I am in a situation where I have few terabytes of data on azure blobs. I have recently added SAS (shared access signature) functionality while returning the direct blob urls back to client and added an expiry date of 2 minutes for all the images coming out. So, now the resource is guaranteed the read access for 2 minutes and expires after it.
All good up to now. I also have social media buttons to share my content to facebook, google+ or twitter etc. When a user shares the content to facebook lets say, the thumbnail remains with the link for specified time but expires after that time giving a bad user experience.
The example of URL generated is
https://storageaccount.blob.core.windows.net/images/2016-May-23-03-28-56_14.jpg?sv=2015-07-08&sr=c&sig=LENHRuvX1Q1d8r2DCJTdMp6fB8mv82DJsMAN8ZAM3C4%3D&se=2016-06-14T22%3A55%3A37Z&sp=r
I want to ask if there is any way out to avoid this situation? I don't have a clue right now.
Related
i have an Azure Blob Storage with blobs that are pdf that are categorized by client number. So for each client, they have multiple pdf reports. I only want the client to be able to access the blobs for their client number. (There are hundreds of clients.)
I've researched, but only see shared access signatures, but this doesn't look like what i need.
There is no user-level blob permissions, other than Shared Access Signatures (and Policies).
It's going to be up to you to manage access to specific user content (and how you manage that is really up to you and your app, and how you manage a user's content metadata).
When providing a link to a user's content: if you assume all content is always private, then simply create an on-demand SAS link when requested. There's no way for the user to modify a SAS link to guess sequential numbers or neighboring blobs, since the SAS is for a specific URL.
As Andrés suggested, you could also use your app to stream blob content, and never worry about SAS. However, you will now be consuming resources of your web app (network, CPU, memory), and this will have an impact on your app's scale requirements. You will no longer be able to offload this to the storage service.
Sounds like you already have the users authenticate, and you know which pdfs belong to them. My suggestion is to add to your current application a simple proxy (for instance if you have an MVC application, you could add a new controller and action method that will retrieve the pdfs on behalf of the user).
This way you don't need to use shared access signature and can keep the blob container private. Your controller/action method would simply use the storage SDK to retrieve the blob. An added bonus is that you could check to make sure that they are requesting their own PDF file and reject the request if they guess the ID of someone else's file.
Project ASP.NET
I will use Azure for my storage. Questions (requirements):
In my project I let my registered users download files. But I don't want the user to share this download link to unregistered people (example : the download link what I gave the registered user shall only be downloadable on their computer) .
I show only for registered users the download link, the registered users can download the files that I gave them
No one can delete my files
Question 1: This is really up to your app but... if you're giving direct links to blobs in Azure Storage, you would need to protect them with a Shared Access Signature (or policy). This way, you can give someone a link that expires (for example, 10 minutes after you issue the link). Then, if someone gives away the link, it won't work for very long. There's no way to limit a link's use to a specific computer. Now, if you simply stream content from blob to your app, and then from your app to the user, you will probably have a bit more control, since you wouldn't really be generating a reusable link. But this will have downsides (such as running all content through your web tier, requiring more resources in your web tier).
Question 2: This is completely up to your app, how you manage assets and present them to a registered user. No way to answer this for you, since we know nothing about your app.
Question 3: Azure Storage is accessible by a secret key, which should stay secret, and only you should ever have access to it (for example, it would be used by your code on the server). As long as you don't publish this key anywhere, then nobody would be able to delete your content.
Hello I have a web page that loads a blob resource using a SAS Policy everytime a hyper link is clicked. Meaning if I click twice or more on the link I will generate two or more different signed URLs to the same blob resource. My question is: is there a way of overwrite or cancel the previously generated SAS policies and keeping only the URL generated when the user clicks last?
Thank you in advance.
Technically it is possible to do so however it is not a recommended approach. Reason being, there can only be 5 access policies on a blob container at any point of time and the process to change access policies would require a round trip to storage (i.e. a network call). Assuming there are 100s of users on your website and all of them accessing the same resource. By changing access policy on the fly would result in errors for some of the users plus because it involves a network call, the overall experience may be degraded.
One thing you could do is keep the SAS expiry time short so that the SAS URL is valid for a short amount of time so that there are less chances of it being misused.
To change the access policy, you would 1st need to fetch the existing access policies on a container. Then you could either update the access policy identifier or remove that access policy + create a new access policy and then save the access policies.
Heres three questions for you!
Is it possible to revoke an active SAS URI without refreshing storage key or using Stored Access Policy?
In my application, all users share the same blob container. Because of this, using stored access policy, (max 5 per container), or refreshing storage key, (will result in ALL SAS URI'S being deleted), is not an option for me.
Is it possible to show custom errors if the SAS URI is incorrect or expired?
