Flaws in Zimbra could allow to takeover webmail server of a targeted organization

Researchers discovered flaws in Zimbra email collaboration software that could allow attackers to compromise email accounts by sending a malicious email.

Cybersecurity researchers have discovered multiple security vulnerabilities, tracked as CVE-2021-35208 and CVE-2021-35208, in Zimbra email collaboration software.

An unauthenticated attacker could chain these vulnerabilities to fully takeover a Zimbra webmail server of a targeted organization. An attacker could trigger the flaws to potentially compromise email accounts by sending a malicious message. The vulnerabilities were discovered by SonarSource vulnerability researcher, Simon Scannell.

The impact of the flaws could be severe because Zimbra is used by over 200,000 businesses and over a thousand government & financial institutions. 

The CVE-2021-35208 flaw is stored XSS issue that resides in the ZmMailMsgView.js of the Calendar Invite component in Zimbra Collaboration Suite 8.8.x before 8.8.15 Patch 23, it was rated with a CVSS score of 5.4.

“The first vulnerability is a Cross-Site Scripting bug (CVE-2021-35208) that can be triggered in a victim’s browser when viewing an incoming email. The malicious email would contain a crafted JavaScript payload that, when executed, would provide an attacker with access to all emails of the victim, as well as to their webmail session.” reads the post published by Sonarsource. “With this, other features of Zimbra could be accessed and further attacks could be launched.”

“An attacker could place HTML containing executable JavaScript inside element attributes. This markup becomes unescaped, causing arbitrary markup to be injected into the document.” reads the description for the vulnerability.

The second flaw, tracked as CVE-2021-35209 (CVSS score: 6.1) is a proxy Servlet Open Redirect vulnerability that resides in ProxyServlet.java in the /proxy servlet in Zimbra Collaboration Suite 8.8 before 8.8.15 Patch 23 and 9.x before 9.0.0 Patch 16.

“The second vulnerability is an interesting bypass of an allow-list that leads to a powerful Server-Side Request Forgery vulnerability (CVE-2021-35209). It can be exploited by an authenticated member of an organization with any permission role, which means that it can be combined with the first vulnerability. A remote attacker is then able to extract, for example, Google Cloud API Tokens or AWS IAM credentials from instances within the cloud infrastructure.”states Sonarsource.

“The value of the X-Host header overwrites the value of the Host header in proxied requests. The value of X-Host header is not checked against the whitelist of hosts Zimbra is allowed to proxy to (the zimbraProxyAllowedDomains setting).” reads the advisory for this issue.

The vulnerabilities have been addressed with the release of Zimbra versions 8.8.15 Patch 23 and 9.0.0 Patch 16.

“A combination of these vulnerabilities could enable an unauthenticated attacker to compromise a complete Zimbra webmail server of a targeted organization,” said Scannell. “As a result, an attacker would gain unrestricted access to all sent and received emails of all employees.”

Zimbra also published an alert to warn its customers about these vulnerabilities.

Below the timeline for these flaws:

Date	Action
2021-05-19	We reached out to the Zimbra Security team and exchanged PGP keys
2021-05-19	The vendor responded with a PGP key
2021-05-20	We sent the vendor an advisory regarding the SSRF vulnerability
2021-05-22	We sent the vendor an advisory regarding the XSS vulnerability
2021-05-24	The vendor confirmed receipt of the details
2021-06-28	Zimbra released patches for both vulnerabilities

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Pierluigi Paganini

(SecurityAffairs – hacking)

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