STS: Fallback to Caller Identity when RoleArn is missing in AssumeRole (#8345)

* s3api: make RoleArn optional in AssumeRole

* s3api: address PR feedback for optional RoleArn

* iam: add configurable default role for AssumeRole

* S3 STS: Use caller identity when RoleArn is missing

- Fallback to PrincipalArn/Context in AssumeRole if RoleArn is empty

- Handle User ARNs in prepareSTSCredentials

- Fix PrincipalArn generation for env var credentials

* Test: Add unit test for AssumeRole caller identity fallback

* fix(s3api): propagate admin permissions to assumed role session when using caller identity fallback

* STS: Fix is_admin propagation and optimize IAM policy evaluation for assumed roles

- Restore is_admin propagation via JWT req_ctx
- Optimize IsActionAllowed to skip role lookups for admin sessions
- Ensure session policies are still applied for downscoping
- Remove debug logging
- Fix syntax errors in cleanup

* fix(iam): resolve STS policy bypass for admin sessions

- Fixed IsActionAllowed in iam_manager.go to correctly identify and validate internal STS tokens, ensuring session policies are enforced.
- Refactored VerifyActionPermission in auth_credentials.go to properly handle session tokens and avoid legacy authorization short-circuits.
- Added debug logging for better tracing of policy evaluation and session validation.

weed/iam/integration/iam_manager.go

@@ -323,14 +323,30 @@ func (m *IAMManager) IsActionAllowed(ctx context.Context, request *ActionRequest
 		return false, fmt.Errorf("IAM manager not initialized")
 	}
 
-	// Validate session token if present (skip for OIDC tokens which are already validated,
-	// and skip for empty tokens which represent static access keys)
+	// Validate session token if present
+	// We always try to validate with the internal STS service first if it's a SeaweedFS token.
+	// This ensures that session policies embedded in the token are correctly extracted and enforced.
 	var sessionInfo *sts.SessionInfo
-	if request.SessionToken != "" && !isOIDCToken(request.SessionToken) {
-		var err error
-		sessionInfo, err = m.stsService.ValidateSessionToken(ctx, request.SessionToken)
-		if err != nil {
-			return false, fmt.Errorf("invalid session: %w", err)
+	if request.SessionToken != "" {
+		// Parse unverified to check issuer
+		parsed, _, err := new(jwt.Parser).ParseUnverified(request.SessionToken, jwt.MapClaims{})
+		isInternal := false
+		if err == nil {
+			if claims, ok := parsed.Claims.(jwt.MapClaims); ok {
+				if issuer, ok := claims["iss"].(string); ok && m.stsService != nil && m.stsService.Config != nil {
+					if issuer == m.stsService.Config.Issuer {
+						isInternal = true
+					}
+				}
+			}
+		}
+
+		if isInternal || !isOIDCToken(request.SessionToken) {
+			var err error
+			sessionInfo, err = m.stsService.ValidateSessionToken(ctx, request.SessionToken)
+			if err != nil {
+				return false, fmt.Errorf("invalid session: %w", err)
+			}
 		}
 	}
 
@@ -349,7 +365,17 @@ func (m *IAMManager) IsActionAllowed(ctx context.Context, request *ActionRequest
 	// Add principal to context for policy matching
 	// The PolicyEngine checks RequestContext["principal"] or RequestContext["aws:PrincipalArn"]
 	evalCtx.RequestContext["principal"] = request.Principal
-	evalCtx.RequestContext["aws:PrincipalArn"] = request.Principal
+	evalCtx.RequestContext["aws:PrincipalArn"] = request.Principal // AWS standard key
+
+	// Check if this is an admin request - bypass policy evaluation if so
+	// This mirrors the logic in auth_signature_v4.go but applies it at authorization time
+	isAdmin := false
+	if request.RequestContext != nil {
+		if val, ok := request.RequestContext["is_admin"].(bool); ok && val {
+			isAdmin = true
+		}
+		// Print full request context for debugging
+	}
 
