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Use deny policies rather than conditions to reject insecure access #21

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54 changes: 32 additions & 22 deletions buckup/bucket_creator.py
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Original file line number Diff line number Diff line change
Expand Up @@ -16,17 +16,6 @@

POLICY_NAME_FORMAT = "{bucket_name}-owner-policy"

REQUIRE_HTTPS_CONDITION = {
"Bool": {
# Require HTTPS
"aws:SecureTransport": "true"
},
"NumericGreaterThanEquals": {
# Require TLS >= 1.2
"s3:TlsVersion": ["1.2"]
},
}


class BucketCreator:
def __init__(self, profile_name=None, region_name=None):
Expand Down Expand Up @@ -77,11 +66,36 @@ def format_path(path):
"Principal": "*",
"Action": ["s3:GetObject"],
"Resource": paths_resources,
# Require HTTPS for public requests
"Condition": REQUIRE_HTTPS_CONDITION,
}

def get_bucket_policy_statements_for_user_access(self, bucket, user):
# Deny access using a version of TLS older than 1.2.
# This is also enforced by S3, but included here for visibility.
yield {
"Sid": "DenyOutdatedTLS",
"Effect": "Deny",
"Principal": "*",
"Action": "s3:*",
"Resource": [
f"arn:aws:s3:::{bucket.name}",
f"arn:aws:s3:::{bucket.name}/*",
],
"Condition": {"NumericLessThan": {"s3:TlsVersion": "1.2"}},
}

# Deny access which isn't over HTTPS
yield {
"Sid": "DenyInsecureTransport",
"Effect": "Deny",
"Principal": "*",
"Action": "s3:*",
"Resource": [
f"arn:aws:s3:::{bucket.name}",
f"arn:aws:s3:::{bucket.name}/*",
],
"Condition": {"Bool": {"aws:SecureTransport": "false"}},
}

# Create policy statement giving the created user access to
# non-destructive actions on the bucket.
yield {
Expand All @@ -95,8 +109,6 @@ def get_bucket_policy_statements_for_user_access(self, bucket, user):
"s3:ListBucketVersions",
],
"Resource": "arn:aws:s3:::{bucket_name}".format(bucket_name=bucket.name),
# Require HTTPS for API
"Condition": REQUIRE_HTTPS_CONDITION,
}
# Create policy statement giving the created user full access over the
# objects.
Expand All @@ -106,14 +118,11 @@ def get_bucket_policy_statements_for_user_access(self, bucket, user):
"Principal": {"AWS": user.arn},
"Action": "s3:*",
"Resource": "arn:aws:s3:::{bucket_name}/*".format(bucket_name=bucket.name),
# Require HTTPS for API
"Condition": REQUIRE_HTTPS_CONDITION,
}

def set_bucket_policy(
self, bucket, user, allow_public_acls, public_get_object_paths=None
):
policy_statement = []
public_access = bool(public_get_object_paths)

# NB: This API doesn't exist on a `Bucket`
Expand All @@ -129,15 +138,16 @@ def set_bucket_policy(
if public_access or allow_public_acls:
print("Configured public access to bucket.")

policy_statement = list(
self.get_bucket_policy_statements_for_user_access(bucket, user)
)

if public_access:
policy_statement.append(
self.get_bucket_policy_statement_for_get_object(
bucket, public_get_object_paths
)
)
policy_statement.extend(
list(self.get_bucket_policy_statements_for_user_access(bucket, user))
)
policy = json.dumps(
{
"Version": "2012-10-17",
Expand All @@ -148,7 +158,7 @@ def set_bucket_policy(
try:
bucket.Policy().put(Policy=policy)
except ClientError as e:
if e.response["Error"]["Code"] == "MalformedPolicy":
if e.response["Error"]["Message"] == "Invalid principal in policy":
print(
"Waiting for the user to be available to be "
"attached to the policy (wait 5s)."
Expand Down