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  5. Bucket Policy

Bucket Policy

Bucket policies provide centralized, access control to buckets and objects based on a variety of conditions, including OBS operations, requesters, resources, and aspects of the request (e.g., IP address). The permissions attached to a bucket apply to all of the objects in that bucket.

Individuals as well as companies can use bucket policies. When companies register with OBS they create an account. Thereafter, the company becomes synonymous with the account. Accounts are financially responsible for the resources they (and their employees) create. Accounts have the power to grant bucket policy permissions and assign employees permissions based on a variety of conditions. For example, an account could create a policy that gives a user write access:

To a particular bucket

From an account's corporate network

From an account's custom application

Unlike access control lists (ACL), which can add (grant) permissions only on individual objects, policies can either add or deny permissions across all (or a subset) of objects within a bucket. With one request an account can set the permissions of any number of objects in a bucket. An account can use wildcards (similar to regular expression operators) on Amazon resource names (ARNs) and other values, so that an account can control access to groups of objects.

A bucket owner can perform the PUT Bucket policy operation to set a bucket access policy. However, a new policy will overwrite the existing one. A bucket owner can also perform the Get Bucket policy or DELETE Bucket policy operation to obtain or delete a bucket policy. After a policy is set for a bucket, all subsequent access to the bucket is controlled by the policy. Requests for accessing the bucket are accepted or denied according to the policy.

In the following example bucket policy, accounts 783fc6652cf246c096ea836694f71855 (Domain ID) and 219d520ceac84c5a98b237431a2cf4c2 (Domain ID) are granted the GetObject permission for all objects in mybucket.

{ 
   "Version":"2008-10-17", 
   "Id":"aaaa-bbbb-cccc-dddd", 
   "Statement" : [ 
      { 
       "Effect":"Allow", 
       "Sid":"1",  
       "Principal" : { 
         "AWS":["arn:aws:iam::783fc6652cf246c096ea836694f71855:root","arn:aws:iam::219d520ceac84c5a98b237431a2cf4c2:root"] 
       }, 
       "Action":["s3:GetObject"], 
       "Resource":"arn:aws:s3:::mybucket/*" 
      } 
   ]  
 }

Policy Format

A policy is described in the JSON format, as shown in the following syntax:

{ 
 "Version" : "version", 
 "Id" : "id", 
 "Statement" : [statement] 
 }

The Id and Version parameters are optional.

Table 1 Policy

Element

Description

Required

Id

Indicates the ID of a policy. The value is a string that describes the policy.

No

Version

Possible values are 2008-10-17.

No

A policy can contain multiple statements and each statement contains the following elements:

Table 2 statement elements

Element

Description

Required

Sid

Indicates the ID of a statement. The value is a string that describes the statement.

No

Principal

Indicates the grantee of a statement. The value can be a wildcard character (*) that indicates all users. When the grantee is a domain, Principal is in the AWS:domainidAWS:arn:aws:iam::domainid:root, or CanonicalUser: format. When the grantee is a user, Principal is in the AWS:arn:aws:iam::domainid:user/userId or AWS:arn:aws:iam::domainid:user/userName format.

A statement grantee can specify whether the statement grantors are agency users or federated users. For agency users, the principal format is AWS:arn:aws:iam::domainid:agency/agencyName. For federated users, the principal format is Federated:arn:aws:iam::domainid:identity-provider/providername or Federated:arn:aws:iam::domainid:group/groupname.

No, Choose either Principal or NotPrincipal

NotPrincipal

Specifies an exception to a list of principals in the statement. You can deny access to all principals except the one named in the NotPrincipal element. The value of this element is similar to that of Principal.

No, Choose either Principal or NotPrincipal

Action

Specifies operations that a grantee can perform. The value is a case-insensitive string consisting of a set of operations supported by OBS.

The value can be a wildcard character (*) that indicates all operations.

Example:

"Action":["s3:List*", "s3:Get*"]

No, Choose either Action or NotAction

NotAction

Specifies an exception to a list of actions in the statement. All actions are performed except the one specified in NotAction. The value of this element is similar to Action.

