Role trusts Signature validation Trust policy Source Identity Rules Revocation

Trust model in AWS Identity and Access Management Roles Anywhere

AWS Identity and Access Management Roles Anywhere works by bridging the trust model of IAM and Public Key Infrastructure (PKI). The model connects the role, the IAM Roles Anywhere service principal, and identities encoded in X509 certificates, that are issued by a Certificate Authority (CA).

Role trusts

To use an IAM role with IAM Roles Anywhere, you must create a trust relationship with the IAM Roles Anywhere service principal rolesanywhere.amazonaws.com. To create the trust relationship, you create a trust policy, a JSON policy document. The policy must grant the permissions:

sts:AssumeRole
sts:SetSourceIdentity
sts:TagSession

Sessions issued by IAM Roles Anywhere have the source identity set to the common name(s) (CN) of the subject(s) you use in end-entity certificate(s) authenticating to the target role. IAM Roles Anywhere extracts values from the subject, issuer, and Subject Alternative Name (SAN) fields of the authenticating certificate and makes them available for policy evaluation via the sourceIdentity and principal tags. The sts:SourceIdentity key is present in the request when IAM Roles Anywhere initially sets the source identity while assuming a role. You can apply more authorization constraints by using condition clauses in the policy statement. The tags will also be evaluated against policies on any resources accessed with the issued session.

Additionally, sessions issued by AWS have a Session Name property that is used as part of the assumed role ARN. IAM Roles Anywhere sets this automatically, using the hex-encoded serial number of the authenticating certificate.

To examine the contents of a certificate, use the following command:


$openssl x509 -text -noout -in certificate.pem


      Certificate:
      Data:
          Version: 3 (0x2)
          Serial Number:
              1f:71:c5:11:4a:11:9f:c0:cc:5a:5a:52:fb:37:20:ad
      Signature Algorithm: sha256WithRSAEncryption
          Issuer: C=US, O=Amazon, OU=IAM, ST=Washington, CN=Roles Anywhere, L=Seattle
          Validity
              Not Before: Apr 20 14:13:43 2022 GMT
              Not After : Jan 10 15:13:43 2023 GMT
          Subject: CN=awsUser
          # remainder omitted...

In this case, the value awsUser becomes the source identity. The values from the Issuer field become principal tags in the resulting session.

For more information, see:

Signature validation

To authenticate a request for credentials, IAM Roles Anywhere validates the incoming signature by using the signature validation algorithm required by the key type of the certificate, for example RSA or ECDSA. After validating the signature, IAM Roles Anywhere checks that the certificate was issued by a certificate authority configured as a trust anchor in the account using algorithms defined by public key infrastructure X.509 (PKIX) standards.

End entity certificates must satisfy the following constraints to be used for authentication:

The certificates MUST be X.509v3.
Basic constraints MUST include CA: false.
The key usage MUST include Digital Signature.
The signing algorithm MUST include SHA256 or stronger. MD5 and SHA1 signing algorithms are rejected.

Certificates used as trust anchors must satisfy the same requirements for signature algorithm, but with the following differences:

The key usage MUST include Certificate Sign, and MAY include CRL Sign. Certificate Revocation Lists (CRLs) are an optional feature of IAM Roles Anywhere.
Basic constraints MUST include CA: true.

Trust policy

Temporary credentials for IAM roles are issued to IAM Roles Anywhere clients via the API method CreateSession. In order for the call to be authorized, the target role of the CreateSession API call must have an Assume Role Policy Document to trust the IAM Roles Anywhere service principal.

Important

It is also recommended to have additional condition statements to further restrict authorization based on attributes that are extracted from the X.509 certificate.

The Subject, Issuer, and Subject Alternative Name (SAN) fields from X509 certificates are extracted and used as PrincipalTag elements in the session.

Note

These tags are also available to be used in conditions in the identity-based policy attached to the role.

Certificate Subject fields mapping

IAM Roles Anywhere maps Relative Distinguished Names (RDNs) from an authenticating certificate's Subject into distinct PrincipalTag elements in the session.

