Security flaws in email systems can cause trouble for both businesses and everyday users. In 2023, CVE-2023-26430 exposed a serious risk in SIEVE mail-filter rules that powers the OX App Suite email server. Attackers could exploit this to sneak in forbidden filtering extensions or break individual mail processing for users.

In this post, I’ll break down the vulnerability in plain language, show how it could have been exploited, and explain what’s been done to fix it. You’ll also find original sources and sample code along the way.


> TL;DR:
> If someone got access to your account, they could sneak weird coding characters into your filter settings and mess up your mail. This is now fixed—so keep things updated!

Summary:

Attackers with access to a user’s account could insert (inject) special control characters into their SIEVE mail-filter rules. This could cause two big problems:

1. Let attackers use SIEVE filter features (extensions) that are normally not allowed, or

2. Break filter parsing so badly that user filters stopped working and had to be manually cleaned up.

Disclosure: Feb 2023

Reference:
- NIST NVD Record
- OX App Suite Security Advisory

What’s SIEVE?

SIEVE is a small filtering language for email messages. Programs like Open-Xchange (OX App Suite) use it to manage stuff like auto-replies and spam rules.

Example SIEVE Rule

# Auto-file newsletters
if header :contains "List-Id" "newsletter@example.com" {
    fileinto "Newsletters";
    stop;
}

The Flaw: Control Character Injection

*Control characters* are invisible codes like newline (\n) or carriage return (\r). Hackers love them because they can trick parsers and code.

The bug: When a user updated their mail filter rules through the web or API, it was possible to include such control characters as part of the SIEVE code—*and the server did not clean them out*.

Why Is That Dangerous?

- Bypass SIEVE Extension Restrictions: Certain SIEVE extensions are blocked for regular users—but with a clever injection, a hacker could sneak them in.
- Wreck Filtering Logic: Extra control chars could “break” individual user filters so no mail gets filtered correctly, making clean-up hard.

Exploit Walkthrough (for Awareness)

Note: There are no known public exploits, but here’s how it would have looked.

Assume attacker has basic user access (through phishing or weak password).

Step 1: Craft a Malicious SIEVE Rule

Let’s say the admin has blocked the “vacation” SIEVE extension (auto-replies).

Normal (Rejected) Rule

require ["vacation"];
vacation :days 1 :subject "Out of Office" "I'll reply soon.";

You can sneak in a control char, such as a newline or special byte, like this (in Python)

malicious = 'require ["vacation' + '\x01' + '"];\nvacation :days 1 :subject "Out of Office" "Injected!";'

When the filter system parses that rule, the control char may break the extension check—potentially letting the forbidden “vacation” extension be used.

Attackers with access could POST this broken rule to the filter API using curl or similar

curl -X POST -H "Authorization: Bearer <token>" \
     -d "script=REPLACE_WITH_MALICIOUS_RULE_HERE" \
     https://mail.example.com/api/mailfilter

Step 3: Abuse or Break

- If successful, attacker could enable advanced SIEVE extensions (like custom redirects, file delivery, or autoresponders), or

Code Reference: Sanitizing Control Characters

The OX developers fixed this by making sure no control characters go into SIEVE handling.

Before

public void createFilter(String script) {
    // Used raw script—risk of control chars
    filterEngine.parse(script);
}

After

public void createFilter(String script) {
    String cleaned = script.replaceAll("[\\p{Cntrl}&&[^\r\n\t]]", "");
    filterEngine.parse(cleaned);
}

*(Example in Java, real implementation may differ; actual OX code is proprietary)*

How Was It Fixed?

- Sanitization: All mail-filter API input is cleaned so no control characters are passed down to the SIEVE subsystem.

Update: All supported versions of OX App Suite have the patch.

If you run OX App Suite:
> Update your mail server!
> Get the latest release from the OX download portal.

References

- CVE-2023-26430 on NVD
- OX Security Advisory 2023-03
- Intro to SIEVE Language (RFC 5228)

For email admins, reviewing filter API and logs for strange SIEVE rules is recommended after this fix.


Stay safe out there!
If you have any questions about SIEVE, email security, or this vulnerability, let me know in the comments. For urgent support, always contact your vendor directly.

Timeline

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