CVE-2026-50055

Executive Summary

CVE-2026-50055 is a policy-enforcement flaw in Zimbra Collaboration Suite that permits an authenticated user to circumvent a disabled mail-forwarding policy. By abusing a Sieve notify action, an attacker can exfiltrate copies of email content and headers to an arbitrary external address despite administrative restrictions. The vulnerability affects Zimbra deployments where mail forwarding has been administratively disabled, allowing an authenticated low-privileged user to silently leak sensitive mailbox data. With a CVSS score of 6.5 (AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N), the primary impact is confidentiality loss; the EPSS score is not yet available, no public PoC or exploit code is known, and the issue remains unpatched.

Technical Analysis

CWE-863 (Incorrect Authorization) is the root cause: the system fails to enforce the mail-forwarding policy at the Sieve execution layer. Zimbra’s mail filtering supports Sieve scripts, which can contain a notify action to generate user notifications — typically used for out-of-office messages. However, notify can also carry a copy of the original message, including headers and body, to an arbitrary recipient address specified in the script.

When an administrator disables mail forwarding (e.g., to prevent data exfiltration or comply with retention policies), the enforcement logic likely blocks the standard forwarding mechanisms (redirect, or UI-level forwarding flags). However, the Sieve notify action is not covered by the same policy check. As a result, an authenticated user with the ability to upload or edit a Sieve script can craft a filter that triggers on incoming mail and notifies an external address, effectively forwarding the entire message.

The attack requires authentication (PR:L) and no user interaction (UI:N). The network attack vector (AV:N) and low complexity (AC:L) make this a straightforward exploit for a motivated insider or compromised account. The confidentiality impact is high (C:H) because the leaked data includes email content and headers, potentially exposing credentials, business secrets, or PII. There is no integrity or availability impact (I:N/A:N).

Exploitation Scenarios

1. Reconnaissance and target identification: The attacker identifies a Zimbra collaboration server (e.g., via Shodan searches for http.title:"Zimbra"). They must obtain valid credentials — through phishing, credential stuffing, or as a malicious insider — to authenticate to the webmail interface.

2. Triggering the policy-enforcement flaw: The attacker accesses the Sieve filter management functionality (typically under Preferences → Filters). They create a new filter rule or edit an existing one to include a notify action. The specific syntax is not detailed in the NVD description, but the class of the flaw is a Sieve script that uses notify to send message content and headers to an arbitrary email address. This bypasses the disabled forwarding policy because the policy check does not extend to the Sieve notify action.

3. Payload/execution at the vulnerability-class level: The Sieve script is saved and activated. When a new email arrives in the mailbox, the Sieve interpreter executes the filter, triggering the notify action. The notification includes the original message (content and headers) and is sent to the attacker-controlled address — no further user interaction is required.

4. Post-exploitation impact: The attacker receives a continuous stream of all incoming email, including password reset links, confidential attachments, and sensitive communications. This enables further lateral movement, data theft, and long-term espionage. Because the forwarding occurs via Sieve, it may leave a minimal footprint in standard mail logs.

No public PoC is available; the attack chain is inferred from the vulnerability class and description. Specific function names, endpoint parameters, or Sieve syntax are not provided and must not be assumed.

Impact Assessment

• Business impact: High confidentiality loss for affected organizations. Attackers can silently exfiltrate all incoming email, leading to data breaches, regulatory violations (GDPR, HIPAA), and intellectual property theft.
• Affected systems: Any Zimbra Collaboration Suite instance where administrators have disabled mail forwarding but not Sieve notify actions. All Zimbra versions that support Sieve and are configured with such policies are potentially vulnerable.
• Blast radius: An authenticated user can exfiltrate data from their own mailbox only; however, if the account belongs to an executive, finance, or IT administrator, the blast radius expands significantly. The vulnerability does not grant access to other users’ mailboxes directly.
• CVSS subscores: AV:N (remotely exploitable), AC:L (no special conditions), PR:L (requires authentication), UI:N (no user action), S:U (scope unchanged), C:H (high confidentiality impact), I:N (no integrity impact), A:N (no availability impact). Base score: 6.5 (Medium).

