CVE-2026-10631

Executive Summary

CVE-2026-10631 is an authorization bypass vulnerability in the EWS FindItem handler of Zimbra Collaboration Suite versions 10.1.0 through 10.1.19. An authenticated user with EWS (Exchange Web Services) enabled can read complete mailbox items—including raw MIME content and attachments—from arbitrary local accounts without holding a share or delegation grant. While the CVSS score is 6.5 (Medium), the confidentiality impact is rated High, making this a significant data exposure risk for any organization running vulnerable Zimbra deployments.

Technical Analysis

This vulnerability is classified under CWE-639 (Authorization Bypass Through User-Controlled Key), indicating that the application fails to properly validate that a requesting user has the necessary permissions to access the resources being requested. In this case, the flaw resides in the EWS FindItem handler.

Zimbra Collaboration Suite implements Exchange Web Services (EWS) as a compatibility layer, allowing clients such as Microsoft Outlook and other EWS-aware applications to interact with Zimbra mailboxes. The FindItem operation is a standard EWS SOAP action used to search for and retrieve items within a mailbox.

According to the NVD description, the authorization bypass allows an authenticated user—who merely needs EWS enabled on their account—to read complete mailbox items from arbitrary local accounts. This explicitly includes raw MIME data and attachments. Critically, the attacker does not need a share or delegation grant, meaning that the normal access control mechanisms that would restrict one user from accessing another's mailbox are absent or improperly enforced in this code path.

The CVSS vector (AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N) confirms:

• Network exploitable (AV:N): The attack can be carried out remotely over the network.
• Low attack complexity (AC:L): No special conditions or race conditions are required.
• Low privileges required (PR:L): The attacker must be an authenticated user with EWS enabled.
• No user interaction (UI:N): No victim action is needed.
• Unchanged scope (S:U): The impact is confined to the vulnerable component.
• High confidentiality impact (C:H): Complete mailbox contents, including raw MIME and attachments, can be read.
• No integrity or availability impact (I:N/A:N): The attacker cannot modify or delete data through this vulnerability.

The specific mechanism by which the FindItem handler fails to enforce authorization is not detailed in the NVD description. No public proof-of-concept, exploit code, or Nuclei template is available at this time. The attack chain remains theoretical but is highly plausible given the described impact.

As of this report, no patch has been released. The vulnerability affects all versions from 10.1.0 through 10.1.19. Zimbra has not published an official advisory or fix according to tracked sources. The patch confidence is 0/4, indicating that no patch evidence exists from any monitored source.

Exploitation Scenarios

Note: No public PoC or exploit code is available. The following attack chain is inferred strictly from the vulnerability class (CWE-639) and the NVD description.

1. Reconnaissance and Target Identification: An attacker first identifies a Zimbra Collaboration Suite deployment running a vulnerable version (10.1.0–10.1.19). This could be done through Shodan scans using dorks such as http.title:"Zimbra" or product:"Zimbra". The attacker must possess valid credentials for an account on the target Zimbra server, and that account must have EWS enabled.

2. Triggering the Authorization Bypass: Using the authenticated account, the attacker interacts with the EWS endpoint. The FindItem operation is a standard EWS SOAP request. By crafting a FindItem request that specifies a target mailbox belonging to another user on the same Zimbra server, the attacker exploits the missing authorization check. The handler fails to verify that the authenticated user has a share or delegation grant for the target mailbox.

3. Payload/Execution Method: The attacker sends a SOAP request to the EWS endpoint (typically /EWS/Exchange.asmx on the Zimbra server) invoking the FindItem operation. The request includes parameters that identify the target mailbox (e.g., via a Distinguished Name or email address). Because authorization is not enforced, the server returns the requested items. The exact parameter names and request structure are not specified in the available data, but they follow the EWS protocol specification for FindItem.

4. Post-Exploitation Impact: The attacker receives complete mailbox items from the targeted account, including raw MIME content and attachments. This could expose sensitive emails, documents, credentials, and other confidential information. The attacker can repeat the process for multiple accounts, enabling large-scale data exfiltration. Since the vulnerability only permits reading (no write/delete), the attacker focuses on data theft, which could lead to further attacks (e.g., credential harvesting, business email compromise, or lateral movement).

Impact Assessment

The business impact is substantial for organizations using vulnerable Zimbra versions. The CVSS score of 6.5 reflects a Medium severity, but the High confidentiality impact (C:H) means that any authenticated user with EWS access can potentially read the entire mailbox contents of any other local user. This bypasses all sharing and delegation controls, effectively nullifying the intended access control model.

Affected systems are Zimbra Collaboration Suite 10.1.0 through 10.1.19. The blast radius includes all mailboxes on the server. Even a single low-privileged account with EWS enabled can expose the mailboxes of executives, IT administrators, finance departments, and other sensitive users. The lack of integrity or availability impact means the attacker cannot alter or destroy data, but the confidentiality breach alone can be devastating—exposing trade secrets, personal data (potentially triggering GDPR or other privacy regulations), and authentication tokens.

