mod_http_upload_external

Introduction

This module implements XEP-0363, which lets clients upload files over HTTP to an external web server.

This module generates URLs that are signed using a HMAC. Any web service that can authenticate these URLs can be used.

Implementations

To implement your own service compatible with this module, check out the implementation notes below (and if you publish your implementation - let us know!).

Configuration

Add "http_upload_external" to modules_enabled in your global section, or under the host(s) you wish to use it on.

External URL

You need to provide the path to the external service. Ensure it ends with ‘/’.

For example, to use the PHP implementation linked above, you might set it to:

Secret

Set a long and unpredictable string as your secret. This is so the upload service can verify that the upload comes from mod_http_upload_external, and random strangers can’t upload to your server.

You need to set exactly the same secret string in your external service.

Limits

A maximum file size can be set by:

Default is 100MB (100*1024*1024).

Compatibility

Works with Prosody 0.9.x and later.

Implementation

To implement your own external service that is compatible with this module, you need to expose a simple API that allows the HTTP GET, HEAD and PUT methods on arbitrary URLs located on your service.

For example, if http_upload_external_base_url is set to https://example.com/upload/ then your service might receive the following requests:

Upload a new file:

PUT https://example.com/upload/foo/bar.jpg?v=49e9309ff543ace93d25be90635ba8e9965c4f23fc885b2d86c947a5d59e55b2

Recipient checks the file size and other headers:

HEAD https://example.com/upload/foo/bar.jpg

Recipient downloads the file:

GET https://example.com/upload/foo/bar.jpg

The only tricky logic is in validation of the PUT request. Firstly, don’t overwrite existing files (return 409 Conflict).

Then you need to validate the auth token.

Validating the auth token

Version Supports
v Validates only filename and size. Does not support file type restrictions by the XMPP server.
v2 Validates the filename, size and MIME type. This allows the server to implement MIME type restrictions.

It is probable that a future v3 will be specified that allows carrying information about the uploader identity, allowing the implementation of per-user quotas and limits.

Implementations may implement one or more versions of the protocol simultaneously. The XMPP server generates the URLs and ultimately selects which version will be used.

XMPP servers MUST only generate URLs with one of the versions listed here. However in case multiple parameters are present, upload services MUST only use the token from the highest parameter version that they support.

Version 1 (v)

The token will be in the URL query parameter ‘v’. If it is absent, fail with 403 Forbidden.

Calculate the expected auth token by reading the value of the Content-Length header of the PUT request. E.g. for a 1MB file will have a Content-Length of ‘1048576’. Append this to the uploaded file name, separated by a space (0x20) character.

For the above example, you would end up with the following string: “foo/bar.jpg 1048576”

The auth token is a SHA256 HMAC of this string, using the configured secret as the key. E.g.

calculated_auth_token = hmac_sha256("foo/bar.jpg 1048576", "secret string")

If this is not equal to the ‘v’ parameter provided in the upload URL, reject the upload with 403 Forbidden.

Security note: When comparing calculated_auth_token with the token provided in the URL, you must use a constant-time string comparison, otherwise an attacker may be able to discover your secret key. Most languages/environments provide such a function, such as hash_equals() in PHP, hmac.compare_digest() in Python, or ConstantTimeCompare() from crypto/subtle in Go.

Version 2 (v2)

The token will be in the URL query parameter ‘v2’. If it is absent, fail with 403 Forbidden.

Input Example Read from
file_path foo/bar.jpg The URL of the PUT request, with the service’s base prefix removed.
content_size 1048576 Content-Size header
content_type image/jpeg Content-Type header

The parameters should be joined into a single string, separated by NUL bytes (\0):

  signed_string = ( file_path + '\0' + content_size + '\0' + content_type )
  signed_string = "foo/bar.jpg\01048576\0image/jpeg"

The expected auth token is the SHA256 HMAC of this string, using the configured secret key as the key. E.g.:

calculated_auth_token = hmac_sha256(signed_string, "secret string")

If this is not equal to the ‘v2’ parameter provided in the upload URL, reject the upload with 403 Forbidden.

Security note: When comparing calculated_auth_token with the token provided in the URL, you must use a constant-time string comparison, otherwise an attacker may be able to discover your secret key. Most languages/environments provide such a function, such as hash_equals() in PHP, hmac.compare_digest() in Python, or ConstantTimeCompare() from crypto/subtle in Go.

Security considerations

HTTPS

All uploads and downloads should only be over HTTPS. The security of the served content is protected only by the uniqueness present in the URLs themselves, and not using HTTPS may leak the URLs and contents to third-parties.

Implementations should consider including HSTS and HPKP headers, with consent of the administrator.

MIME types

If the upload Content-Type header matches any of the following MIME types, it MUST be preserved and included in the Content-Type of any GET requests made to download the file:

It is recommended that other MIME types are preserved, but served with the addition of the following header:

Content-Disposition: attachment

This prevents the browser interpreting scripts and other resources that may potentially be malicious.

Some browsers may also benefit from explicitly telling them not to try guessing the type of a file:

X-Content-Type-Options: nosniff
Security headers

The following headers should be included to provide additional sandboxing of resources, considering the uploaded content is not understood or trusted by the upload service:

Content-Security-Policy: default-src 'none'
X-Content-Security-Policy: default-src 'none'
X-WebKit-CSP: default-src 'none'