No, Choose either Action or NotAction

Effect

Indicates whether permission in a statement is allowed or denied. The value is Allow or Deny.

Yes

Resource

Specifies resources that the statement covers. A resource is defined in the Amazon Resource Name (ARN) format. You can use a wildcard character (*) to represent all resources.

No, Choose either Resource or NotResource

NotResource

Specifies an exception to a list of resources in the statement. A policy is not applied to resources specified in NotResource. The value of this parameter is similar to that of Resource.

No, Choose either Resource or NotResource

Condition

Indicates the conditions for a statement to take effect.

No

NOTE:

A statement contains either:

  • Action or NotAction
  • Resource or NotResource
  • Principal or NotPrincipal

In OBS, Action can be the following operations on buckets:

  • s3:DeleteBucket
  • s3:ListBucket
  • s3:ListBucketVersions
  • s3:ListBucketMultipartUploads
  • s3:GetBucketAcl
  • s3:PutBucketAcl
  • s3:GetBucketCORS
  • s3:PutBucketCORS
  • s3:GetBucketVersioning
  • s3:PutBucketVersioning
  • s3:GetBucketLocation
  • s3:GetBucketLogging
  • s3:PutBucketLogging
  • s3:GetBucketWebsite
  • s3:PutBucketWebsite
  • s3:DeleteBucketWebsite
  • s3:GetLifecycleConfiguration
  • s3:PutLifecycleConfiguration
  • s3:GetBucketNotification
  • s3:PutBucketNotification
  • s3:PutBucketPolicy
  • s3:GetBucketPolicy
  • s3:DeleteBucketPolicy
  • s3:PutBucketQuota
  • s3:GetBucketQuota
  • s3:PutBucketStoragePolicy
  • s3:GetBucketStoragePolicy
  • s3:GetBucketStorage
  • s3:PutBucketTagging
  • s3:GetBucketTagging

In OBS, Action can be the following operations on objects:

  • s3:GetObject (applies to GET Object and HEAD Object)
  • s3:GetObjectVersion
  • s3:PutObject (applies to PUT Object, POST Object, Initiate Multipart Upload, Upload Part, and Complete Multipart Upload)
  • s3:GetObjectAcl
  • s3:GetObjectVersionAcl
  • s3:PutObjectAcl
  • s3:PutObjectVersionAcl
  • s3:DeleteObject
  • s3:DeleteObjectVersion
  • s3:ListMultipartUploadParts
  • s3:AbortMultipartUpload
  • s3:RestoreObject

OBS supports S3 resources in the ARN format:

  • arn:aws:s3:::bucketname (operations on buckets)
  • arn:aws:s3:::bucketname/path/objectname (operations on objects)

The following policy grants all permissions (including bucket and object operations) for bucket examplebucket to 71f3901173514e6988115ea2c26d1999 (user ID) in b4bf1b36d9ca43d984fbcb9491b6fce9 (domain ID).

{
    "Statement":[
    {
      "Sid":"test",
      "Effect":"Allow",
      "Principal": {"AWS": ["arn:aws:iam::b4bf1b36d9ca43d984fbcb9491b6fce9:user/71f3901173514e6988115ea2c26d1999"]},
      "Action":["s3:*"],
      "Resource":[
        "arn:aws:s3:::examplebucket/*",
        "arn:aws:s3:::examplebucket"
      ]
    }
  ]
}

or

{ 
    "Statement":[ 
    { 
      "Sid":"test", 
      "Effect":"Allow", 
      "Principal": {"AWS": ["arn:aws:iam::b4bf1b36d9ca43d984fbcb9491b6fce9:user/user1"]}, 
      "Action":["s3:*"], 
      "Resource":[ 
        "arn:aws:s3:::examplebucket/*", 
        "arn:aws:s3:::examplebucket" 
      ] 
    } 
  ] 
}
NOTE:

If you do not specify a path when uploading an object, omit /path in the ARN.

