Add support for GitHub OAuth2 authentication

[Copilot]

Pelican assumed OIDC with full discovery support. GitHub implements OAuth2 without OIDC extensions, causing three failure modes: metadata discovery overriding explicit endpoints, ID token parsing crashes, and required OIDC claims (sub, iss) failing authentication.

Changes

Respect explicit endpoint configuration

• config/oidc_metadata.go: Check IsSet() before overwriting endpoints from discovery metadata
• Enables manual configuration of OAuth2 endpoints when auto-discovery fails or is unsupported

Handle missing OIDC ID tokens

• web_ui/oauth2_client.go: Validate ID token is non-empty string before JWT parsing
• OAuth2 providers don't issue OIDC ID tokens; previous code crashed on empty string

Make OIDC claims optional with fallbacks

• New params Issuer.OIDCSubjectClaim and Issuer.OIDCIssuerClaim (defaults: sub, iss)
• Subject: support numeric claims (GitHub user IDs), fallback to username if missing
• Issuer: fallback to OIDC.Issuer config or authorization endpoint hostname
• Numeric claim handling uses int64 conversion to avoid float precision loss

Example Configuration

OIDC:
  Issuer: https://github.com
  AuthorizationEndpoint: https://github.com/login/oauth/authorize
  TokenEndpoint: https://github.com/login/oauth/access_token
  UserInfoEndpoint: https://api.github.com/user

Issuer:
  OIDCAuthenticationUserClaim: login  # GitHub uses "login", not "sub"
  OIDCSubjectClaim: id                # GitHub's numeric user ID

See docs/github-oauth-config-example.yaml for complete configuration.

Backward compatible: existing OIDC providers (CILogon, Globus) work unchanged.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• abc123
  • Triggering command: /tmp/go-build857477745/b845/config.test /tmp/go-build857477745/b845/config.test -test.testlogfile=/tmp/go-build857477745/b845/testlog.txt -test.paniconexit0 -test.timeout=10m0s -m64 -mtune=generic -march=x86-64 -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -ato�� g_.a om/!azure/azure-sdk-for-go/sdk/a-ifaceassert ux_amd64/vet -errorsas /storage/remote/--64 -nilfunc ux_amd64/vet (dns block)
  • Triggering command: /tmp/go-build1685999187/b845/config.test /tmp/go-build1685999187/b845/config.test -test.testlogfile=/tmp/go-build1685999187/b845/testlog.txt -test.paniconexit0 -test.timeout=10m0s -m64 -mtune=generic -march=x86-64 -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -p pkg/mod/go.mongodb.org/[email protected]/bson-p pkg/mod/go.mongodb.org/[email protected]/bsonmain ux_amd64/compile -I /tmp/go-build202--64 -I ux_amd64/compile/tmp/ccGGdC8B.o (dns block)
• cilogon.org
  • Triggering command: /tmp/go-build2144026736/b001/config.test /tmp/go-build2144026736/b001/config.test -test.testlogfile=/tmp/go-build2144026736/b001/testlog.txt -test.paniconexit0 -test.timeout=10m0s -test.v=true -test.run=TestGetMetadataRespectsExplicitEndpoints --local ux_amd64/compile credential.helpe/opt/hostedtoolcache/go/1.24.11/x64/pkg/tool/linux_amd64/vet (dns block)
  • Triggering command: /tmp/go-build857477745/b845/config.test /tmp/go-build857477745/b845/config.test -test.testlogfile=/tmp/go-build857477745/b845/testlog.txt -test.paniconexit0 -test.timeout=10m0s -m64 -mtune=generic -march=x86-64 -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -ato�� g_.a om/!azure/azure-sdk-for-go/sdk/a-ifaceassert ux_amd64/vet -errorsas /storage/remote/--64 -nilfunc ux_amd64/vet (dns block)
  • Triggering command: /tmp/go-build1685999187/b845/config.test /tmp/go-build1685999187/b845/config.test -test.testlogfile=/tmp/go-build1685999187/b845/testlog.txt -test.paniconexit0 -test.timeout=10m0s -m64 -mtune=generic -march=x86-64 -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -p pkg/mod/go.mongodb.org/[email protected]/bson-p pkg/mod/go.mongodb.org/[email protected]/bsonmain ux_amd64/compile -I /tmp/go-build202--64 -I ux_amd64/compile/tmp/ccGGdC8B.o (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for authenticating via GitHub</issue_title>
<issue_description>## Summary

Currently, a Pelican service can (in theory) use any identity provider that implements OIDC to authenticate users. Unfortunately, the GitHub identity provider implements only OAuth2, and there seems to be no way of using Pelican's current configuration knobs to make it authenticate users via GitHub. Which is to say, there are several bugs and assumptions in Pelican that need to be addressed in order to use GitHub as an identity provider.

For reference, when attempting to set this up: Documentation for GitHub's OAuth2 and related API endpoints.

Details

Bug: OIDC.Issuer overrides explicitly set OIDC endpoints

Given the following configuration

OIDC:                                                                           
  AuthorizationEndpoint: https://github.com/login/oauth/authorize               
  DeviceAuthEndpoint: https://github.com/login/device/code                      
  TokenEndpoint: https://github.com/login/oauth/access_token                    
  UserInfoEndpoint: https://api.github.com/user

the default value of https://cilogon.org for OIDC.Issuer overrides OIDC.AuthorizationEndpoint via what I assume is CILogon's metadata document at https://cilogon.org/.well-known/openid-configuration. I must explicitly set

OIDC:
  Issuer: null

in order for the explicitly set endpoints to be respected — which strikes me as odd and unnecessary (why is Pelican trying to look up a value I already gave it?).

Bug: Bad check for missing OIDC ID token

The next bug is that the auth process crashes out trying to parse an ID token that does not actually exist:

pelican/web_ui/oauth2_client.go

Line 390 in 816a58e

if idTokenRaw := token.Extra("id_token"); idTokenRaw != nil {

When the token does not contain the id_token field, .Extra is seemingly returning an empty string (""). We need to add a check for that.

Unfortunate assumption: The user info endpoint returns a sub claim

Following that, even if we configure the "user" claim to something reasonable for GitHub

Issuer:
  OIDCAuthenticationUserClaim: login

we still end up crashing out of the auth process because Pelican assumes that the user info endpoint is OIDC-compliant and responds with a sub claim:

pelican/web_ui/oauth2_client.go

Lines 231 to 236 in 816a58e

	subIface, ok := claimsSource["sub"]
	if !ok {
	log.Errorln("User info endpoint did not return a value for the sub claim")
	err = errors.New("identity provider did not return a subject for logged-in user")
	return
	}

Another unfortunate assumption: The user info endpoint returns "groups"

In the case of GitHub, we actually need to query another API endpoint and parse the response to get something resembling "group memberships."

Other remarks

GitHub does have an OIDC provider, but it is aimed at authenticating workflow actions. For example, the issuer's metadata includes no endpoints.</issue_description>

<agent_instructions>Please try to address as many of these concerns as possible. It appears one problem is that the code assumes OIDC with full discovery while GitHub might only do OAuth2 (or perhaps it assumes some shortcuts for OIDC such as tokens that are JWT!).

Review GitHub documentation to try and determine any other potential problems. The reporter is not a developer so we'll need to give them a fairly good approach. Include regression tests for the items they identified.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add support for authenticating via GitHub #2898

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