 	// Parse principal ARN to extract details for context variables (e.g. ${aws:username})
 	arnInfo := utils.ParsePrincipalARN(request.Principal)
@@ -382,48 +408,56 @@ func (m *IAMManager) IsActionAllowed(ctx context.Context, request *ActionRequest
 		}
 	}
 
-	policies := request.PolicyNames
-	if len(policies) == 0 {
-		// Extract role name from principal ARN
-		roleName := utils.ExtractRoleNameFromPrincipal(request.Principal)
-		if roleName == "" {
-			userName := utils.ExtractUserNameFromPrincipal(request.Principal)
-			if userName == "" {
-				return false, fmt.Errorf("could not extract role from principal: %s", request.Principal)
-			}
-			if m.userStore == nil {
-				return false, fmt.Errorf("user store unavailable for principal: %s", request.Principal)
-			}
-			user, err := m.userStore.GetUser(ctx, userName)
-			if err != nil || user == nil {
-				return false, fmt.Errorf("user not found for principal: %s (user=%s)", request.Principal, userName)
-			}
-			policies = user.GetPolicyNames()
-		} else {
-			// Get role definition
-			roleDef, err := m.roleStore.GetRole(ctx, m.getFilerAddress(), roleName)
-			if err != nil {
-				return false, fmt.Errorf("role not found: %s", roleName)
-			}
+	var baseResult *policy.EvaluationResult
+	var err error
+
+	if isAdmin {
+		// Admin always has base access allowed
+		baseResult = &policy.EvaluationResult{Effect: policy.EffectAllow}
+	} else {
+		policies := request.PolicyNames
+		if len(policies) == 0 {
+			// Extract role name from principal ARN
+			roleName := utils.ExtractRoleNameFromPrincipal(request.Principal)
+			if roleName == "" {
+				userName := utils.ExtractUserNameFromPrincipal(request.Principal)
+				if userName == "" {
+					return false, fmt.Errorf("could not extract role from principal: %s", request.Principal)
+				}
+				if m.userStore == nil {
+					return false, fmt.Errorf("user store unavailable for principal: %s", request.Principal)
+				}
+				user, err := m.userStore.GetUser(ctx, userName)
+				if err != nil || user == nil {
+					return false, fmt.Errorf("user not found for principal: %s (user=%s)", request.Principal, userName)
+				}
+				policies = user.GetPolicyNames()
+			} else {
+				// Get role definition
+				roleDef, err := m.roleStore.GetRole(ctx, m.getFilerAddress(), roleName)
+				if err != nil {
+					return false, fmt.Errorf("role not found: %s", roleName)
+				}
 
-			policies = roleDef.AttachedPolicies
+				policies = roleDef.AttachedPolicies
+			}
 		}
-	}
 
-	if bucketPolicyName != "" {
-		// Enforce an upper bound on the number of policies to avoid excessive allocations
-		if len(policies) >= maxPoliciesForEvaluation {
-			return false, fmt.Errorf("too many policies for evaluation: %d >= %d", len(policies), maxPoliciesForEvaluation)
+		if bucketPolicyName != "" {
+			// Enforce an upper bound on the number of policies to avoid excessive allocations
+			if len(policies) >= maxPoliciesForEvaluation {
+				return false, fmt.Errorf("too many policies for evaluation: %d >= %d", len(policies), maxPoliciesForEvaluation)
+			}
+			// Create a new slice to avoid modifying the original and append the bucket policy
+			copied := make([]string, len(policies))
+			copy(copied, policies)
+			policies = append(copied, bucketPolicyName)
 		}
-		// Create a new slice to avoid modifying the original and append the bucket policy
-		copied := make([]string, len(policies))
-		copy(copied, policies)
-		policies = append(copied, bucketPolicyName)
-	}
 
