Reported by @pieterphilippaerts
PKCE downgrade attack in authentik
PKCE is a very important countermeasure in OAuth2 , both for public and confidential clients. It protects against CSRF attacks and code injection attacks. Because of this bug, an attacker can circumvent the protection PKCE offers.
authentik 2023.8.7 and 2023.10.7 fix this issue.
There is a bug in our implementation of PKCE that allows an attacker to circumvent the protection that PKCE offers. PKCE adds the
code_challenge’ parameter to the authorization request and adds the code_verifier’ parameter to the token request. We recently fixed a downgrade attack (in v2023.8.5 and 2023.10.4) where if the attacker removed the
code_verifier’ parameter in the token request, authentik would allow the request to pass, thus circumventing PKCE’s protection. However, in the latest version of the software, another downgrade scenario is still possible: if the attacker removes the code_challenge’ parameter from the authorization request, authentik will also not do the PKCE check.
Note that this type of downgrade enables an attacker to perform a code injection attack, even if the OAuth client is using PKCE (which is supposed to protect against code injection attacks). To start the attack, the attacker must initiate the authorization process without that
code_challenge’ parameter in the authorization request. But this is easy to do (just use a phishing site or email to trick the user into clicking on a link that the attacker controls – the authorization link without that code_challenge’ parameter).
The OAuth BCP (https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics) explicitly mentions this particular attack in section 2.1.1: “Authorization servers MUST mitigate PKCE Downgrade Attacks by ensuring that a token request containing a code_verifier parameter is accepted only if a code_challenge parameter was present in the authorization request, see Section 4.8.2 for details.”
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