Azure Functions
An Azure service that provides an event-driven serverless compute platform.
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Intermittent 403 Forbidden Error on Azure Function App (Linux Plan, Private Endpoint, Premium V3)
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Oriol Trujillo
0
Reputation points
I'm experiencing an intermittent 403 Forbidden error on my Azure Function App, and I'm hoping someone can help me troubleshoot this issue.
- The Function App usually works fine, but occasionally it becomes inaccessible for a specific browser session.
- When this happens, I get a 403 Forbidden error (Azure blue screen) in one browser session, while at the same time, the same request works fine from Incognito mode or another browser (check out the screenshot )
Clearing cookies in the original browser session fixes the issue.
The issue does not seem related to the function's execution, as the API is still accessible from other sessions.
My setup:
- Azure Function App running on a Linux Plan (Premium v3, P0V3).
- Inbound traffic configuration:
- Public network access: Disabled
- Private endpoints: 1 private endpoint
- Inbound addresses: 10.5.11.6
- Optional inbound services: Azure Front Door (enabled)
- Outbound traffic configuration:
- Virtual network integration: Enabled (VNET-PIA-PROD/SNET-PIA-PROD-FUNCTIONS)
- Hybrid connections: Not configured
- Outbound DNS: Inherited (from virtual network)
- Outbound addresses: A set of Azure IPs (listed in screenshot)
- Integration subnet configuration:
- NAT gateway: Not configured
- Network security group: Configured (NSG-PIA-BASE )
- User-defined route: Configured (ROUTE-PIA-BASE)
What could be causing session-specific 403 Forbidden errors when the same API works in Incognito mode or another browser at the same time?
Would really appreciate any insights! Thanks in advance people 🚀
{count} votes
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Khadeer Ali 2,770 Reputation points Microsoft Vendor2025-01-30T16:20:55.16+00:00 Welcome to Microsoft Q&A Platform!
Thanks for reaching out.
The intermittent 403 Forbidden error on your Azure Function App, specifically tied to browser sessions and resolved by clearing cookies, strongly suggests an authentication or authorization issue related to session management. Even though the API works in other contexts, the problem lies in how the browser is presenting credentials or tokens.
Your Function App likely uses some form of authentication (e.g., Azure AD, API Keys). The browser caches these tokens. If a cached token expires or becomes invalid, the request fails with a 403. Incognito mode or a different browser starts with a fresh session, acquiring a new, valid token. This is the most likely cause.
Which browser are you using?Are you getting the error on refreshing the page?
Are you seeing the issue in other browsers as well?
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Oriol Trujillo 0 Reputation points
2025-01-31T10:16:13.49+00:00 Hi @Khadeer Ali ,
Thanks a lot for your early reply! So glad to find a helping hand for this.
- Yes, i'm still getting the error on refreshing the page
- It happened to me in all the 3 browser that I tried: Chrome, Firefox & Edge.
And that's the thing, for some endpoints (like the one I'm using to report this error) I don't use any type of authentication per se, it's like auth / tokens free but the access is restricted because only inbound traffic from a Private Endpoint of the specified Virtual Network is allowed, and can only be accessed from a VPN (with its firewall and so on).
It's like the whole webapp (front, backend & other services) runs on an "intranet" only accessible from that VPN.
That's why I can't figure out why the access i being not permitted from certain browser sessions intermittenly.
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Khadeer Ali 2,770 Reputation points Microsoft Vendor
2025-01-31T14:57:52.71+00:00 Thanks for the details.
You're getting intermittent 403 Forbidden errors on your Azure Function App, even though it doesn't require authentication and is restricted by Private Endpoint/VNET. The fact that it works sometimes, including in Incognito, tells us it's probably not a simple DNS issue. It's more likely a transient network problem or something related to how your browser is handling connections.
While a simple network problem would likely affect Incognito too, more subtle network issues (router problems, firewall rules, or VPN-specific problems) could explain why Incognito sometimes works. It's still worthwhile to try restarting your network devices (router, computer, VPN) and checking for any network congestion. Even if Incognito works sometimes, a network problem is still a very plausible explanation for intermittent connectivity issues.
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