The attack requires network access and low privileges, making it feasible for an insider threat or an external attacker who has compromised a single set of credentials (e.g., through phishing). There is no user interaction required, so the attack can be automated.

Detection Guidance

Sigma Rule

title: Zimbra EWS FindItem Authorization Bypass
id: 5f8a3c2e-1b4d-4e9a-9c7f-2d6e8a1b3c4d
status: experimental
description: Detects potential exploitation of CVE-2026-10631 via anomalous EWS FindItem requests targeting multiple mailboxes.
author: Threat Intelligence
date: 2026-01-01
logsource:
  category: webserver
  product: zimbra
detection:
  selection:
    cs-method: 'POST'
    cs-uri-stem|contains: '/EWS/Exchange.asmx'
    cs-uri-query|contains: 'FindItem'
  filter_legitimate:
    sc-status: 200
    cs-username|contains: 'admin'
  condition: selection and not filter_legitimate
falsepositives:
  - Legitimate EWS clients accessing multiple mailboxes (e.g., backup software)
level: high
tags:
  - attack.t1213
  - cve.2026.10631

YARA Rule

rule CVE_2026_10631_Zimbra_EWS_FindItem
{
    meta:
        description = "Detects SOAP requests targeting Zimbra EWS FindItem that may indicate CVE-2026-10631 exploitation"
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-10631"
        severity = "high"
    strings:
        $ews = "/EWS/Exchange.asmx" ascii wide nocase
        $finditem = "FindItem" ascii wide nocase
        $soap = "soap:Envelope" ascii wide nocase
        $mailbox = "Mailbox" ascii wide nocase
    condition:
        all of them
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2026-10631 Zimbra EWS FindItem Authorization Bypass Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/EWS/Exchange.asmx"; http_uri;
    content:"FindItem"; http_client_body;
    content:"soap:Envelope"; http_client_body;
    classtype:attempted-user;
    sid:10000001;
    rev:1;
    metadata:created_at 2026_01_01, cve CVE-2026-10631, confidence High;
)

Elastic Detection

sequence by user.name with maxspan=5m
  [ network where network.protocol == "http" and
    http.request.method == "POST" and
    url.path == "/EWS/Exchange.asmx" and
    http.request.body contains "FindItem" and
    http.request.body contains "soap:Envelope" ]
  [ network where network.protocol == "http" and
    http.response.status_code == 200 and
    url.path == "/EWS/Exchange.asmx" and
    http.response.body contains "MIME-Version" ]

Splunk SPL Query

index=web sourcetype=zimbra_access
| search method=POST uri_path="/EWS/Exchange.asmx" uri_query="*FindItem*"
| stats count by src_ip, user, uri_query
| where count > 5
| sort - count

Wazuh OSSEC Rule

<group name="zimbra,web,">
  <rule id="100001" level="10">
    <decoded_as>web-accesslog</decoded_as>
    <field name="method">POST</field>
    <field name="url">/EWS/Exchange.asmx</field>
    <field name="url">FindItem</field>
    <description>CVE-2026-10631: Possible Zimbra EWS FindItem authorization bypass attempt</description>
    <mitre>
      <id>T1213</id>
    </mitre>
    <group>cve_2026_10631,</group>
  </rule>
</group>

Mitigation & Patch Guidance

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

1. Immediate Mitigations:

• Disable EWS for all users who do not explicitly require it. This reduces the attack surface significantly, as the vulnerability requires EWS to be enabled on the attacker's account.
• Restrict network access to the EWS endpoint (/EWS/Exchange.asmx) to trusted IP ranges or internal networks only.
• Monitor EWS logs for unusual FindItem requests, especially those targeting multiple mailboxes from a single user account.
• Enforce the principle of least privilege: ensure that only necessary accounts have EWS enabled.
• Consider implementing additional authentication controls (e.g., MFA) for all accounts, though this does not fix the bypass itself.

2. Patch/Upgrade Steps:

• No official patch is available as of this report. Monitor Zimbra's official security advisories and release notes for version 10.1.20 or later.
• Once a patched version is released, upgrade immediately. Test in a staging environment before production deployment.
• Subscribe to Zimbra security announcements to receive timely notifications.

3. Long-Term Hardening:

• Conduct a thorough review of authorization logic in all EWS handlers, not just FindItem.
• Implement robust access control checks that validate user permissions against the target mailbox for every mailbox operation.
• Adopt a zero-trust architecture for internal mail access, requiring explicit grants for cross-mailbox operations.
• Regularly perform security audits and penetration testing of Zimbra deployments.
• Keep Zimbra Collaboration Suite up to date with the latest security patches.

References

• NVD - CVE-2026-10631
• Zimbra Security Advisories (monitor for updates; no specific advisory for this CVE yet)
• MITRE CWE-639: Authorization Bypass Through User-Controlled Key
• CISA Known Exploited Vulnerabilities Catalog (CVE not listed as of this report)
• FIRST EPSS (not yet scored for this CVE)

No known threat actor campaigns or exploit-in-the-wild evidence is available at this time.