PKCE Verification Failed: 5 Causes and How to Debug Each One

OAuth 2.0 applications, particularly mobile and single-page applications, increasingly depend on Proof Key for Code Exchange (PKCE) as a primary security measure to prevent authorization code interception. Despite its apparent simplicity, PKCE implementations can fail due to several common issues, including incorrect verifier length, inappropriate encoding, or mismanagement of session storage across context switches. These failures manifest as server-side errors such as "invalid_grant" when the code_verifier does not match the registered code_challenge. The article explores the typical causes of PKCE failures, offering practical debugging tips and solutions in JavaScript, Swift, and Kotlin. It highlights that many bugs arise from hand-rolled PKCE implementations, suggesting the use of libraries like AppAuth-iOS, AppAuth-Android, and openid-client, which adhere to RFC 7636 standards, to ensure correct PKCE handling.

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