A new security vulnerability, identified as CVE-2023-26430, has been discovered that potentially impacts mail servers running the SIEVE mail-filtering extension. Attackers who have access to user accounts can inject arbitrary control characters into SIEVE mail-filter rules, potentially bypassing restrictions and causing disruption to mail-filtering operations. In this post, we will discuss the details of this vulnerability, how it can be exploited, and the efforts that have been made to mitigate it.

The Vulnerability: CVE-2023-26430

The Common Vulnerabilities and Exposures (CVE) system has assigned the ID CVE-2023-26430 to this vulnerability. This issue is particularly serious because attackers with access to user accounts can inject arbitrary control characters into SIEVE mail-filter rules.

Access SIEVE extensions that are not allowed by App Suite, thereby bypassing security restrictions.

- Inject rules that would break per-user filter processing, resulting in a disruption of mail-filtering services and requiring manual cleanup of the affected rules.

The following code snippet illustrates how injecting control characters could be abused

# This is an example of a SIEVE mail-filter rule with injected control characters
require ["fileinto"];
if header :contains "Subject" "[MALICIOUS_CONTROL_CHAR]Sales Report" {fileinto "Reports"; }

Mitigation Efforts

To address this vulnerability, sanitization has been added to all mail-filter APIs. This ensures that control characters are not forwarded to subsystems, reducing the potential for malicious actors to exploit CVE-2023-26430.

Original References

For more information about CVE-2023-26430 and the related mitigation steps, you can refer to the following original references:

- CVE-2023-26430 - National Vulnerability Database
- SIEVE Mail-Filter Rule Exploit - Security Mailing List

Exploit Details

As of now, there are no publicly available exploits known for CVE-2023-26430. However, it is important to stay vigilant and apply necessary security patches and updates to prevent potential exploitation of this vulnerability.

Conclusion

CVE-2023-26430 poses a serious threat to mail servers running the SIEVE mail-filtering extension, as attackers with access to user accounts can inject arbitrary control characters into mail-filter rules and bypass security restrictions. Fortunately, security experts have responded to this vulnerability by adding sanitization to all mail-filter APIs, which helps to prevent the forwarding of control characters to subsystems.

It is essential for system administrators and users to remain aware of this vulnerability and ensure they promptly apply security patches and keep their systems up to date to minimize the risk of exploitation.

Timeline

Published on: 08/02/2023 13:15:00 UTC
Last modified on: 08/08/2023 13:58:00 UTC