This is the default page:
If I let users create their own SAS URI for uploading/downloading, do I need to think about setting restrictions? Can this be abused?
Currently, in my application, there are restrictions on how much you are allowed to upload, but no restrictions on how many SAS URIS you are allowed to create. Users can aquire how many SAS URIS as they like as long as the don't complete their upload or exceed the allowed stored bytes.
How does real filesharing websites deal with this?
How much does a SAS URI cost to create?
Edit - Clarification of question 3.
Before you can upload or download a blob you must first get the SAS URI. I was wondering if it's "expensive" to create a SAS URI. Imagine a user exploiting this, creating a SAS URI over and over again without finishing the upload/download.
I was also wondering how real filesharing websites deal with this. It's easy to store information about how much storage the user is using and with that information put restrictions etc, but... If a user keeps uploading files to 99% and then cancel and restarts again and do the same thing, i imagine it would cost alot for the host
To answer your questions:
No, ad-hoc SAS tokens (i.e. tokens without Storage Access Policy) can't be revoked other than changing the storage key or access policy.
No, at this time it is not possible to customize error message. Standard error returned by storage service will be shown.
You need to provide more details regarding 3. As it stands, I don't think we have enough information to comment.
UPDATE
Regarding your question about how expensive creating a SAS URI is, one thing is that creating a SAS URI does not involve making a REST API call to storage service so there's no storage transaction involved. So from the storage side, there's no cost involved in creating a SAS URI. Assuming your service is a web application, only cost I could think of is user making call to your service to create a SAS URI.
Regarding your comment about how real file sharing websites deal with it, I think unless someone with a file sharing website answers it, it would be purely speculative.
(My Speculative response :)) If I were running a file sharing website, I would not worry too much about this kind of thing simply because folks don't have time to "mess around" with your site/application. It's not that the users would come to your website with an intention of "let's just upload files till the upload is 99%, cancel the upload and do that again" :). But again, it is purely a speculative response :).
I'm developing a filesharing website and I have a couple of questions regarding Windows Azure Shared Access Signatures.
About my website: Registered users can upload, share and store their files using blob storage. The files can be up to 2GB in size so I want the upload and download to be as fast as possible. It's
also important that the administartion cost for me as a host is at its minimum. User stored files must be private.
I'm OK with using SAS URI for uploads, but for downloads I'm abit spooked.
Questions:
1. Users can store files on their account and these files should only be accessed by that user. If I were to use SAS URI download here, the files will always be available with an URI as long as
the URI lives, (doesnt require you to be logged in if you know the URI, you can just download the file). This is quite scary if you want the file to be private. I know the signature in the SAS URI
is "HMAC computed over a string-to-sign and key using the SHA256 algorithm, and then encoded using Base64 encoding", is this safe? Is it acceptable to use SAS URI for downloads even if
the files are private? Should I instead stream the file between the server and website, (this will be much more safe but the speed will suffer and the administration cost will rise).
2. How much slower and how much more will it cost if I stream the downloads between (server, website, user) instead of using SAS, (server directly to user)?
3. If I set the SAS URI expiry time to 1 hour and the download takes longer than 1 hour, will the download cancel if the download started before the expiry time?
4. If my website is registered at x.azurewebsites.net and I'm using a purchased domain so I can access my website at www.x.com, is it possible to make the SAS URI's look somethinglike this:
https://x.com/blobpath instead of https://x.blob.core.windows.net/blobpath, (my guess is no..).
Sorry for the wall of text!
There's nothing that stops someone from sharing a URI, whether with or without a SAS. So from a safety perspective, if you leave the expiry date far-off into the future, the URI will remain accessible with the SAS-encoded URI. From an overall security perspective: Since your blob is private, nobody else will have access to the blob without a SAS-encoded URI. To limit SAS use: If, instead of being issued a long-standing SAS URI, the user visited a web page (or API) to request file access, you could generate a new SAS URI for a smaller time window; at this point, the end user would still be able to direct-access the blob without streaming the content through the VM (this just adds an extra network hop for obtaining the URI, along with whatever is needed to host the web / API server). Also related to security: If you use a stored access policy, you have the ability to modify access after issuing the SAS, rather than embedding start+end time directly into the SAS URI itself (see here info about access policies).
You'll incur the cost of the VM(s) used for fronting the URI requests. Outbound bandwidth costs are the same as using blob access directly: You pay for outbound bandwidth only. Performance will be affected by many things if going through a VM: VM size, VM resource use (e.g. if your VM is running at 100% CPU, you might see performance degredation), number of concurrent accesses, etc.
Yes, if the user hits expiry time, the link is no longer valid.
Yes, you can use a SAS combined with custom domain names used with storage. See here for more information about setting up custom domain names for storage.