We are using Amazon S3 for images on our website and users upload the images/files directly to S3 through our website. In our policy file we ensure it "begins-with" "upload/". Anyone is able to see the full urls of these images since they are publicly readable images after they are uploaded. Could a hacker come in and use the policy data in the javascript and the url of the image to overwrite these images with their data? I see no way to prevent overwrites after uploading once. The only solution I've seen is to copy/rename the file to a folder that is not publicly writeable but that requires downloading the image then uploading it again to S3 (since Amazon can't really rename in place)
If I understood you correctly The images are uploaded to Amazon S3 storage via your server application.
So the Amazon S3 write permission has only your application. Clients can upload images only throw your application (which will store them on S3). Hacker can only force your application to upload image with same name and rewrite the original one.
How do you handle the situation when user upload a image with a name that already exists in your S3 storage?
Consider following actions:
First user upload a image some-name.jpg
Your app stores that image in S3 under name upload-some-name.jpg
Second user upload a image some-name.jpg
Will your application overwrite the original one stored in S3?
I think the question implies the content goes directly through to S3 from the browser, using a policy file supplied by the server. If that policy file has set an expiration, for example, one day in the future, then the policy becomes invalid after that. Additionally, you can set a starts-with condition on the writeable path.
So the only way a hacker could use your policy files to maliciously overwrite files is to get a new policy file, and then overwrite files only in the path specified. But by that point, you will have had the chance to refuse to provide the policy file, since I assume that is something that happens after authenticating your users.
So in short, I don't see a danger here if you are handing out properly constructed policy files and authenticating users before doing so. No need for making copies of stuff.
actually S3 does have a copy feature that works great
Copying Amazon S3 Objects
but as amra stated above, doubling your space by copying sounds inefficient
mybe itll be better to give the object some kind of unique id like a guid and set additional user metadata that begin with "x-amz-meta-" for some more information on the object, like the user that uploaded it, display name, etc...
on the other hand you could always check if the key exists already and prompt for an error
Related
I have a bucket that has multiple users, and would like to pre-sign urls for the client to upload to s3 (some files can be large, so I'd rather they not pass through the Node server. My question is this: Until the mongo database is hit, there is no mongo Object Id to tag as a prefix for the file. (I'm separating the files in this structure: (UserID/PostID/resource) so you can check all of a user's pictures by looking under /UserID, and you can target a specific post by also adding the PostID. Conversely, there is no Object URL until the client uploads the file, so I'm at a bit of an impasse.
Is it bad practice to rename files after they touch the bucket? I just can't pre-know the ObjectID (the post has to be created in Mongo first) - but the user has to select what files they want to upload before the object is created. I was thinking the best flow could be one of two situations:
Client sets files -> Mongo created Document -> Responds to client with ObjectID and pre-signed urls for each file with the key set to /UserID/PostID/name. After successful upload, it re-triggers an update function on the server to edit the urls of the post. after update, send success to client.
Client uploads files to root of bucket -> Mongo doc created where urls of uploaded s3 files are being stored -> iterate over list and prepend the UserID and newly-created PostID, updating mongo document -> success response to client
Is there another approach that I don't know about?
Answering your question:
Is it bad practice to rename files after they touch the server?
If you are planing to use S3 to save your files, there is no server, so there is no problems to change these files after you upload them.
The only thing that you need to understand is renaming a object you need to two requests:
copy the object with a new name
delete the old object with the old name
And this means that maybe can be a problem in costs/latency if you have a huge number of changes (but I can say for most of cases this will not be a problem)
I can say that the first option will be a good option for you, and the only thing that I would change is adding a Serverless processing for your object/files, using the AWS Lambda service will be a good option .
In this case instead of updating the files on the server, you will update using a Lambda function, you only need to add a trigger for your bucket in the PutObject event on S3, this way will can change the name of your files in the best processing time for your client and with low costs.
I'm trying to better understand how the overall flow should work with AWS Lambda and my Web App.
I would like to have the client upload a file to a public bucket (completely bypassing my API resources), with the client UI putting it into a folder for their account based on a GUID. From there, I've got lambda to run when it detects a change to the public bucket, then resizing the file and placing it into the processed bucket.