Table 3 lists the general types of Condition that you can specify.

Table 3 Condition

Type

Element

Description

String

StringEquals

Strict matching

Short version: streq

StringNotEquals

Strict negated matching

Short version: strneq

StringEqualsIgnoreCase

Strict matching, ignoring case

Short version: streqi

StringNotEqualsIgnoreCase

Strict negated matching, ignoring case

Short version: strneqi

StringLike

Loose case-insensitive matching. The values can include a multi-character match wildcard (*) or a single-character match wildcard (?) anywhere in the string.

Short version: strl

StringNotLike

Negated loose case-insensitive matching. The values can include a multi-character match wildcard (*) or a single-character match wildcard (?) anywhere in the string.

Short version: strnl

Numeric

NumericEquals

Strict matching

Short version: numeq

NumericNotEquals

Strict negated matching

Short version: numneq

NumericLessThan

"Less than" matching

Short version: numlt

NumericLessThanEquals

"Less than or equals" matching

Short version: numlteq

NumericGreaterThan

"Greater than" matching

Short version: numgt

NumericGreaterThanEquals

"Greater than or equals" matching

Short version: numgteq

Date

DateEquals

Strict matching

Short version: dateeq

DateNotEquals

Strict negated matching

Short version: dateneq

DateLessThan

A point in time at which a key stops taking effect

Short version: datelt

DateLessThanEquals

A point in time at which a key stops taking effect

Short version: datelteq

DateGreaterThan

A point in time at which a key starts to take effect

Short version: dategt

DateGreaterThanEquals

A point in time at which a key starts to take effect

Short version: dategteq

Boolean

Bool

Strict Boolean matching

IP address

IpAddress

Approved based on the IP address or range

NotIpAddress

Denial based on the IP address or range

NOTE:

Elements in Condition are case-sensitive. Date conditions must be in the ISO 8601 format. For details, see http://www.w3.org/TR/NOTE-datetime.

A Condition block (element) can contain multiple key value pairs. The following example Condition block specifies requests initiated between 2009-04-16T12:00:00Z and 2009-04-16T15:00:00Z from IP addresses on network segment 192.168.176.0/24 or 192.168.143.0/24:

"Condition" : { 
   "DateGreaterThan" : { 
   "aws:CurrentTime" : "2009-04-16T12:00:00Z" 
   },
   "DateLessThan": { 
   "aws:CurrentTime" : "2009-04-16T15:00:00Z" 
   },
   "IpAddress" : { 
   "aws:SourceIp" : ["192.168.176.0/24","192.168.143.0/24"] 
   } 
 }

A Condition block can contain two types of keys:

  • General keys that have nothing to do with Action.
  • S3 service-specific keys associated with Action.

Table 4 lists the general keys.

Table 4 Common Condition Key

Condition Key

Condition Type

aws:CurrentTime

Date

aws:EpochTime

Numeric

aws:SecureTransport

Boolean

aws:SourceIp

IP address

aws:UserAgent

String

aws:Referer

String

Table 5 lists the OBS service-specific keys.

Table 5 OBS Action Condition Key

Action

Key

Description

s3:CreateBucket

s3:x-amz-acl

x-amz-acl can contain the canned ACL.

Valid values: private| public-read| public-read-write|authenticated-read|bucket-owner-read|bucket-owner-full-control|log-delivery-write

s3:x-amz-grant-permission

Not supported

s3:LocationConstraint

Not supported

s3:ListBucket

s3:prefix

String

s3:delimiter

String

s3:max-keys

Numeric

s3:ListBucketVersions

s3:prefix

String

s3:delimiter

String

s3:max-keys

Numeric

s3:PutBucketAcl

s3:x-amz-acl

x-amz-acl can contain the canned ACL.

Valid values: private| public-read| public-read-write|authenticated-read|bucket-owner-read|bucket-owner-full-control|log-delivery-write

s3:x-amz-grant-permission

Not supported

s3:PutObject

s3:x-amz-acl

x-amz-acl can contain the canned ACL.

