CVE-2026-50054: Zimbra Auth Bypass Vulnerability

CVE-2026-50054 Overview

CVE-2026-50054 is an authorization flaw in Zimbra Collaboration Suite's GrantRightsRequest SOAP handler. An attacker with access to an authenticated account can grant another local account the loginAs right. The granted right creates persistent mailbox access and mail-sending authority on the targeted account. This access survives password resets and session expiry, defeating common incident response actions. The weakness is tracked under [CWE-269: Improper Privilege Management].

Critical Impact
An authenticated attacker can establish durable cross-account mailbox access in Zimbra, allowing continued mail reading and sending even after the victim rotates credentials or terminates sessions.

Affected Products

Zimbra Collaboration Suite (versions listed in the vendor advisory)
Deployments exposing the Zimbra SOAP API to authenticated users
Multi-tenant Zimbra environments where accounts share a domain or server

Discovery Timeline

2026-10-08 - CVE CVE-2026-50054 published to NVD
2026-10-08 - Last updated in NVD database

Technical Details for CVE-2026-50054

Vulnerability Analysis

Zimbra's SOAP service exposes GrantRightsRequest to let account owners delegate rights such as sendAs, sendOnBehalfOf, and loginAs. The vulnerable handler does not correctly enforce that the caller owns the target account before persisting a grant for the loginAs right. An attacker who controls one authenticated session can issue a crafted GrantRightsRequest that assigns loginAs on a second account to a grantee of their choice. Because grants are stored as account attributes in the Zimbra LDAP directory, the authorization persists across logout, password changes, and token invalidation. The result is a durable backdoor into the targeted mailbox usable through the standard web client or any IMAP, POP, or SOAP client that honors delegation.

Root Cause

The root cause is improper privilege management in the grant handler. The code path trusts caller-supplied target identifiers instead of verifying that the authenticated principal has administrative authority over the target account. This matches the [CWE-269] pattern of granting privileges without validating the actor's entitlement to confer them.

Attack Vector

Exploitation requires network access to the Zimbra SOAP endpoint and a single set of valid user credentials. The attacker submits a GrantRightsRequest referencing the victim account and specifying the loginAs right for a grantee under their control. No user interaction is required on the victim side. See the Zimbra Security Advisories for proof-of-concept details and affected build numbers.

Detection Methods for CVE-2026-50054

Indicators of Compromise

Unexpected zimbraACE attributes on user accounts containing loginAs grants to unrelated local accounts.
Audit log entries recording GrantRightsRequest SOAP calls where the actor and target account differ and the right is loginAs.
Mail client sessions authenticating to a mailbox using a different user's credentials via delegated access.

Detection Strategies

Enumerate all accounts with zmprov getAccount <user> zimbraACE and flag any loginAs grants not approved by administrators.
Parse mailbox.log and audit.log for GrantRightsRequest operations and correlate the authenticated principal with the targeted account identifier.
Alert on SOAP traffic where the request body contains <grantee> and right="loginAs" originating from non-administrative users.

Monitoring Recommendations

Forward Zimbra audit logs to a centralized SIEM and build a rule for cross-account grant operations.
Baseline legitimate delegation relationships and alert on new loginAs grants outside that baseline.
Track IMAP, POP, and web client logins where the authenticating identity differs from the mailbox owner.

How to Mitigate CVE-2026-50054

Immediate Actions Required

Apply the Zimbra security patch referenced in the Zimbra Security Advisories as soon as it is available for your release train.
Audit every account for unauthorized loginAs grants and revoke them using zmprov revokeRight.
Rotate credentials and invalidate active auth tokens for any account that previously held an unexpected grant.

Patch Information

Zimbra tracks fixes for this issue through the vendor advisory portal. Administrators should consult the Zimbra Security Advisories page for the exact patch level, backport availability, and upgrade instructions corresponding to their Zimbra Collaboration Suite version.

Workarounds

Restrict access to the Zimbra SOAP endpoint using network ACLs so only trusted clients can reach /service/soap.
Disable or heavily restrict user-initiated delegation features until the patch is applied.
Schedule a recurring job that queries zimbraACE attributes and reverts unapproved loginAs grants.

bash

# Audit and revoke unauthorized loginAs grants
zmprov -l getAllAccounts | while read user; do
  zmprov getAccount "$user" zimbraACE | grep -i loginAs \
    && echo "Review account: $user"
done

# Revoke a specific grant
zmprov revokeRight account <victim@example.com> usr <grantee@example.com> loginAs