A flaw was found in oauthlib. A timing side-channel vulnerability in the Proof Key for Code Exchange (PKCE) validation process allows a remote attacker to recover secret verification codes. Because comparison operations do not execute in constant time, an attacker who has intercepted an authorization code can measure response latency differences across repeated requests to infer the expected verification code character-by-character. Successful exploitation allows the attacker to exchange the authorization code for an access token and impersonate the victim user.