Detection Guidance

Sigma Rule

title: Zimbra Sieve Notify Action Used to Bypass Disabled Forwarding Policy
id: 8a1b2c3d-4e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects the creation or modification of a Sieve filter that includes a notify action, potentially bypassing a disabled mail forwarding policy.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-50055
author: Threat Intelligence Team
date: 2026/04/15
logsource:
  product: zimbra
  service: mailbox
  definition: Requires Zimbra mailbox audit logging enabled to capture Sieve script changes.
detection:
  selection:
    EventID: 'SIEVE_SCRIPT_MODIFIED'  # Placeholder; adjust to actual Zimbra audit event ID
    ScriptContent|contains: 'notify'
  condition: selection
fields:
  - User
  - SourceIP
  - ScriptContent
falsepositives:
  - Legitimate out-of-office notifications configured by users with proper authorization.
level: high

YARA Rule

rule CVE_2026_50055_Zimbra_Sieve_Notify_Exfil
{
    meta:
        description = "Detects Sieve scripts abusing notify action to exfiltrate email content, related to CVE-2026-50055"
        author = "Threat Intelligence Team"
        date = "2026-04-15"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-50055"
        severity = "high"

    strings:
        $sieve_notify = "notify" ascii wide nocase
        $sieve_action = "action" ascii wide nocase
        $sieve_method = "method" ascii wide nocase
        $sieve_options = "options" ascii wide nocase
        $sieve_message = "message" ascii wide nocase
        $sieve_headers = "headers" ascii wide nocase

    condition:
        uint32(0) == 0x53696576 or // "Siev" (RFC 5228)
        ($sieve_notify and $sieve_action and $sieve_method) or
        ($sieve_notify and $sieve_options and 2 of ($sieve_message, $sieve_headers))
}

Suricata IDS Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 25 (msg:"CVE-2026-50055 Zimbra Sieve Notify Exfiltration Attempt"; flow:to_server,established; content:"notify"; nocase; content:"method"; distance:0; nocase; content:"options"; distance:0; nocase; reference:cve,2026-50055; reference:url,https://nvd.nist.gov/vuln/detail/CVE-2026-50055; classtype:attempted-recon; sid:10000001; rev:1;)

Elastic Detection

sequence by user.name
  [ authentication where event.action == "logon" and event.outcome == "success" ]
  [ file where event.action == "modification" and file.path : "*sieve*" and file.Ext.content : "notify" ]

Splunk SPL Query

index=zimbra sourcetype=zimbra:audit OR sourcetype=zimbra:mailbox
( EventType="SIEVE_SCRIPT_MODIFIED" OR EventType="FILTER_UPDATE" )
| search script_content="*notify*"
| stats count values(user) as users values(src_ip) as source_ips by _time, host
| where count > 0

Wazuh OSSEC Rule

<group name="zimbra,">
  <rule id="100200" level="12">
    <decoded_as>json</decoded_as>
    <field name="EventType">SIEVE_SCRIPT_MODIFIED</field>
    <field name="script_content" type="pcre2">(?i)notify</field>
    <description>CVE-2026-50055: Possible Zimbra mail forwarding bypass via Sieve notify action detected</description>
    <mitre>
      <id>T1114</id>
    </mitre>
    <group>data_exfiltration,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations:

   • Disable the Sieve notify extension for all users, if Zimbra allows such configuration. This may require modifying Sieve capabilities via zimbraSieveNotifyEnabled or similar LDAP attributes.
   • Restrict user ability to upload custom Sieve scripts; enforce server-side validation that rejects scripts containing notify actions when forwarding is disabled.
   • Implement network egress filtering to block SMTP traffic to external addresses from the Zimbra server, except for authorized relays.
   • Monitor Sieve script changes and alert on any notify action creation.

2. Patch/upgrade steps: No vendor patch is available. Monitor Zimbra’s official security advisories and apply updates as soon as they are released. Until a fix is issued, consider the mitigations above as compensating controls.

3. Long-term hardening:

   • Adopt a least-privilege model for Sieve script management, limiting access to trusted administrators only.
   • Integrate DLP (Data Loss Prevention) solutions to inspect outbound email for unauthorized forwarding patterns.
   • Conduct regular audits of mailbox rules and Sieve scripts for unauthorized exfiltration mechanisms.
   • Enable comprehensive audit logging for all Sieve modifications and forwarding policy changes.

References

1. NVD – CVE-2026-50055: https://nvd.nist.gov/vuln/detail/CVE-2026-50055
2. Zimbra Security Advisories: https://wiki.zimbra.com/wiki/Security_Center
3. CWE-863: Incorrect Authorization: https://cwe.mitre.org/data/definitions/863.html
4. RFC 5228 – Sieve: An Email Filtering Language: https://datatracker.ietf.org/doc/html/rfc5228
5. MITRE ATT&CK T1114 – Email Collection: https://attack.mitre.org/techniques/T1114/ No threat actor campaigns or exploit-in-the-wild evidence are currently known for this CVE.