However, I need to update a row in my RDS Database.
Issue
I'm struggling to understand the best practice to use for identifying the row to update. Should I be uploading another file with the necessary details (where every image upload consists really of two files - an image and a json config)? Should the image be processed, and then the client receives some data and it makes an API request to update the row in the database? What is the right flow for this step?
Thanks.
You should use a pre-signed URL for the upload. This allows your application to put restrictions on the upload, such as file type, directory and size. It means that, when the file is uploaded, you already know who did the upload. It also prevents people from uploading randomly to the bucket, since it does not need to be public.
The upload can then use an Amazon S3 Event to trigger the Lambda function. The filename/location can be used to identify the user, so the database can be updated at the time that the file is processed.
See: Uploading Objects Using Presigned URLs - Amazon Simple Storage Service
I'd avoid uploading a file directly to S3 bypassing the API. Uploading file from your API allows you to control type of file, size etc as well as you will know who exactly is uploading the file (API authid or user id in API body). This is also a security risk to open a bucket to public for writes.
Your API clients can then upload the file via API, which then can store file on S3 (trigger another lambda for processing) and then update your RDS with appropriate meta-data for that user.
The users in my site need to be able to upload a bunch of files and folders into S3 while maintaining the folder structure.
Say they have the following files in their local boxes.
/file1.jpg
/some_folder/file2.jpg
After upload, I need their s3 urls to be
http://s3bucket.amazon.com/usersfolder/file1.jpg
http://s3bucket.amazon.com/usersfolder/some_folder/file2.jpg
How can i do this ? To make matters a little more complicated, Upload from client side can be initiated only after they download an upload policy.
Edit: I would like to know a solution for the front end part of this question. Looks like on server i can use a wildcard character to specify access permissions, so i am good on that part.
I am using Node.JS/Express JS as a backend
I would like to upload images from users and upload to S3. The problem is, that the images should only be display from the user itself and not invoke by other people who know the url.
I think it is possible to return the image from a url in my application an check the session data if the user is valid. Maybe http://img.domain.com/31hh21ej12he/
But is there a way in S3 to do this?
You can generate pre-signed URLs, they will be much harder to "guess" or share and expire after a while. See: http://docs.aws.amazon.com/AmazonS3/latest/dev/ShareObjectPreSignedURLJavaSDK.html
if i upload a file on azure blob in the same container where the file is existing already, it is over-writing the file, how to avoid overwriting the same? below i am mentioning the scenario...
step1 - upload file "abc.jpg" on azure in container called say "filecontainer"
step2 - once it gets uploaded, try uploading some different file with the same name to the same container
Output - it will overwrite existing file with the latest uploaded
My Requirement - i want to avoid this overwrite, as different people may upload files having same name to my container.
Please help
P.S.
-i do not want to create different containers for different users
-i am using REST API with Java
Windows Azure Blob Storage supports conditional headers using which you can prevent overwriting of blobs. You can read more about conditional headers here: http://msdn.microsoft.com/en-us/library/windowsazure/dd179371.aspx.
Since you want that a blob should not be overwritten, you would need to specify If-None-Match conditional header and set it's value to *. This would cause the upload operation to fail with Precondition Failed (412) error.
Other idea would be to check for blob's existence just before uploading (by fetching it's properties) however I would not recommend this approach as it may lead to some concurrency issues.
You have no control over the name your users upload their files with. You, however, have control over the name you store those files with. The standard way is to generate a Guid and name each file accordingly. The chances of conflict is almost zero.
A simple pseudocode looks like this:
//generate a Guid and rename the file the user uploaded with the generated Guid
//store the name of the file in a dbase or what-have-you with the Guid
//upload the file to the blob storage using the name you generated above
Hope that helps.
Let me put it that way:
step one - user X uploads file "abc1.jpg" and you save it io a local folder XYZ
step two - user Y uploads another file with same name "abc1.jpg", and now you save it again in a local folder XYZ
What do you do now?
With this I am illustrating that your question does not relate to Azure in any way!
Just do not rely on original file names when saving files. Where-ever you are saving them. Generate random names (GUIDs for example) and "attach" the original name as meta-data.