Amazon AWS S3 browser-based upload using POST -

I am building a web application that includes a file upload feature. My goal is to initiate upload from users directly to an S3 bucket. The strategy is to pre-sign a POST request that will get submitted as a form.

The roadblock is a SignatureDoesNotMatch error - as far as I can tell I've conformed to the documentation, and have explored a lot of options, but still unable to resolve. I am able to generate presigned download links.

Referencing:

AWS POST documentation

Example

boto3 generate_presigned_post reference

Generate signed request:

def s3_upload_creds(name, user):
    s3 = boto3.client('s3')
    key = '${filename}'
    region = 'us-east-1'
    date_short = datetime.datetime.utcnow().strftime('%Y%m%d')
    date_long = datetime.datetime.utcnow().strftime('%Y%m%dT000000Z')
    fields = { 
        'acl': 'private',
        'date': date_short,
        'region': region,
        'x-amz-algorithm': 'AWS4-HMAC-SHA256',
        'x-amz-date': date_long
    }

    return s3.generate_presigned_post(
        Bucket = 'leasy',
        Fields = fields,
        Key = key,
        Conditions = [
            {'acl': 'private'},
            {'x-amz-algorithm': 'AWS4-HMAC-SHA256'},
            {'x-amz-credential': '/'.join(['AKI--snip--', date_short, region, 's3', 'aws4_request'])},
            {'x-amz-date': date_long}
        ]
    )

Upload form (populated with fields above):

<html>
  <head>

    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />

  </head>
  <body>
  {{ creds }}
  <form action="{{ creds.url }}" method="post" enctype="multipart/form-data">
    Key to upload: 
    <input type="input"  name="key" value="${filename}" /><br />
    <input type="input"  name="acl" value="{{ creds.fields.acl }}" />
    <input type="hidden" name="Policy" value="{{ creds.fields.policy }}" />
    <input type="text"   name="X-Amz-Algorithm" value="{{ creds.fields['x-amz-algorithm'] }}" />
    <input type="input"   name="X-Amz-Credential" value="{{ creds.fields.AWSAccessKeyId }}/{{ creds.fields.date }}/us-east-1/s3/aws4_request" />
    <input type="input"   name="X-Amz-Date" value="{{ creds.fields['x-amz-date'] }}" />
    <input type="input" name="X-Amz-Signature" value="{{ creds.fields.signature }}" />
    File: 
    <input type="file"   name="file" /> <br />
    <!-- The elements after this will be ignored -->
    <input type="submit" name="submit" value="Upload to Amazon S3" />
  </form>

</html>

Relevant portion of response:

<Error>
<Code>SignatureDoesNotMatch</Code>
<Message>
The request signature we calculated does not match the signature you provided. Check your key and signing method.
</Message>
<AWSAccessKeyId>AKI--snip--</AWSAccessKeyId>
<StringToSign>
eyJjb25kaXRpb25zIjogW3siYWNsIjogInByaXZhdGUifSwgeyJ4LWFtei1hbGdvcml0aG0iOiAiQVdTNC1ITUFDLVNIQTI1NiJ9LCB7IngtYW16LWNyZWRlbnRpYWwiOiAiQUtJQUlDVjRNVlBUUlFHU1lLV1EvMjAxNTEyMTgvdXMtZWFzdC0xL3MzL2F3czRfcmVxdWVzdCJ9LCB7IngtYW16LWRhdGUiOiAiMjAxNTEyMThUMDAwMDAwWiJ9LCB7ImJ1Y2tldCI6ICJsZWFzeSJ9LCBbInN0YXJ0cy13aXRoIiwgIiRrZXkiLCAiIl1dLCAiZXhwaXJhdGlvbiI6ICIyMDE1LTEyLTE4VDA1OjEwOjU2WiJ9
</StringToSign>
<SignatureProvided>wDOjsBRc0iIW7JNtz/4GHgfvKaU=</SignatureProvided>

Base64 decode of StringToSign in above error:

{u'conditions': [{u'acl': u'private'},
                 {u'x-amz-algorithm': u'AWS4-HMAC-SHA256'},
                 {u'x-amz-credential': u'AKI--snip--/20151218/us-east-1/s3/aws4_request'},
                 {u'x-amz-date': u'20151218T000000Z'},
                 {u'bucket': u'leasy'},
                 [u'starts-with', u'$key', u'']],
 u'expiration': u'2015-12-18T04:59:32Z'}

Upvotes: 13

Answers (3)

Pranav Nandan

Reputation: 361

In my case, I was generating a form with Base64-encoded.

