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Fix: Send the PKCE code verifier instead of the challenge in the OAuth2 token request - #167
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…h2 token request The OAuth2 session sent the S256 code challenge as both the code_challenge in the authorization request and the code_verifier in the token request, so the secret verifier RFC 7636 relies on was never used. Generate a random verifier, derive the challenge from it as BASE64URL(SHA256(verifier)), and send the verifier with the token request. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
rafaeljusto
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Tested this change with our MCP and the DCR flow looks good 👍
chrisrollins65
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Nov 6, 2026
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rollins wanna run claude through it ? lgtm anyway
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Yeah I always run claude on PRs before I approve them now, so already done. |
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What
session.OAuth2sent the same value as both thecode_challengein the authorization request and thecode_verifierin the token request. It hashed random bytes into an S256 challenge, used that challenge for both, and discarded the random input. The secret verifier that PKCE (RFC 7636) depends on was never sent. Anyone who saw the authorization request had everything needed to redeem the code, and a server that checks PKCE correctly rejects the token request.Change
generatePKCEreturns a verifier and its challenge:BASE64URL(SHA256(verifier)), per §4.2code_challenge_method=S256, as before), and the verifier in the token request.Tests
New
session/oauth2_test.go:TestGeneratePKCE: the verifier matches the RFC 7636 character set and length; the challenge is the S256 transform of the verifier; the verifier differs from the challenge and between calls.TestRetrieveTokenSendsVerifier: the token request posts the verifier ascode_verifier, checked against a local test server.go test ./...,go vet ./...andgolangci-lint run ./...pass.🤖 Generated with Claude Code