Attachment types blocked by the institute's mail server

The institute's mail server blocks several kinds of attachment types to improve security against malware. This page provides an overview over the blocked file types and the reasons for blocking them. Permanent Link:

Handling emails with blocked attachments

The mail server will notify you if

• ... you sent an email with one or more blocked attachments. Emails with blocked attachments will not be forwarded to intended recipients!
• ... you received an email with a blocked attachment. This is the case if
  • ... you were attacked and the mail server protected you.
  • ... someone sent you an outdated or dangerous file type.

Solution:

• Avoid using blocked file types. None of them is irreplaceable!
• If you receive an error message, stating that an email was blocked instead of put into your mailbox, please ask the sender to re-send the email with attachment(s) in a different format.
• .doc should be sent as .docx , .xls should be sent as .xlsx. Conversion can be done by using the "Save as..." function in MS Office. A complete list of critical file formats can be found at #list .

List of blocked file types

This list is updated automatically. It reflects the current state of the mailserver's blocking rule set.

Extention	Explanation
dll	Binary MS Windows library.
doc	Deprecated MS Office document format - superseded by .docx
docm	Document format which contains programs. Please use .docx instead!
dot	Deprecated document template format - superseded by .dotx
dotm	Document template format which contains programs. Please use .dotx instead!
dqy	Security sensitive database query description for MS Excel
exe	Binary MS Windows program
hta	Microsoft specific HTML archive format which might contain programs
iqy	Security sensitive web query description for MS Excel.
ocx	Dangerous deprecated browser extension for MS Internet Explorer.
oqy	Security sensitive database query description for MS Excel
pot	Deprecated Powerpoint presentation template format - superseded by .potx
potm	Powerpoint presentation template format which contains programs. Please use .pptx instead!
ppt	Deprecated Powerpoint presentation format - superseded by .pptx.
pptm	Powerpoint presentation format which contains programs. Please use .pptx instead!
qry	Security sensitive database query description for MS Excel
scr	Specific binary program ("Screensaver") for MS Windows
xls	Deprecated MS Office sheet format - superseded by .xlsx
xlsm	Sheet format which contains programs. Please use .xlsx instead!
xlt	Deprecated MS Office sheet template format - superseded by .xltx
xltm	Sheet template format which contains programs. Please use .xltx instead!

Reasons for blocking

Running custom programs on your computer is the goal of malware authors and attackers. Preventing malicious programs from entering the institute is a prime goal of IT security.

Most of today's malware is transferred via email to potential victims. Since most of the malware hides in a limited set of file types, it's common practice to block these file types whenever there's an alternative available. Keep in mind that this is not a measure to protect the mail server but a mitigation against security weaknesses in applications with MS Office being the most prominent one.

FAQ

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Why can you not simply scan the files?

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• It is scientifically proven to be impossible to test a program automatically for malicious effects. Anti-Virus software is only able to test, if a program looks like a malicious piece of software that was encountered before.
• Ransomware attacks make a lot of money today which means, a lot of money can be invested by bad guys into making brand new programs specifically for one institute that no Anti-Virus software ever encountered before.
• Anti-Virus software is only effective against attackers that have little money.

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Why is .rtf among the blocked file types?

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There is a severe security problem in MS Word which enables attackers to get full control over a your running MS Word if you open a specially crafted .rtf file. There's no patch available at this time.

Please use .odt or .docx instead.

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Why is .doc, .xls and .ppt among the blocked file types?

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.doc and .xls were the default file formats of MS Word and MS Excel several years ago. For good reason they are no longer, as they can contain custom programs that are run when the file is opened.

Current MS Word and MS Excel releases are using .docx and .xlsx file formats which are default for over a decade now. To convert a .doc or .xls file into a recent format, just open it and save it under a new filename with the proper modern file extension.

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The email was sent by someone I trust. What's the danger?

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Here are some examples:

1. Your trusted contact didn't sent the email themselves. The address might be forged or you might misinterpret the origin address (see Groupware: B1, B2, B3).
2. Your trusted contact didn't sent the email themselves because the account was hijacked (e.g. by password phishing) and an attacker sent the email.
3. Your trusted contact forwarded a document that they just received.

Those are not theoretical scenarios - we've already seen all of them at the institute! If this is about an office document (.xls,.doc), please ask the sender to use the "Save as..." function to convert the document to .xlsx or .docx. It's that easy to avoid 99.99% of the possible attacks.

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My mail contained several attachments. Why is my whole mail blocked?

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In German jurisdiction it's quite problematic to tamper with mail or comparable things, e.g. email. We're therefore unable to remove single attachments while letting the rest of the mail pass. It is considered a good solution to block the whole mail and notify both the sender and the receiver about this action.

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What to do, if I need to do complex Excel calculations?

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Formulas in Excel are not a problem and can be used in .xlsx files. However, if VBA is being utilized, you'd have to use an .xlsm file which cannot be transmitted by our mail server. It's perfectly fine to use .xlsm files internally. Even having it stored in a StorageUnified folder and used by multiple users is perfectly fine.

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