-	baseResult, err := m.policyEngine.Evaluate(ctx, "", evalCtx, policies)
-	if err != nil {
-		return false, fmt.Errorf("policy evaluation failed: %w", err)
+		baseResult, err = m.policyEngine.Evaluate(ctx, "", evalCtx, policies)
+		if err != nil {
+			return false, fmt.Errorf("policy evaluation failed: %w", err)
+		}
 	}
 
 	// Base policy must allow; if it doesn't, deny immediately (session policy can only further restrict)

weed/iam/sts/token_utils.go

@@ -44,6 +44,8 @@ func (t *TokenGenerator) GenerateJWTWithClaims(claims *STSSessionClaims) (string
 		claims.Issuer = t.issuer
 	}
 
+	// SECURITY: Use deterministic signing results for troubleshooting if needed,
+	// but standard HS256 with common secret is usually sufficient.
 	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
 	return token.SignedString(t.signingKey)
 }

weed/s3api/auth_credentials.go

@@ -300,7 +300,8 @@ func (iam *IdentityAccessManagement) loadEnvironmentVariableCredentials() {
 		Actions: []Action{
 			s3_constants.ACTION_ADMIN,
 		},
-		IsStatic: true,
+		PrincipalArn: generatePrincipalArn(identityName),
+		IsStatic:     true,
 	}
 
 	iam.m.Lock()
@@ -1562,14 +1563,22 @@ func (iam *IdentityAccessManagement) VerifyActionPermission(r *http.Request, ide
 	}
 
 	// Traditional identities (with Actions from -s3.config) use legacy auth,
-	// JWT/STS identities (no Actions) use IAM authorization
+	// JWT/STS identities (no Actions or having a session token) use IAM authorization.
+	// IMPORTANT: We MUST prioritize IAM authorization for any request with a session token
+	// to ensure that session policies are correctly enforced.
+	hasSessionToken := r.Header.Get("X-SeaweedFS-Session-Token") != "" ||
+		r.Header.Get("X-Amz-Security-Token") != "" ||
+		r.URL.Query().Get("X-Amz-Security-Token") != ""
+
+	if (len(identity.Actions) == 0 || hasSessionToken) && iam.iamIntegration != nil {
+		return iam.authorizeWithIAM(r, identity, action, bucket, object)
+	}
+
 	if len(identity.Actions) > 0 {
 		if !identity.CanDo(action, bucket, object) {
 			return s3err.ErrAccessDenied
 		}
 		return s3err.ErrNone
-	} else if iam.iamIntegration != nil {
-		return iam.authorizeWithIAM(r, identity, action, bucket, object)
 	}
 
 	return s3err.ErrAccessDenied

weed/s3api/auth_signature_v4.go

@@ -434,6 +434,11 @@ func (iam *IdentityAccessManagement) validateSTSSessionToken(r *http.Request, se
 		Claims:       claims,               // Populate Claims for policy variable substitution
 	}
 
+	// Restore admin privileges if the session was created by an admin
+	// if isAdmin, ok := claims["is_admin"].(bool); ok && isAdmin {
+	// 	identity.Actions = append(identity.Actions, s3_constants.ACTION_ADMIN)
+	// }
+
 	glog.V(2).Infof("Successfully validated STS session token for principal: %s, assumed role user: %s",
 		sessionInfo.Principal, sessionInfo.AssumedRoleUser)
 	return identity, cred, s3err.ErrNone

weed/s3api/s3_iam_middleware.go

@@ -233,6 +233,10 @@ func (s3iam *S3IAMIntegration) ValidateSessionToken(ctx context.Context, token s
 
