Uploading to S3 Through CloudFront Pre-Signed URLs

CloudFront signed URLs let you upload to S3 through a custom domain—useful when direct S3 pre-signed URLs are not an option.

Takahiro Iwasa

2024-07-07

3 min read

CloudFront supports the feature of generating signed URLs. While S3 also offers similar functionality, CloudFront provides the added benefit of enabling uploads through your custom domain, making it especially useful for domain-restricted environments.

Serve private content with signed URLs and signed cookies

Architecture Diagram

Specifying Trusted Signers

To begin, you must create a trusted key group for use as a trusted signer.

Specify signers that can create signed URLs and signed cookies

❗ Important

While you can use your AWS account as a trusted signer, AWS recommends using a key group. Refer to Choose between trusted key groups (recommended) and AWS accounts for details.

Key pairs must adhere to the following requirements:

Type: SSH-2 RSA key pair
Format: Base64-encoded PEM
Key Size: 2048-bit

Use the following commands to create a key pair:

openssl genrsa -out private_key.pem 2048
openssl rsa -pubout -in private_key.pem -out public_key.pem

Building

Pass the public key to the PublicKey parameter (line 5) and use it (line 38).
Ensure the S3 bucket policy allows the s3:PutObject action (line 27).
Use the AllViewerExceptHostHeader origin request policy (line 85).

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AWSTemplateFormatVersion: 2010-09-09
2
Description: Example of CloudFront pre-signed URLs to upload files to S3 Bucket
3

4
Parameters:
5
  PublicKey:
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    Type: String
7

8
Resources:
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  S3Bucket:
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    Type: AWS::S3::Bucket
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    Properties:
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      BucketName: !Sub uploaded-files-${AWS::AccountId}-${AWS::Region}
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  S3BucketPolicy:
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    Type: AWS::S3::BucketPolicy
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    Properties:
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      Bucket: !Ref S3Bucket
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      PolicyDocument:
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        Version: 2008-10-17
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        Id: PolicyForCloudFrontPrivateContent
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        Statement:
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          - Sid: AllowCloudFrontServicePrincipal
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            Effect: Allow
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            Principal:
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              Service: cloudfront.amazonaws.com
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            Action:
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              - s3:PutObject
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            Resource: !Sub ${S3Bucket.Arn}/*
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            Condition:
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              StringEquals:
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                "AWS:SourceArn": !Sub arn:aws:cloudfront::${AWS::AccountId}:distribution/${CloudFrontDistribution}
32

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  CloudFrontPublicKey:
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    Type: AWS::CloudFront::PublicKey
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    Properties:
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      PublicKeyConfig:
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        Name: signer1
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        EncodedKey: !Ref PublicKey
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        CallerReference: cloudfront-caller-reference-example
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  CloudFrontKeyGroup:
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    Type: AWS::CloudFront::KeyGroup
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    Properties:
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      KeyGroupConfig:
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        Name: cloudfront-key-group-1
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        Items:
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          - !Ref CloudFrontPublicKey
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  CloudFrontOriginAccessControl:
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    Type: AWS::CloudFront::OriginAccessControl
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    Properties:
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      OriginAccessControlConfig:
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        Name: !Ref S3Bucket
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        OriginAccessControlOriginType: s3
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        SigningBehavior: always
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        SigningProtocol: sigv4
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  CloudFrontDistribution:
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    Type: AWS::CloudFront::Distribution
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    Properties:
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      DistributionConfig:
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        Enabled: true
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        HttpVersion: http2and3
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        Origins:
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          - Id: !GetAtt S3Bucket.DomainName
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            DomainName: !GetAtt S3Bucket.DomainName
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            OriginAccessControlId: !Ref CloudFrontOriginAccessControl
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            S3OriginConfig:
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              OriginAccessIdentity: ''
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        DefaultCacheBehavior:
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          AllowedMethods:
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            - HEAD
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            - DELETE
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            - POST
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            - GET
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            - OPTIONS
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            - PUT
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            - PATCH
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          Compress: true
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          # CachingDisabled
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          # See https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/using-managed-cache-policies.html#managed-cache-policy-caching-disabled
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          CachePolicyId: 4135ea2d-6df8-44a3-9df3-4b5a84be39ad
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          # AllViewerExceptHostHeader
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          # https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/using-managed-origin-request-policies.html#managed-origin-request-policy-all-viewer-except-host-header
85
          OriginRequestPolicyId: b689b0a8-53d0-40ab-baf2-68738e2966ac
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          TargetOriginId: !GetAtt S3Bucket.DomainName
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          TrustedKeyGroups:
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            - !Ref CloudFrontKeyGroup
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          ViewerProtocolPolicy: https-only
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Outputs:
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  CloudFrontDistributionDomainName:
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    Value: !GetAtt CloudFrontDistribution.DomainName
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  CloudFrontPublicKeyId:
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    Value: !Ref CloudFrontPublicKey
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  S3BucketName:
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    Value: !Ref S3Bucket

Deploy the CloudFormation stack with the following command:

PUBLIC_KEY=$(cat public_key.pem)
aws cloudformation deploy \
  --template-file template.yaml \
  --stack-name cloudfront-presigned-urls-example \
  --parameter-overrides PublicKey=$PUBLIC_KEY

Check the deployed resources:

aws cloudformation describe-stacks \
  --stack-name cloudfront-presigned-urls-example \
| jq ".Stacks[0].Outputs"

Testing

Set the following variables:

CLOUDFRONT_DOMAIN=<CloudFront domain>
KEYPAIR_ID=<Key pair ID>
UTC_OFFSET=+9

Generate a URL:

PRESIGNED_URL=$(aws cloudfront sign \
  --url https://$CLOUDFRONT_DOMAIN/upload-test.txt \
  --key-pair-id $KEYPAIR_ID \
  --private-key file://private_key.pem \
  --date-less-than $(date -v +5M "+%Y-%m-%dT%H:%M:%S$UTC_OFFSET"))

echo $PRESIGNED_URL
# https://<distribution-id>.cloudfront.net/upload-test.txt?Expires=...&Signature=...Key-Pair-Id=...

Upload a file:

echo 'Hello World' > example.txt
curl -X PUT -d "$(cat example.txt)" $PRESIGNED_URL

Confirm the file is uploaded:

aws s3 cp s3://uploaded-files-<AWS::AccountId>-<AWS::Region>/upload-test.txt ./
cat ./upload-test.txt

Cleaning Up

Clean up all the AWS resources provisioned during this example with the following command:

ℹ️ Note

Disabling the CloudFront distribution may take several minutes.

aws s3 rm s3://uploaded-files-<AWS::AccountId>-<AWS::Region>/upload-test.txt
aws cloudformation delete-stack --stack-name cloudfront-presigned-urls-example