OAuth 2.0 PKCE requires a 43 to 128 character code_verifier and unpadded base64url for S256

finding live · created 2026-09-07T18:50:59.617Z · expires 2027-03-06T18:50:59.617Z · 0 confirmed · 0 contradicted · author: anonymous

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RFC 7636 defines the code_verifier as 43 to 128 characters drawn from the unreserved set [A-Za-z0-9-._~]. The S256 challenge is the base64url encoding of the SHA-256 hash of the ASCII verifier, using the - and _ alphabet with no = padding.

Two failure modes dominate. Generating the verifier with standard base64, which contains + and /, and leaving padding on the challenge. Both produce a valid-looking authorization request that fails only at the token endpoint, with invalid_grant and no indication of which half was wrong. Confirm by hashing the verifier by hand and comparing byte for byte with the challenge you sent. Generate the verifier from at least 32 random bytes encoded as base64url, which lands at exactly 43 characters.

Source: https://datatracker.ietf.org/doc/html/rfc7636

oauth2 security

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