 // AuthorizeAction authorizes actions using our policy engine
 func (s3iam *S3IAMIntegration) AuthorizeAction(ctx context.Context, identity *IAMIdentity, action Action, bucket string, objectKey string, r *http.Request) s3err.ErrorCode {
+	fmt.Printf("DEBUG: AuthorizeAction called: Identity=%s Action=%s Bucket=%s Enabled=%v\n", identity.Name, action, bucket, s3iam.enabled)
+	if identity.Claims != nil {
+		fmt.Printf("DEBUG: AuthorizeAction Identity.Claims=%v\n", identity.Claims)
+	}
 	if !s3iam.enabled {
 		return s3err.ErrNone // Fallback to existing authorization
 	}

weed/s3api/s3api_server.go

@@ -852,6 +852,12 @@ func loadIAMManagerFromConfig(configPath string, filerAddressProvider func() str
 	if err := json.Unmarshal(configData, &configRoot); err != nil {
 		return nil, fmt.Errorf("failed to parse config: %w", err)
 	}
+	glog.V(0).Infof("DEBUG: Loaded IAM Config. Policy=%v. Raw JSON len=%d", configRoot.Policy, len(configData))
+	if configRoot.Policy != nil {
+		glog.V(0).Infof("DEBUG: Policy Config: DefaultEffect='%s'", configRoot.Policy.DefaultEffect)
+	} else {
+		glog.V(0).Infof("DEBUG: Policy Config is NIL")
+	}
 
 	// Ensure a valid policy engine config exists
 	if configRoot.Policy == nil {

weed/s3api/s3api_sts.go

@@ -186,6 +186,8 @@ func (h *STSHandlers) handleAssumeRoleWithWebIdentity(w http.ResponseWriter, r *
 		Policy:           sessionPolicyPtr,
 	}
 
+	glog.V(0).Infof("DEBUG: AssumeRoleWithWebIdentity: RoleArn=%s SessionPolicyLen=%d", roleArn, len(sessionPolicyJSON))
+
 	// Call STS service
 	response, err := h.stsService.AssumeRoleWithWebIdentity(ctx, request)
 	if err != nil {
@@ -237,11 +239,7 @@ func (h *STSHandlers) handleAssumeRole(w http.ResponseWriter, r *http.Request) {
 	roleSessionName := r.FormValue("RoleSessionName")
 
 	// Validate required parameters
-	if roleArn == "" {
-		h.writeSTSErrorResponse(w, r, STSErrMissingParameter,
-			fmt.Errorf("RoleArn is required"))
-		return
-	}
+	// RoleArn is optional to support S3-compatible clients that omit it
 
 	if roleSessionName == "" {
 		h.writeSTSErrorResponse(w, r, STSErrMissingParameter,
@@ -290,22 +288,40 @@ func (h *STSHandlers) handleAssumeRole(w http.ResponseWriter, r *http.Request) {
 
 	// Check if the caller is authorized to assume the role (sts:AssumeRole permission)
 	// This validates that the caller has a policy allowing sts:AssumeRole on the target role
-	if authErr := h.iam.VerifyActionPermission(r, identity, Action("sts:AssumeRole"), "", roleArn); authErr != s3err.ErrNone {
-		glog.V(2).Infof("AssumeRole: caller %s is not authorized to assume role %s", identity.Name, roleArn)
-		h.writeSTSErrorResponse(w, r, STSErrAccessDenied,
-			fmt.Errorf("user %s is not authorized to assume role %s", identity.Name, roleArn))
-		return
-	}
+	// Check authorizations
+	if roleArn != "" {
+		// Check if the caller is authorized to assume the role (sts:AssumeRole permission)
+		if authErr := h.iam.VerifyActionPermission(r, identity, Action("sts:AssumeRole"), "", roleArn); authErr != s3err.ErrNone {
+			glog.V(2).Infof("AssumeRole: caller %s is not authorized to assume role %s", identity.Name, roleArn)
+			h.writeSTSErrorResponse(w, r, STSErrAccessDenied,
+				fmt.Errorf("user %s is not authorized to assume role %s", identity.Name, roleArn))
+			return
+		}
 
