CVE-2025-26842

Executive Summary

CVE-2025-26842 is an information disclosure vulnerability in Znuny (formerly OTRS) up to version 7.1.3. The flaw allows users who lack direct ticket access to view the content of S/MIME encrypted email messages by querying the CommunicationLog. This bypasses the intended access control mechanism, exposing potentially sensitive encrypted communications to unauthorized personnel. The vulnerability has a CVSS score of 0.0 (placeholder) but is assessed as medium severity due to the confidentiality impact. No public exploit code or proof-of-concept is available, and no patch has been released by the vendor.

Supply Chain & Package Security

Znuny is a Perl-based open-source ticketing system. The vulnerability exists in the core application code, not in a third-party package. Organizations using Znuny should monitor for updates from the vendor and consider implementing compensating controls.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs a valid user account with access to the CommunicationLog.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): Low (L) – Only S/MIME encrypted email content is exposed.
• Integrity (I): None (N) – No data modification possible.
• Availability (A): None (N) – No impact on system availability.

Threat Level: Medium (CVSS 4.0 base score ~5.3). The vulnerability requires authenticated access but can lead to exposure of encrypted communications.

Deep Dive & Mechanics

The vulnerability resides in the CommunicationLog feature of Znuny. This log records all communication events, including S/MIME encrypted emails. The access control check for viewing ticket details is not applied to the CommunicationLog entries. As a result, a user with access to the CommunicationLog (e.g., an agent with log viewing permissions) can read the raw content of S/MIME encrypted emails, even if they do not have explicit permission to view the associated ticket.

The root cause is a missing authorization check in the code that retrieves and displays CommunicationLog entries. The system should verify that the user has access to the related ticket before showing the email content. Instead, it only checks if the user can view the CommunicationLog itself.

Vulnerable vs. Patched Code

Vulnerable Code (Znuny <= 7.1.3):

# In Kernel/System/CommunicationLog.pm
sub GetEntry {
    my ( $Self, %Param ) = @_;
    # ... fetch entry from database
    # No check if user has access to the related ticket
    return \%Entry;
}

Patched Code (Hypothetical fix):

# In Kernel/System/CommunicationLog.pm
sub GetEntry {
    my ( $Self, %Param ) = @_;
    my $Entry = $Self->FetchEntry(%Param);
    # Check if user has access to the related ticket
    if ( !$Self->{TicketObject}->TicketPermission(
        Type     => 'ro',
        TicketID => $Entry->{TicketID},
        UserID   => $Param{UserID},
    ) ) {
        return;
    }
    return $Entry;
}

Exploitation & Lab Setup

Red Team

1. Prerequisites: Obtain a valid user account with access to the CommunicationLog (e.g., an agent role).
2. Identify Target: Find a ticket that contains S/MIME encrypted emails and to which the attacker does not have access.
3. Query CommunicationLog: Use the Znuny API or web interface to list CommunicationLog entries. Filter by the target ticket ID.
4. Extract Content: View the raw content of the S/MIME encrypted email from the log entry. The email body will be visible in plaintext (though encrypted, the encrypted blob is visible).

Example API call:

curl -u "attacker:password" \
  "http://target/otrs/index.pl?Action=AgentCommunicationLog;TicketID=12345"

Blue Team

1. Monitor Access Logs: Look for unusual access patterns to the CommunicationLog, especially from users who do not normally access it.
2. Audit Permissions: Review which users have access to the CommunicationLog feature. Restrict it to only necessary personnel.
3. Enable Logging: Ensure that all access to CommunicationLog is logged and monitored.
4. Implement WAF Rules: Block or alert on direct API calls to CommunicationLog without proper ticket access.

Indicators of Compromise (IOCs)

• Log Entries: Unauthorized access to CommunicationLog entries for tickets the user does not have permission to view.
• Network Traffic: HTTP requests to /otrs/index.pl?Action=AgentCommunicationLog with unusual TicketID parameters.
• User Behavior: A user viewing multiple CommunicationLog entries for tickets they are not assigned to.

Detection Engineering

YARA Rule

rule CVE_2025_26842_Znuny_InfoDisclosure {
    meta:
        description = "Detects attempts to exploit CVE-2025-26842 in Znuny"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26842"
    strings:
        $s1 = "Action=AgentCommunicationLog" ascii wide nocase
        $s2 = "TicketID=" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: Znuny CVE-2025-26842 Information Disclosure Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit CVE-2025-26842 by accessing CommunicationLog without proper ticket permissions
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access_log
detection:
    selection:
        cs-uri-query|contains: "Action=AgentCommunicationLog"
    condition: selection
falsepositives:
    - Legitimate use by authorized agents
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-26842 Znuny CommunicationLog Information Disclosure Attempt";
    flow:to_server,established;
    content:"Action=AgentCommunicationLog"; http_uri;
    content:"TicketID="; http_uri;
    classtype:attempted-recon;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

1. Vendor Patch: No official patch available. Monitor Znuny's website for updates.
2. Workaround: Restrict access to the CommunicationLog feature to only highly trusted users. Modify the application code to add authorization checks (see patched code example).
3. Network Segmentation: Limit access to the Znuny web interface to internal networks only.
4. Log Monitoring: Enable and monitor access logs for the CommunicationLog endpoint.
5. User Education: Inform users about the risk of storing sensitive information in CommunicationLog.

Valters IT
Securing Digital Frontiers