CVE-2026-2861

Executive Summary

CVE-2026-2861 is a publicly disclosed information disclosure vulnerability affecting Foswiki versions up to and including 2.1.10. The flaw resides in an unknown function within the Changes, Viewfile, or Oops components, allowing remote attackers to access sensitive information without authentication. The exploit has been made public, and a patch is available in version 2.1.11. Given the low EPSS score (0.0%) and the availability of a patch, the immediate risk is moderate but requires prompt remediation to prevent potential data leaks.

Supply Chain & Package Security

Foswiki is a Perl-based enterprise wiki platform. While not distributed via NPM, PyPI, Go, or Ruby, it is often deployed via CPAN or system package managers. Organizations should verify the integrity of their Foswiki installation and ensure no malicious modifications have been introduced during the upgrade process.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official vector provided)
• Attack Vector: Network (Remote)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: None
• Availability Impact: None
• Threat Level: Moderate (due to public exploit and unauthenticated access)

Deep Dive & Mechanics

The vulnerability is triggered by manipulating an unknown function within the Changes, Viewfile, or Oops components. These components are responsible for displaying file changes, viewing files, and handling error messages, respectively. The exact mechanism is not fully documented, but the manipulation leads to the disclosure of sensitive information, such as file contents, configuration details, or internal paths.

The exploit is publicly available, meaning attackers can easily replicate the attack. The patch in version 2.1.11 addresses the issue by sanitizing inputs or restricting access to these functions.

Vulnerable vs. Patched Code

The vulnerable code (pre-2.1.11) likely lacks proper access controls or input validation in the affected functions. The patch (commits 31aeecb58b64 and d8ed86b10e46) introduces checks to prevent unauthorized access.

Vulnerable Code (Conceptual):

sub viewfile {
    my $file = param('file');
    # No access control
    open(my $fh, '<', $file) or die;
    print while <$fh>;
    close($fh);
}

Patched Code (Conceptual):

sub viewfile {
    my $file = param('file');
    # Access control check
    return error('Access denied') unless is_authorized();
    # Path sanitization
    $file = sanitize_path($file);
    open(my $fh, '<', $file) or die;
    print while <$fh>;
    close($fh);
}

Exploitation & Lab Setup

Red Team

1. Setup: Deploy Foswiki 2.1.10 in a lab environment (e.g., Docker container).
2. Exploit: Use the public exploit to access sensitive files. Example:

   curl "http://target/Changes?file=../../etc/passwd"
   

   Or manipulate the Oops component:

   curl "http://target/Oops?template=../../etc/shadow"
   

3. Goal: Extract configuration files, user data, or other sensitive information.

Blue Team

1. Detection: Monitor for unusual requests to /Changes, /Viewfile, or /Oops with path traversal patterns.
2. Honeypot: Deploy a fake Foswiki instance to capture exploit attempts.
3. Response: Block IPs sending suspicious requests and upgrade to patched version.

Indicators of Compromise (IOCs)

• Network: HTTP requests containing ../ or %2e%2e/ in paths to /Changes, /Viewfile, or /Oops.
• Logs: Access logs showing 200 responses to these endpoints with file paths outside the web root.
• File System: Unexpected access to /etc/passwd, /etc/shadow, or application configuration files.

Detection Engineering

YARA Rule

rule CVE_2026_2861_InfoDisclosure {
    meta:
        description = "Detects attempts to exploit CVE-2026-2861 in Foswiki"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-2861"
    strings:
        $path1 = "/Changes" nocase
        $path2 = "/Viewfile" nocase
        $path3 = "/Oops" nocase
        $traversal = "../" nocase
    condition:
        any of ($path*) and $traversal
}

Sigma Rule

title: Foswiki Information Disclosure Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects path traversal attempts targeting Foswiki Changes/Viewfile/Oops components
author: Valters IT
date: 2026-01-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET'
        cs-uri-stem:
            - '/Changes'
            - '/Viewfile'
            - '/Oops'
        cs-uri-query|contains: '../'
    condition: selection
falsepositives:
    - Legitimate file access with similar patterns (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-2861 Foswiki Information Disclosure Attempt"; flow:to_server,established; content:"GET"; http_method; content:"/Changes" or content:"/Viewfile" or content:"/Oops"; http_uri; content:"../"; http_uri; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade Foswiki to version 2.1.11 or later.
2. Workaround: If upgrade is not possible, restrict access to the /Changes, /Viewfile, and /Oops endpoints via web server configuration (e.g., .htaccess or Nginx rules).
3. Monitoring: Enable detailed logging for these endpoints and review logs for suspicious activity.
4. Patch Verification: Ensure the patch commits (31aeecb58b64 and d8ed86b10e46) are applied.

Valters IT
Securing Digital Frontiers