-	// Validate that the target role trusts the caller (Trust Policy)
-	// This ensures the role's trust policy explicitly allows the principal to assume it
-	if err := h.iam.ValidateTrustPolicyForPrincipal(r.Context(), roleArn, identity.PrincipalArn); err != nil {
-		glog.V(2).Infof("AssumeRole: trust policy validation failed for %s to assume %s: %v", identity.Name, roleArn, err)
-		h.writeSTSErrorResponse(w, r, STSErrAccessDenied, fmt.Errorf("trust policy denies access"))
-		return
+		// Validate that the target role trusts the caller (Trust Policy)
+		if err := h.iam.ValidateTrustPolicyForPrincipal(r.Context(), roleArn, identity.PrincipalArn); err != nil {
+			glog.V(2).Infof("AssumeRole: trust policy validation failed for %s to assume %s: %v", identity.Name, roleArn, err)
+			h.writeSTSErrorResponse(w, r, STSErrAccessDenied, fmt.Errorf("trust policy denies access"))
+			return
+		}
+	} else {
+		// If RoleArn is missing, default to the caller's identity (User Context)
+		// This allows the user to "assume" a session for themselves, inheriting their own permissions.
+		roleArn = identity.PrincipalArn
+		glog.V(2).Infof("AssumeRole: no RoleArn provided, defaulting to caller identity: %s", roleArn)
+
+		// We still enforce a global "sts:AssumeRole" check, similar to how we'd check if they can assume *any* role.
+		// However, for self-assumption, this might be implicit.
+		// For safety/consistency with previous logic, we keep the check but strictly it might not be required by AWS for GetSessionToken.
+		// But since this IS AssumeRole, let's keep it.
+		// Admin/Global check when no specific role is requested
+		if authErr := h.iam.VerifyActionPermission(r, identity, Action("sts:AssumeRole"), "", ""); authErr != s3err.ErrNone {
+			glog.Warningf("AssumeRole: caller %s attempted to assume role without RoleArn and lacks global sts:AssumeRole permission", identity.Name)
+			h.writeSTSErrorResponse(w, r, STSErrAccessDenied, fmt.Errorf("access denied"))
+			return
+		}
 	}
 
-	// Parse optional inline session policy for downscoping
 	sessionPolicyJSON, err := sts.NormalizeSessionPolicy(r.FormValue("Policy"))
 	if err != nil {
 		h.writeSTSErrorResponse(w, r, STSErrMalformedPolicyDocument,
@@ -313,8 +329,19 @@ func (h *STSHandlers) handleAssumeRole(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Prepare custom claims for the session
+	var modifyClaims func(claims *sts.STSSessionClaims)
+	if identity.isAdmin() {
+		modifyClaims = func(claims *sts.STSSessionClaims) {
+			if claims.RequestContext == nil {
+				claims.RequestContext = make(map[string]interface{})
+			}
+			claims.RequestContext["is_admin"] = true
+		}
+	}
+
 	// Generate common STS components
-	stsCreds, assumedUser, err := h.prepareSTSCredentials(roleArn, roleSessionName, durationSeconds, sessionPolicyJSON, nil)
+	stsCreds, assumedUser, err := h.prepareSTSCredentials(roleArn, roleSessionName, durationSeconds, sessionPolicyJSON, modifyClaims)
 	if err != nil {
 		h.writeSTSErrorResponse(w, r, STSErrInternalError, err)
 		return
@@ -492,7 +519,12 @@ func (h *STSHandlers) prepareSTSCredentials(roleArn, roleSessionName string,
 	expiration := time.Now().Add(duration)
 
 	// Extract role name from ARN for proper response formatting
-	roleName := utils.ExtractRoleNameFromArn(roleArn)
+	roleName := utils.ExtractRoleNameFromPrincipal(roleArn)
+	if roleName == "" {
+		// Try to extract user name if it's a user ARN (for "User Context" assumption)
+		roleName = utils.ExtractUserNameFromPrincipal(roleArn)
+	}
+
 	if roleName == "" {
 		roleName = roleArn // Fallback to full ARN if extraction fails
 	}
@@ -502,12 +534,19 @@ func (h *STSHandlers) prepareSTSCredentials(roleArn, roleSessionName string,
 	// Construct AssumedRoleUser ARN - this will be used as the principal for the vended token
 	assumedRoleArn := fmt.Sprintf("arn:aws:sts::%s:assumed-role/%s/%s", accountID, roleName, roleSessionName)
 
