CVE-2026-3108: Mattermost Terminal Injection Vulnerability

CVE-2026-3108 Overview

CVE-2026-3108 is a terminal escape sequence injection vulnerability affecting multiple versions of Mattermost. The vulnerability exists because Mattermost fails to sanitize user-controlled post content in the mmctl commands terminal output. This allows attackers to manipulate administrator terminals via crafted messages containing ANSI and OSC escape sequences that enable screen manipulation, fake prompts, and clipboard hijacking.

Critical Impact

Attackers can inject malicious ANSI and OSC escape sequences through crafted messages, potentially hijacking administrator clipboards, displaying fake prompts, and manipulating terminal screens to facilitate credential theft or mislead system administrators.

Affected Products

• Mattermost 11.2.x versions <= 11.2.2
• Mattermost 10.11.x versions <= 10.11.10
• Mattermost 11.4.x versions <= 11.4.0
• Mattermost 11.3.x versions <= 11.3.1

Discovery Timeline

• 2026-03-26 - CVE-2026-3108 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-3108

Vulnerability Analysis

This vulnerability is classified under CWE-150 (Improper Neutralization of Escape, Meta, or Control Sequences). The core issue stems from inadequate input sanitization when rendering user-controlled post content within the mmctl command-line interface. Terminal emulators interpret ANSI escape codes and Operating System Command (OSC) sequences as control instructions rather than displayable text. When Mattermost outputs unsanitized post content containing these sequences to the terminal, attackers can leverage this behavior to execute various terminal manipulation attacks.

The attack requires network access but depends on an administrator or privileged user executing mmctl commands that display malicious post content. The scope is changed (S:C in CVSS vector), indicating the vulnerability can impact resources beyond the vulnerable component—specifically, the terminal session and potentially the system clipboard.

Root Cause

The root cause is improper neutralization of escape sequences in user-controlled input. The mmctl command-line tool outputs post content directly to the terminal without stripping or escaping ANSI control sequences, OSC sequences, or other terminal control characters. This violates the principle of treating all user input as untrusted data that requires sanitization before being rendered in security-sensitive contexts.

Attack Vector

An attacker with low privileges (such as a standard Mattermost user) can craft a malicious message containing embedded ANSI escape codes and OSC sequences. When an administrator uses mmctl to view or process posts containing this malicious content, the terminal interprets these sequences. Exploitation scenarios include:

• Screen Manipulation: Using ANSI sequences to clear the screen, reposition the cursor, or overwrite displayed content to hide malicious activity or display misleading information
• Fake Prompts: Crafting sequences that display fake command prompts, potentially tricking administrators into entering credentials or executing unintended commands
• Clipboard Hijacking: Using OSC 52 sequences to write arbitrary content to the system clipboard, which could be pasted into sensitive contexts like password fields or command lines

The exploitation mechanism involves embedding control sequences within legitimate-looking message content. When processed by mmctl and output to a vulnerable terminal emulator, these sequences execute their intended terminal manipulation functions.

Detection Methods for CVE-2026-3108

Indicators of Compromise

• Mattermost posts containing unusual character sequences such as \\x1b[, \033[, or \\x9b which indicate embedded ANSI escape codes
• Messages containing OSC sequences identifiable by patterns like \\x1b] followed by numeric codes (particularly OSC 52 for clipboard operations)
• Unexpected terminal behavior when administrators execute mmctl commands, including screen flickering, cursor movement, or clipboard content changes
• User reports of strange display artifacts when viewing posts through command-line tools

Detection Strategies

• Implement log analysis to identify posts containing control character patterns indicative of escape sequence injection attempts
• Monitor mmctl command execution for anomalous terminal output behavior through endpoint detection and response (EDR) solutions
• Deploy content filtering rules on Mattermost servers to flag or block messages containing suspicious escape sequence patterns
• Utilize SentinelOne Singularity platform to monitor for clipboard manipulation and suspicious terminal activity on endpoints running mmctl

Monitoring Recommendations

• Enable verbose logging for mmctl operations to capture full post content for security analysis
• Configure alerting on pattern matches for known malicious escape sequence structures in message content
• Implement periodic audits of Mattermost message content for embedded control characters
• Monitor administrator workstations for unexpected clipboard changes during mmctl usage

How to Mitigate CVE-2026-3108

Immediate Actions Required

• Upgrade Mattermost to patched versions: 11.2.3 or later, 10.11.11 or later, 11.4.1 or later, or 11.3.2 or later
• Configure terminal emulators used by administrators to disable or restrict OSC 52 clipboard operations until patches are applied
• Advise administrators to avoid using mmctl to view untrusted post content until the system is patched
• Consider using terminal emulators with built-in escape sequence sanitization or restricted mode options

Patch Information

Mattermost has released security updates addressing this vulnerability. Refer to the Mattermost Security Updates page for detailed patch information and download links. The security advisory reference is MMSA-2026-00599.

Organizations should prioritize upgrading to the following minimum versions:

• 11.2.x series: Upgrade to 11.2.3 or later
• 10.11.x series: Upgrade to 10.11.11 or later
• 11.4.x series: Upgrade to 11.4.1 or later
• 11.3.x series: Upgrade to 11.3.2 or later

Workarounds

• Pipe mmctl output through a sanitization utility such as cat -v or sed to neutralize escape sequences before terminal rendering
• Use terminal emulators in safe mode or configure them to ignore OSC sequences (particularly OSC 52)
• Implement network-level content filtering to strip control characters from Mattermost API responses consumed by command-line tools
• Restrict mmctl usage to isolated environments or containers where clipboard hijacking poses minimal risk

# Example: Sanitize mmctl output to neutralize escape sequences
mmctl post list --channel mychannel | cat -v

# Alternative: Use sed to strip ANSI escape codes
mmctl post list --channel mychannel | sed 's/\\x1b\[[0-9;]*[a-zA-Z]//g'

# Disable OSC 52 clipboard in common terminals (add to terminal config)
# For xterm: Add to ~/.Xresources
# XTerm*disallowedWindowOps: 20,21,SetXprop