The problem was due to Firefox inherently encoding the Policy and Security token values into Base64-encoded on top of it.

Thus there was double encoding and therefore the signature did not match as required.

Upvotes: 0

KC54

Reputation: 251

Been a few years since the last response, but I've been stuck on this for the last day or two so I'll share my experience for anyone it may help.

I had been getting the error: "403: The AWS Access Key Id you provided does not exist in our records" when trying to upload to an s3 bucket via my presigned url.

I was able to successfully generate a presigned url similarly to above, using the server-side code:

signed_url_dict = self.s3_client.generate_presigned_post(
                self.bucket_name,
                object_name,
                ExpiresIn=300

This returned a dictionary with the structure:

{
    url: "https://___",
    fields: {
        key: "___",
        AWSAccesKeyId: "___",
        x-amz-security-token: "___",
        policy: "___",
        signature: "___"
    }
}

This lead to the part where things were a little different now in 2019 with the browser-side javascript, where the required form inputs seem to have changed. Instead of setting up the form as OP did, I had to create my form as seen below:

<form action="https://pipeline-poc-ed.s3.amazonaws.com/" method="post" enctype="multipart/form-data" name="upload_form">
            <!-- Copy ALL of the 'fields' key:values returned by S3Client.generate_presigned_post() -->
            <input type="hidden" name="key" value="___" />
            <input type="hidden" name="AWSAccessKeyId" value="___" />
            <input type="hidden" name="policy" value="___"/>
            <input type="hidden" name="signature" value="___" />
            <input type="hidden" name="x-amz-security-token" value="___" />
        File:
            <input type="file"   name="file" /> <br />
            <input type="submit" name="submit" value="Upload to Amazon S3" />
        </form>

My error was that I followed an example in the boto3 1.9.138 docs and left out "x-amz-security-token" on the form, which turned out to be quite necessary. A thoughtless oversight on may part, but hopefully this will help someone else.

EDIT: My results above were based on a N. Virginia Lambda Function. When I ran generate_presigned_post(...) in Ohio (the region containing my bucket), I got results similar to OP:

{
  "url": "https://__",
  "fields": {
    "key": "___",
    "x-amz-algorithm": "___",
    "x-amz-credential": "___",
    "x-amz-date": "___",
    "x-amz-security-token": "___",
    "policy": "___",
    "x-amz-signature": "___"
  }
}

Perhaps the results of the function are region specific?

Upvotes: 3

A---

Reputation: 2593

Found a solution: had to explicitly configure the s3 client to use Amazon's new signature v4. The error occurs since it defaults to an older version, causing the mismatch. Bit of a facepalm - at the time this wasn't written in boto3 docs, although folks at Amazon say it should be soon.

The method is simplified since it now returns exactly the fields required:

def s3_upload_creds(name):
    BUCKET = 'mybucket'
    REGION = 'us-west-1'
    s3 = boto3.client('s3', region_name=REGION, config=Config(signature_version='s3v4'))
    key = '${filename}'
    return s3.generate_presigned_post(
        Bucket = BUCKET,
        Key = key
    )

Which means the form can be easily generated:

<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
  </head>
  <body>
    {{ creds }}
      <form action="https://mybucket.s3.amazonaws.com" method="post" enctype="multipart/form-data">
        {% for key, value in creds.fields.items() %}
          <input type="hidden" name="{{ key }}" value="{{ value }}" />
        {% endfor %}
      File:
      <input type="file"   name="file" /> <br />
    <input type="submit" name="submit" value="Upload to Amazon S3" />
  </form>
</html>

Cheers

Upvotes: 12

Amazon AWS S3 browser-based upload using POST -

Answers (3)

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