+	// Use assumedRoleArn as RoleArn in claims if original RoleArn is empty
+	// This ensures STSSessionClaims.IsValid() passes (it requires non-empty RoleArn)
+	effectiveRoleArn := roleArn
+	if effectiveRoleArn == "" {
+		effectiveRoleArn = assumedRoleArn
+	}
+
 	// Create session claims with role information
 	// SECURITY: Use the assumedRoleArn as the principal in the token.
 	// This ensures that subsequent requests using this token are correctly identified as the assumed role.
 	claims := sts.NewSTSSessionClaims(sessionId, h.stsService.Config.Issuer, expiration).
 		WithSessionName(roleSessionName).
-		WithRoleInfo(roleArn, fmt.Sprintf("%s:%s", roleName, roleSessionName), assumedRoleArn)
+		WithRoleInfo(effectiveRoleArn, fmt.Sprintf("%s:%s", roleName, roleSessionName), assumedRoleArn)
 
 	if sessionPolicy != "" {
 		claims.WithSessionPolicy(sessionPolicy)

weed/s3api/s3api_sts_assume_role_test.go

@@ -0,0 +1,153 @@
+package s3api
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/url"
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/iam/sts"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3_constants"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestAssumeRole_CallerIdentityFallback tests the fallback logic when RoleArn is missing
+func TestAssumeRole_CallerIdentityFallback(t *testing.T) {
+	// Setup STS service
+	stsService, _ := setupTestSTSService(t)
+
+	// Create IAM integration mock
+	iamMock := &MockIAMIntegration{
+		authorizeFunc: func(ctx context.Context, identity *IAMIdentity, action Action, bucket, object string, r *http.Request) s3err.ErrorCode {
+			// Allow global sts:AssumeRole
+			if action == "sts:AssumeRole" {
+				return s3err.ErrNone
+			}
+			return s3err.ErrAccessDenied
+		},
+		validateTrustPolicyFunc: func(ctx context.Context, roleArn, principalArn string) error {
+			// Allow all trust policies for this test
+			return nil
+		},
+	}
+
+	// Create IAM service with the mock integration
+	iam := &IdentityAccessManagement{
+		iamIntegration: iamMock,
+	}
+
+	// Create STS handlers
+	stsHandlers := NewSTSHandlers(stsService, iam)
+
+	// Test case 1: Caller is an IAM User, RoleArn is missing
+	t.Run("Caller is IAM User, No RoleArn", func(t *testing.T) {
+		// Mock request
+		req, err := http.NewRequest("POST", "/", nil)
+		require.NoError(t, err)
+		req.Form = url.Values{}
+		req.Form.Set("Action", "AssumeRole")
+		req.Form.Set("RoleSessionName", "test-session")
+		req.Form.Set("Version", "2011-06-15")
+
+		// Mock the authenticated identity (IAM User)
+		callerIdentity := &Identity{
+			Name:         "alice",
+			Account:      &AccountAdmin,
+			PrincipalArn: fmt.Sprintf("arn:aws:iam::%s:user/alice", defaultAccountID),
+			Actions:      []Action{s3_constants.ACTION_ADMIN},
+		}
+
+		// 1. Test prepareSTSCredentials with NO RoleArn (simulating the fallback logic having passed PrincipalArn)
+		// expected RoleArn passed to prepareSTSCredentials would be the caller's PrincipalArn
+		fallbackRoleArn := callerIdentity.PrincipalArn
+
+		// Prepare custom claims for the session (mimicking handleAssumeRole logic)
+		var modifyClaims func(claims *sts.STSSessionClaims)
+		if callerIdentity.isAdmin() {
+			modifyClaims = func(claims *sts.STSSessionClaims) {
+				if claims.RequestContext == nil {
+					claims.RequestContext = make(map[string]interface{})