Valid values: private| public-read| public-read-write|authenticated-read|bucket-owner-read|bucket-owner-full-control|log-delivery-write

s3:x-amz-copy-source

String

Example format:

/bucketname/keyname

s3:x-amz-metadata-di

rective

Valid values: COPY| REPLACE

s3:x-amz-grant-permission

Not supported

s3:x-amz-storage-class

Not supported

s3:PutObjectAcl

s3:x-amz-acl

x-amz-acl can contain the canned ACL.

Valid values: private| public-read| public-read-write|authenticated-read|bucket-owner-read|bucket-owner-full-control|log-delivery-write

s3:x-amz-grant-permission

Not supported

s3:GetObjectVersion

s3:VersionId

String

s3:GetObjectVersionAcl

s3:VersionId

String

s3:PutObjectVersionAcl

s3:VersionId

String

s3:x-amz-acl

x-amz-acl can contain the canned ACL.

Valid values: private| public-read| public-read-write|authenticated-read|bucket-owner-read|bucket-owner-full-control|log-delivery-write

s3:x-amz-grant-permission

Not supported

s3:DeleteObjectVersion

s3:VersionId

String

s3:*

(Actions or any of the S3 Actions)

s3:signatureversion

Not supported

s3:authType

Not supported

s3:signatureAge

Not supported

s3:x-amz-content-sha256

Not supported

Policy Permission Judgment Logic

A policy results in a default deny if conditions in any statement of the policy are not met. If all conditions in statements are met, the policy results in either an allow or an explicit deny. If a bucket policy contains multiple statements, the policy determines which statement prevails according to the following rules:

1. If conditions in any statement of a policy are not met, the policy results in a default deny.

2. An explicit deny overrides allows.

3. An allow overrides default denies.

4. Statements can be in any order in a policy.

Table 6 Statement results

Name

Description

explicit deny

A statement defines effect="deny".

All requests for resources to which the statement applies are denied. No permission is returned.

allow

A statement defines effect="allow".

All requests for resources to which the statement applies are allowed.

default deny

Conditions defined in a statement are not met. Requests are denied.

URL Validation Settings

OBS is charged based on the services that you use. To prevent user data from being stolen, OBS supports URL validation based on HTTP headers. OBS also supports both whitelist and blacklist settings.

  • Whitelist settings

    Users can set a whitelist to allow requests from the websites added in the whitelist and deny requests from any other website.

    For the requests that are initialized from browsers' address boxes, that is, those HTTP requests with a blank referer, users can add the ${null} field to "aws:Referer" of Condition to specify whether to allow the requests with a blank referer.

    Set a whitelist based on the following policy setting:

    "Statement":[
        {"Sid": "1",
         "Effect": "Allow",
         "Principal": {"CanonicalUser":["*"]},
         "Action": "s3:*",
         "Resource":["arn:aws:s3:::bucket/*"],
        },
        {"Sid":"2",
         "Effect":"Deny",
         "Principal":{"CanonicalUser":["*"]},
         "Action":["s3:*"],
         "Resource":["arn:aws:s3:::bucket/*"],                             "Condition":{
             "StringNotEquals":
             {"aws:Referer":["www.example01.com","${null}"]}
          }
        }
    ]

    If you set a whitelist in this way, you can perform operations on resources in buckets only when the value of the referer parameter is www.example01.com or is blank.

  • Blacklist settings

    You can refer to the following policy settings to set a blacklist for access.

    "Statement":[ 
         {"Sid":"1", 
          "Effect":"Deny", 
          "Principal":{"CanonicalUser":["*"]}, 
          "Action":["s3:*"], 
          "Resource":["arn:aws:s3:::bucket/*"],                             "Condition":{ 
              "StringEquals": 
              {"aws:Referer":["www.example01.com","www.example02.com"]} 
           } 
         } 
     ]

    If you set a blacklist in this way, you cannot perform operations on resources in buckets when the value of the referer parameter is www.example01.com or www.example02.com.