The following example shows a trust policy that adds a condition based on the Subject Common Name (CN) of the certificate. For example, if the identity asserted in the certificate is Alice, a condition can be created on the CN of the Subject.


{
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
          "Service": "rolesanywhere.amazonaws.com"
        },
        "Action": [
          "sts:AssumeRole",
          "sts:TagSession",
          "sts:SetSourceIdentity"
        ],
        "Condition": {
          "StringEquals": {
            "aws:PrincipalTag/x509Subject/CN": "Alice"
          },
          "ArnEquals": {
            "aws:SourceArn": [
              "arn:aws:rolesanywhere:region:account:trust-anchor/TA_ID"
            ]
          }
        }
      }
    ]
  }

Certificate Issuer fields mapping

IAM Roles Anywhere maps RDNs from an authenticating certificate’s Issuer into distinct PrincipalTag elements in the session.

The following example shows a trust policy that adds a condition based on the Issuer Common Name (CN) of the certificate. For example, if the issuer identity asserted in the certificate is Bob, a condition can be created on the CN of the Issuer.


{
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
          "Service": "rolesanywhere.amazonaws.com"
        },
        "Action": [
          "sts:AssumeRole",
          "sts:TagSession",
          "sts:SetSourceIdentity"
        ],
        "Condition": {
          "StringEquals": {
            "aws:PrincipalTag/x509Issuer/CN": "Bob"
          },
          "ArnEquals": {
            "aws:SourceArn": [
              "arn:aws:rolesanywhere:region:account:trust-anchor/TA_ID"
            ]
          }
        }
      }
    ]
  }

Certificate Subject Alternative Name (SAN) fields mapping

X509v3 certificates may include an extension to define additional identities, call Subject Alternative Names (SANs). The SAN can have multiple values, each of which can be one of nine types - Domain Name System (DNS) names, Uniform Resource Indicators (URIs), Directory Names, IP addresses, "Other" names, EDI party names, X400 addresses, RFC 822 names, or registered OIDs. IAM Roles Anywhere will map the first value of the following types: DNS Names, Directory Name (DN), and URI Names. The resulting tags will be prefixed with x509SAN, with a corresponding type code. The type code will be one of DNS, URI, or Name. For values of type DN, the individual RDNs will be parsed, as with Subject and Issuer.


      Certificate:
      Data:
          Version: 3 (0x2)
          Serial Number:
              1f:71:c5:11:4a:11:9f:c0:cc:5a:5a:52:fb:37:20:ad
      Signature Algorithm: sha256WithRSAEncryption
          Issuer: C=US, O=Amazon, OU=IAM, ST=Washington, CN=RolesAnywhere, L=Seattle
          Validity
              Not Before: Apr 20 14:13:43 2022 GMT
              Not After : Jan 10 15:13:43 2023 GMT
          Subject: CN=Alice
          X509v3 extensions:
            X509v3 Subject Alternative Name:
                DNS:example.com, 
                URI:spiffe://example.com/workload/alice, 
                DirName:/CN=Alice
          # remainder omitted...

The complete set of session principal tags, including the SAN fields, will look like the following:



        "aws:PrincipalTag/x509Subject/CN": "Alice"
        "aws:PrincipalTag/x509Issuer/C": "US"
        "aws:PrincipalTag/x509Issuer/O": "Amazon"
        "aws:PrincipalTag/x509Issuer/OU": "IAM"
        "aws:PrincipalTag/x509Issuer/ST": "Washington"
        "aws:PrincipalTag/x509Issuer/L": "Seattle"
        "aws:PrincipalTag/x509Issuer/CN": RolesAnywhere
        "aws:PrincipalTag/x509SAN/DNS": "example.com"
        "aws:PrincipalTag/x509SAN/URI": "spiffe://example.com/workload/alice"
        "aws:PrincipalTag/x509SAN/Name/CN": "Alice"

The following example shows a trust policy that adds a condition based on the SAN fields of the certificate.


