Security

heynemann edited this page Dec 7, 2012 · 19 revisions

thumbor's team is very concerned about security and vulnerabilities of the service. Even though the team strives to cover most scenarios, if you find any flaws or vulnerabilities, please contact the team or create an issue.

URL Tampering

Consider the following URL for an image: http://some.server.com/unsafe/300x300/smart/path/to/image.jpg.

Now let's say that some malicious user wants to overload your service. He can easily ask for other sizes in loops or worse, like:

http://some.server.com/unsafe/300x301/smart/path/to/image.jpg
http://some.server.com/unsafe/300x302/smart/path/to/image.jpg
http://some.server.com/unsafe/300x303/smart/path/to/image.jpg
...
http://some.server.com/unsafe/300x9999/smart/path/to/image.jpg
...
http://some.server.com/unsafe/9999x9999/smart/path/to/image.jpg

And that's not even counting varying the available options.

Other than that, the user can ask for images that do not exist, thus forcing us to perform useless http GET operations or filesystem operations.

We classified both scenarios above as URL Tampering.

Stopping Tampering

In order to prevent users from tampering with the URL, thumbor provides a configuration called SECURITY_KEY. This is the key used to generate a hash-based message authentication code.

The process is very straightforward. The web server that has the page using thumbor's image generates an authentication code for the options and image url, using the SECURITY_KEY.

When end-users access the page and thus load the image, thumbor generates an authentication code for the same options and image url, using the same SECURITY_KEY. If both authentication codes match, thumbor processes it.

The secure endpoint looks like this: /<authentication code with 28 characters>/300x200/smart/path/to/image.jpg.

HMAC method

We intend to supply toolkits in many languages that automate the signing process, but we might need help from the community in this direction.

thumbor uses standard HMAC with SHA1 signing.

Let's use as an example the url http://some.server.com/unsafe/300x200/smart/path/to/image.jpg.

In order to convert that to a "safe" url, we must sign the part 300x200/smart/path/to/image.jpg:

  1. Generate a signature of that part using HMAC-SHA1 with the SECURITY_KEY.
  2. Encode the signature as base64. thumbor uses urlsafe_b64encode method of the native python's base64 module. This method replaces some characters in the base64 string so it becomes more url friendly.
  3. Append the encoded_signature to the beginning of the URL, like: /1234567890123456789012345678/300x200/smart/path/to/image.jpg.

That last part gives you the new url: http://thumbor-server/1234567890123456789012345678/300x200/smart/path/to/image.jpg. Notice that the url includes the options part 300x200/smart. That's required for thumbor to generate an authentication code to match the one that signs the image (1234567890123456789012345678).

The code included in this documentation is illustrational and should not be used for any purposes.

The description of the base64 method is: reference

base64.urlsafe_b64encode(s)
Encode string s using a URL-safe alphabet, which substitutes 
- instead of + and _ instead of / in the standard Base64 alphabet. 
The result can still contain =.

The old way

thumbor used to generate URL's differently using AES encryption/decryption. If you need more info on the old way of generating URLs, read the 3.0.0 release changes.

Libraries

There are implementations of url generators in various languages, take a look at the Libraries page to find information about them.

More Information

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