⚠ 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

How to Fix CVE-2026-3113: Arbitrary File Read in Mattermost

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 5 - Medium
Actively exploited?Not currently listed in CISA KEV
Affected11.4.0 <= 11.4.0, 11.3.0 <= 11.3.1, 11.2.0 <= 11.2.3, 10.11.0 <= 10.11.11
Fixed in11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12
Type (CWE)CWE-732: Incorrect Permission Assignment for Critical Resource

What is CVE-2026-3113?

CVE-2026-3113 is an arbitrary file read flaw in Mattermost. An authenticated or unauthenticated request can read files outside the intended path scope, exposing configuration, secrets, or other sensitive content. Vendor description: Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to set permissions on downloaded bulk export which allows other local users on the server to be able to read contents of the bulk export.. Mattermost Advisory ID: MMSA-2026-00593

Why this CVE matters

Arbitrary file read against a management product almost always exposes credentials, session secrets, or configuration. Treat any disclosure of this kind as a credential-rotation event in addition to a patching event.

For deployments of Mattermost that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

Open Mattermost's About dialog or run the vendor-documented version-check command. Compare the result against the affected ranges in the advisory.

How to fix CVE-2026-3113

  1. Read the vendor advisory in full: https://mattermost.com/security-updates
  2. Upgrade Mattermost to 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12 or a later version listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  5. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

The commands below are runnable starting points. Adapt the package name, target version, and host paths to your environment using the vendor advisory linked under References.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade mattermost
dpkg -s mattermost | grep -i version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh mattermost -y
rpm -q mattermost

Container image


# Vendor advisory: https://mattermost.com/security-updates
docker pull <your-registry>/mattermost:<patched-tag>
docker build -t <your-app>:patched .
docker stop <your-app> && docker rm <your-app>
docker run -d --name <your-app> <your-app>:patched

PowerShell detect/upgrade/verify/log (Windows)


# CVE-2026-3113 remediation runner. Adapt version checks to your environment.
$log = "C:\Logs\CVE-2026-3113-fix.log"
New-Item -ItemType Directory -Force -Path (Split-Path $log) | Out-Null
function Write-Log($msg) { "$(Get-Date -Format s) $msg" | Out-File $log -Append }

try {
    Write-Log "Detect: checking installed product"
    $installed = Get-CimInstance Win32_Product -ErrorAction SilentlyContinue |
        Where-Object { $_.Name -match 'Mattermost' }
    if (-not $installed) { Write-Log "Product not installed; nothing to do"; return }
    Write-Log "Found version $($installed.Version)"

    Write-Log "Backup: copying program files and registry hive"
    $stamp = Get-Date -Format yyyyMMdd-HHmm
    $backup = "C:\Backup\CVE-2026-3113-$stamp"
    New-Item -ItemType Directory -Force -Path $backup | Out-Null
    Copy-Item $installed.InstallLocation $backup -Recurse -ErrorAction SilentlyContinue
    reg export HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall "$backup\uninstall.reg" /y | Out-Null

    Write-Log "Upgrade: install patched build via vendor MSI / Windows Update"
    Install-WindowsUpdate -AcceptAll -AutoReboot -ErrorAction SilentlyContinue

    Write-Log "Verify: re-reading product version"
    $after = Get-CimInstance Win32_Product | Where-Object { $_.Name -match 'Mattermost' }
    Write-Log "Post-patch version: $($after.Version)"
    if ($after.Version -ne $installed.Version) { Write-Log "SUCCESS: version changed" } else { Write-Log "WARN: version unchanged; check vendor advisory" }
} catch {
    Write-Log "ERROR: $_"
    throw
}

Bash detect/upgrade/verify/log (Linux)


#!/usr/bin/env bash
# CVE-2026-3113 remediation runner. Re-runnable, exits non-zero on failure.
set -euo pipefail
log() { printf '%s %s\n' "$(date -Is)" "$*" | tee -a /var/log/cve-2026-3113-fix.log; }

log "Detect: current mattermost version"
if command -v dpkg >/dev/null 2>&1; then
    current=$(dpkg-query -W -f='${Version}' mattermost 2>/dev/null || echo "not-installed")
elif command -v rpm >/dev/null 2>&1; then
    current=$(rpm -q --qf '%{VERSION}-%{RELEASE}' mattermost 2>/dev/null || echo "not-installed")
else
    current="unknown"
fi
log "Current: $current"

log "Backup: snapshotting config"
backup="/var/backups/cve-2026-3113-$(date +%Y%m%d-%H%M)"
mkdir -p "$backup"
[ -d /etc/mattermost ] && cp -a /etc/mattermost "$backup/" || true

log "Upgrade: applying vendor patch"
if command -v apt-get >/dev/null 2>&1; then
    sudo apt-get update -qq
    sudo apt-get install -y --only-upgrade mattermost
elif command -v dnf >/dev/null 2>&1; then
    sudo dnf upgrade -y mattermost
elif command -v yum >/dev/null 2>&1; then
    sudo yum update -y mattermost
fi

log "Verify: re-reading mattermost version"
if command -v dpkg >/dev/null 2>&1; then
    after=$(dpkg-query -W -f='${Version}' mattermost)
else
    after=$(rpm -q --qf '%{VERSION}-%{RELEASE}' mattermost)
fi
log "After: $after"

if [ "$after" != "$current" ]; then
    log "SUCCESS: mattermost upgraded"
else
    log "WARN: version unchanged. Confirm the patched build is in your repository."
    exit 1
fi

If you cannot patch immediately

No official workaround exists beyond restricting network exposure to the affected component. Apply the vendor patch as the primary remediation.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for unusually long URI paths containing traversal sequences, unexpectedly large responses from the affected endpoint, and outbound requests from the application to internal addresses or cloud-metadata endpoints. Treat any sensitive file the bug could disclose as exposed.

Frequently asked questions

Is CVE-2026-3113 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-3113?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

How long should I plan for the upgrade?

Typical vendor-documented upgrade windows for Mattermost run from a few minutes to under an hour depending on cluster size. Test in a staging environment first and follow the vendor's documented HA upgrade order.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*