{
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
          "Service": "rolesanywhere.amazonaws.com"
        },
        "Action": [
          "sts:AssumeRole",
          "sts:TagSession",
          "sts:SetSourceIdentity"
        ],
        "Condition": {
          "StringEquals": {
            "aws:PrincipalTag/x509SAN/DNS": "example.com",
            "aws:PrincipalTag/x509SAN/URI": "spiffe://example.com/workload/alice",
            "aws:PrincipalTag/x509SAN/Name/CN": "Alice"
          }, 
          "ArnEquals": {
            "aws:SourceArn": [
              "arn:aws:rolesanywhere:region:account:trust-anchor/TA_ID"
            ]
          }
        }
      }
    ]
  }

Note

RFC 5280 allows certificates with empty subjects if and only if the SAN extension is present and marked critical. Certificates with empty subjects are NOT yet supported, since IAM Roles Anywhere uses the certificate subject as the key of the Subject resource to visualize and audit activities for certificates that are authenticated with IAM Roles Anywhere. For more information about Subject resource, see Monitoring with IAM Roles Anywhere subjects.

Using the aws:SourceArn or aws:SourceAccount or sts:SourceIdentity condition keys

In general, it is strongly recommended that you use the aws:SourceArn or the aws:SourceAccount global condition keys or the sts:SourceIdentity condition key in your role trust policies. This combination of conditions implements least privilege permissions and prevents IAM Roles Anywhere from acting as a potential confused deputy. When a client obtains temporary security credentials from IAM Roles Anywhere, the aws:SourceArn and aws:SourceAccount will be set based on the ARN of the trust anchor specified in the call to CreateSession.

To use the aws:SourceArn and aws:SourceAccount global condition keys, set the value to the Amazon Resource Name (ARN) or account of the trust anchor that you expect to use for authentication. As an example, consider the following role trust policy.


{
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
          "Service": "rolesanywhere.amazonaws.com"
        },
        "Action": [
          "sts:AssumeRole",
          "sts:TagSession",
          "sts:SetSourceIdentity"
        ],
        "Condition": {
          "ArnEquals": {
            "aws:SourceArn": [
              "arn:aws:rolesanywhere:region:account:trust-anchor/TA_ID"
            ]
          }
        }
      }
    ]
  }

To use the sts:SourceIdentity condition key, set the source identity prefix and source identity value according to the following rules. As an example, consider the following role trust policy.



    {
      "Version": "2012-10-17",
      "Statement": [
        {
          "Effect": "Allow",
          "Principal": {
            "Service": "rolesanywhere.amazonaws.com"
          },
          "Action": [
            "sts:AssumeRole",
            "sts:SetSourceIdentity"
          ],
          "Condition": {
            "StringEquals": {
              "sts:SourceIdentity": [
                "${sourceIdentityPrefix}${sourceIdentityValue}"
              ]
            },
            "ArnEquals": {
              "aws:SourceArn": [
                "arn:aws:rolesanywhere:region:account:trust-anchor/TA_ID"
              ]
            }
          }
        },
        {
          "Effect": "Allow",
          "Principal": {
            "Service": "rolesanywhere.amazonaws.com"
          },
          "Action": [
            "sts:TagSession"
          ]
        }
      ]
    }

Source Identity Rules

We define a source identity prefix as follows:

"CN=": the common name of the subject in the certificate is set and less than or equal to the length threshold.
"ID=": the common name of the subject in the certificate is NOT set.
"": (empty string) the common name of the subject in the certificate is set and greater than the length threshold but less than 64 characters.

The value following a source identity prefix is as follows:

"CN=": the common name of the subject in the certificate.
"ID=": the hex-encoded certificate serial number.
"": the common name of the subject in the certificate.

The length threshold is 61 characters.

Revocation

Certificate revocation is supported through the use of imported certificate revocation lists (CRLs). Currently, certification revocation is only supported by the API and CLI. You can import a CRL that is generated from your CA using ImportCrl API or import-crl CLI command. Certificates used for authentication will be checked for their revocation status.

Callbacks to CRL Distribution Points (CDPs) or Online Certificate Status Protocol (OCSP) endpoints are not supported.

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Document Conventions

Obtaining temporary security credentials

Certificate Attribute Mapping