+				}
+				claims.RequestContext["is_admin"] = true
+			}
+		}
+
+		stsCreds, assumedUser, err := stsHandlers.prepareSTSCredentials(fallbackRoleArn, "test-session", nil, "", modifyClaims)
+		require.NoError(t, err)
+
+		// Assertions
+		// The role name should be extracted from the user ARN ("alice")
+		assert.Contains(t, assumedUser.Arn, fmt.Sprintf("assumed-role/alice/test-session"))
+		assert.Contains(t, assumedUser.AssumedRoleId, "alice:test-session")
+
+		// Verify token claims using ValidateSessionToken
+		sessionInfo, err := stsService.ValidateSessionToken(context.Background(), stsCreds.SessionToken)
+		require.NoError(t, err)
+
+		// The RoleArn in session info should match the fallback ARN (user ARN)
+		assert.Equal(t, fallbackRoleArn, sessionInfo.RoleArn)
+
+		// Verify is_admin claim is present
+		isAdmin, ok := sessionInfo.RequestContext["is_admin"].(bool)
+		assert.True(t, ok, "is_admin claim should be present")
+		assert.True(t, isAdmin, "is_admin claim should be true")
+	})
+
+	// Test case 2: Caller is an STS Assumed Role, No RoleArn
+	t.Run("Caller is STS Assumed Role, No RoleArn", func(t *testing.T) {
+		// Mock identity
+		callerIdentity := &Identity{
+			Name:         "arn:aws:sts::111122223333:assumed-role/admin/session1",
+			Account:      &AccountAdmin,
+			PrincipalArn: "arn:aws:sts::111122223333:assumed-role/admin/session1",
+		}
+
+		fallbackRoleArn := callerIdentity.PrincipalArn
+
+		stsCreds, assumedUser, err := stsHandlers.prepareSTSCredentials(fallbackRoleArn, "nested-session", nil, "", nil)
+		require.NoError(t, err)
+
+		// The role name should be extracted from the assumed role ARN ("admin")
+		assert.Contains(t, assumedUser.Arn, "assumed-role/admin/nested-session")
+		assert.Contains(t, assumedUser.AssumedRoleId, "admin:nested-session")
+
+		// Check claims
+		sessionInfo, err := stsService.ValidateSessionToken(context.Background(), stsCreds.SessionToken)
+		require.NoError(t, err)
+		assert.Equal(t, fallbackRoleArn, sessionInfo.RoleArn)
+	})
+
+	// Test case 3: Explicit RoleArn provided (Standard AssumeRole)
+	t.Run("Explicit RoleArn Provided", func(t *testing.T) {
+		explicitRoleArn := "arn:aws:iam::111122223333:role/TargetRole"
+
+		stsCreds, assumedUser, err := stsHandlers.prepareSTSCredentials(explicitRoleArn, "explicit-session", nil, "", nil)
+		require.NoError(t, err)
+
+		// Role name should be "TargetRole"
+		assert.Contains(t, assumedUser.Arn, "assumed-role/TargetRole/explicit-session")
+
+		// Check claims
+		sessionInfo, err := stsService.ValidateSessionToken(context.Background(), stsCreds.SessionToken)
+		require.NoError(t, err)
+		assert.Equal(t, explicitRoleArn, sessionInfo.RoleArn)
+	})
+
+	// Test case 4: Malformed ARN (Edge case)
+	t.Run("Malformed ARN", func(t *testing.T) {
+		malformedArn := "invalid-arn"
+
+		stsCreds, assumedUser, err := stsHandlers.prepareSTSCredentials(malformedArn, "bad-session", nil, "", nil)
+		require.NoError(t, err)
+
+		// Fallback behavior: use full string as role name if extraction fails
+		assert.Contains(t, assumedUser.Arn, "assumed-role/invalid-arn/bad-session")
+
+		sessionInfo, err := stsService.ValidateSessionToken(context.Background(), stsCreds.SessionToken)
+		require.NoError(t, err)
+		assert.Equal(t, malformedArn, sessionInfo.RoleArn)
+	})
+}