Troubleshooting ‘Access Denied’ Errors When Using Postman With Sharepoint Online
Authenticating Requests
When making requests to the SharePoint Online API using Postman, one of the most common errors you may encounter is “Access Denied”. This indicates there is a problem with the authentication and authorization of the request. To properly authenticate, SharePoint Online uses the OAuth 2.0 protocol to issue access tokens that must be attached to API requests in the Authorization header.
Using OAuth 2.0 for Authorization
OAuth 2.0 is the authentication framework used by SharePoint Online to authorize API requests. When using Postman to call SharePoint APIs, you need to configure it for OAuth 2.0 by registering an application in Azure Active Directory to obtain a client ID and client secret. These credentials can be used in Postman to go through the OAuth 2.0 flow and acquire access tokens.
Obtaining Access Tokens
Once OAuth 2.0 is configured, Postman can request an access token from Azure AD by passing the client ID and secret to the token endpoint. This access token then must be attached in the Authorization header, formatted as “Bearer {token}”, when making API calls to SharePoint. Tokens have an expiration time, after which new tokens must be fetched.
Refreshing Expired Tokens
By default access tokens expire after one hour. Postman provides options to automatically refresh expired tokens by using the refresh token that was provided along with the original access token. The refresh token can be persisted and used to request new access tokens without needing to re-authenticate.
Validating Tokens
To validate that the correct access token is being passed in the Authorization header, you can use an online JWT decoder to decode the token and inspect its contents, such as expiration time and permissions scopes. The decoded token indicates what resources and APIs the token holder is allowed to access.
Analyzing Request Headers
In addition to proper authorization and authentication, calling the SharePoint Online API requires that request headers are properly formatted. Malformed headers will often result in “Access Denied” errors.
Examining Authorization Header
Carefully inspect the Authorization header to ensure the access token is passed correctly, formatted as “Bearer {token}”. Check that the token value is the full access token string without any errors or truncations. Additionally verify the token has not yet expired by validating it with a JWT decoder.
Verifying API Permissions
Decode the access token and examine the “scopes” value in the payload section. This lists the SharePoint API permissions the token was issued for. Compare the requested resource against the token scopes to confirm the token has been granted access to that API.
Checking Resource URLs
Double check that the URL path for the SharePoint resource you are calling in the API request matches what is permitted in the access token. Attempting to access a resource not covered in the token will lead to access denied errors.
Investigating IP Address Restrictions
SharePoint Online requires requests originate from a permitted set of IP addresses or address ranges. Calls from unapproved IPs are blocked, often returning an “Access Denied” error.
Understanding SharePoint IP Ranges
Microsoft defines specific IP ranges representing customer data centers that are allowed to access SharePoint Online APIs. To determine if your Postman client IP is permitted, check Microsoft’s published list of IP addresses against the outbound IP of your request.
Configuring Client IP Addresses
If your IP address falls outside the allowed SharePoint Online ranges, you can configure Postman’s proxy settings to route requests through an approved IP. Microsoft’s Azure environments and other large corporate networks typically have dedicated IP addresses for accessing SharePoint.
Using VPNs and Proxies
As an alternative to proxied IP routing, connecting through a VPN or SOCKS proxy will assign you a virtual IP that conforms to the SharePoint IP allow list. Tunneled IPs present as approved addresses for accessing the SharePoint APIs.
Bypassing Preflight Requests
SharePoint Online uses CORS protections including preflight requests to block calls from unauthorized domains or headers. Malformed requests may trigger these checks resulting in denied access.
Identifying CORS Issues
Examine the request domain, headers, and methods to identify anything that may trigger CORS preflight checks. Delete custom headers not on the allowed list to avoid blocked requests prior to accessing the API.
Customizing Request Headers
Modify any custom headers passed in your Postman requests to values that will properly pass SharePoint preflight checks. For example, custom content types must match expected formats to prevent requests from failing at the OPTIONS preflight stage.
Disabling Preflight Checks
As a last resort, some SharePoint APIs support disabling CORS entirely through headers like “X-MS-Disable-Preflight”. This will bypass preflight checks altogether at the risk of disabling this security measure.
Recreating the Issue
To properly diagnose “Access Denied” errors when calling SharePoint Online APIs with Postman, capturing detailed request traces is crucial for identifying the root cause.
Capturing Network Traces
Use Postman’s logging and network tracing tools to capture full request details including headers, payloads, and server responses. Save request histories and monitor traffic in real-time to recreate failed calls for further inspection.
Inspecting Server Responses
Inspect the response details from blocked API calls to SharePoint. Server response codes, headers, and error payloads will indicate exactly why access was denied, whether for authorization issues, CORS problems, or IP restrictions.
Checking Credentials and Scopes
If authentication is failing, verify OAuth 2.0 settings including the client ID, secret, tenant ID, token endpoint, and configured scopes. Test Postman access by requesting a fresh token with the proper permissions to call SharePoint before re-trying the API.
Troubleshooting Recap
Summary of Common Issues
In summary, “Access Denied” errors most commonly occur due to:
- Expired or invalid access tokens
- Tokens with insufficient permission scopes
- Calls from unapproved IP addresses
- Blocked CORS preflight check failures
Tips for Preventing Errors
Strategies to avoid access denied errors when accessing SharePoint Online APIs:
- Request tokens with necessary scopes
- Automate token renewal for long jobs
- Double check IP allow lists
- Remove unnecessary custom headers
- Trace and inspect failed requests
