---
id: "CVE-2026-93631"
kind: "vulnerability"
aliases: ["CVE-2026-93631"]
title: "Zimbra Collaboration Suite: forged OAuth callback links attacker account to victim mailbox"
severity: "high"
cvss: 7.1
epss: 0.00177
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["calendar", "email", "package", "self-hosted", "zimbra"]
not_affected_if: ["default-config", "no-oauth-provider"]
published_at: "2026-10-23T14:17:57+00:00"
first_seen_at: "2026-10-23T21:57:34+00:00"
updated_at: "2026-10-25T12:38:49+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-93631", "first_seen_at": "2026-10-23T21:57:34+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-93631", "first_seen_at": "2026-10-25T12:38:49+00:00"}]
---

# Zimbra Collaboration Suite: forged OAuth callback links attacker account to victim mailbox

A cross-site request forgery flaw in the OAuth extension of Zimbra Collaboration Suite Network Edition lets an attacker attach an external account they control to a signed-in user's mailbox. The attacker then imports contacts and events from that account and receives calendar appointments through bidirectional CalDAV sync.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Confirm the deployment runs Zimbra Collaboration Suite Network Edition 10.1.x with the OAuth extension and an external OAuth provider configured
- Check the installed version against `10.1.16` and `10.1.20`, which the sources list as affected
- Check whether bidirectional CalDAV synchronization is enabled for linked calendars

## Fix

- Upgrade Zimbra Collaboration Suite Network Edition to a release that fixes CVE-2026-93631 (fixed version unknown from the sources)
- Remove or disable the OAuth external provider configuration until patched if the feature is not needed
- Review existing external data sources and linked calendars for accounts the users did not add

## Timeline

- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-93631
- 2026-10-25T12:38:49+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-93631

## From the sources

A cross-site request forgery vulnerability in the OAuth extension of Zimbra Collaboration Suite (ZCS) Network Edition allows an attacker to link an attacker-controlled external account to a victim's mailbox. The OAuth callback does not bind its state value or authorization code to the initiating Zimbra session and instead selects the destination mailbox from the browser's current authentication cookie. On a deployment with the provider configured, an attacker can induce a signed-in victim to load a callback containing a fresh code for the attacker's provider account. This creates a persistent data source that imports attacker-controlled contacts or events. With bidirectional CalDAV synchronization, appointments subsequently created or edited in the linked calendar are also disclosed to the attacker. ZCS 10.1.16 and 10.1.20 are affected.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_