calendar
rpm -q calendar

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-29052 affecting calendar
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-29052-fix-$(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"
    $pkg = Get-Package -Name "*calendar*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\calendar-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\calendar" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "calendar" --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 "calendar"
    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-2026-29052 affecting calendar
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-29052-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 calendar 2>/dev/null | grep -i ^Version || echo "calendar not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q calendar || echo "calendar not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-29052-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/calendar /etc/calendar.d /etc/calendar.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 calendar
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y calendar
elif command -v yum >/dev/null; then
    yum update -y calendar
elif command -v zypper >/dev/null; then
    zypper --non-interactive update calendar
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s calendar 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q calendar
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-2026-29052" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-29052"

Disable the affected service (Linux)


sudo systemctl stop calendar 2>/dev/null || true
sudo systemctl disable calendar 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 calendar 2>/dev/null | grep -i ^Version
rpm -q calendar 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 "*calendar*" | 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-2026-29052 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-2026-29052?

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-2026-29052?

6.9 (Medium). Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N.

Where is the official advisory for CVE-2026-29052?

The vendor advisory is at https://github.com/humhub/calendar/security/advisories/GHSA-gqj3-pmp2-mrx8. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-29052.

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.*