Sporadic errors with authorization_code and PKCE

[emanuelxm]

Hi all,

So we have an issue with sporadic errors when trying to log in. Something like 3-4 times a day from a few hundred attempts. Checking into the logs, the error message is "Transformed code verifier does not match code challenge".

That is weird since at one point we started logging both verifier and challenge, and tried to test them, and we could log in.

We are using .NET 6 and Duende.IdentityServer 6.3.0, client is set up for authorization_code and required PKCE.
Here are some code verifiers and challenges that caused the issue:

verifier: Q6pFlrnkGBaC0OYhA99WIusVmvkLy-6UkKny8LZ6xlZTq_8oLK4_shf5RbymM8Bw
challenge: P1oY2AV2VfzrmLXx9Y29IUx6luPA-fdiq9oogHtDG34

verifier: xxHM58pmtzHdf98rBF9O1ef84lCLJVn3FxA46_CPNy5mgCBQLCxkh_oeDkdq2GpH
challenge: 02w3hZ531F9ctsKBcEtEQq3UxY8cSWnM_S5nYGe_-OY

verifier: iLyrYkP_1-JnW_mnXtbfrw4ayEEjzyauH45BDZuCsnajtHqa__F0YPLUuSMR13_V
challenge: PE9CFPWxhY2vNZgyjuYIaPoIPo2Y5rcfGxlLGWGhE9o

verifier: 2__1bE5EPFQ2eWrfJ4lm5U3W9TiKTmWPx~pmnUmgg6qbakaWvUrQ7lyMfsBpZTS9
challenge: 687PaDVqY6v13eJtUav2zviuOBz4tBEVkzvrCQtLtwU

challenge: IwfoaUevTNGERh6SquqhQmw64AeWIgYfajs7qTDUKyM
verifier: 4rpkg~V031Bd9jVu6cz1alcuZidA-X2jbWKlW3hynM3O0EJjqA93-JRxVrV8xDCG

Thanks

[wcabus]

We haven't seen this issue before, so I've tried to mimic your scenario by writing a small console application targeting .NET 6 and performing the logic against the verifier/challenge pairs you've provided, but the console app ran every sample flawlessly in multiple iterations (1 billion iterations).

I would definitely recommend upgrading your solution to .NET 8 or higher to exclude any issues related to .NET 6 which is already out of support since November 12, 2024.
Additionally, while you are updating, you should also upgrade Duende IdentityServer to v7 since our support for v6 also ended on the same date as .NET 6's end-of-life date.

[AndersAbel]

@emanuelxm We have another bug report that might be related to this. Having two reports to work on can help in cross-referencing common factors. We are currently investigating if the root cause of this could be something with the .NET version, Operating System and crypto libraries.

Could you please list a bit more about your environment? .NET Version? OpenSSL version (if applicable)? OS version? Hardware?

[AndersAbel]

@emanuelxm Just want to follow up on this, do you still see the error in your logs?

[emanuelxm]

Hi,

The errors are still happening, fewer than 10 per day from tens of thousands of login requests.
So the situation was that we switched from implicit flow to Authorization Code Flow with PKCE on this website.
After the switch, we started getting user Tickets that they can't log in with their account.
Authentication Server:
.NET 6
Duende.IdentityServer 6.3.0
Deployed: Azure Web App - OpenSSL 3.2.2 4 Jun 2024

App using AuthServer
PHP 8.3.27
Symfony 6.4.20
Deployed: Kubernets with Docker Containers (we use a web hosting provider) - OpenSSL 3.0.16 11 Feb 2025

I hope this helps.

[jimeshmandaliyamarutitech]

Hi team,

We are also seeing a very similar issue.

Our setup:

.NET 8
Duende IdentityServer upgraded from 7.2.1 to 7.4.3

After the upgrade, we started getting intermittent authentication failures with the error: "Transformed code verifier does not match code challenge"

This started happening suddenly after the IdentityServer upgrade. There were no changes on the client side — the way we generate and pass code_challenge, code_challenge_method, and code_verifier remains exactly the same.

The failures are rare but consistent (sporadic, hard to reproduce), similar to what’s described earlier in this thread.

[jimeshmandaliyamarutitech]

@wcabus As part of this update, we upgraded the following packages:

Updated
Duende.AccessTokenManagement: 3.0.1 → 4.1.0
Duende.IdentityServer.AspNetIdentity: 7.2.1 → 7.4.3
Duende.IdentityServer: 7.2.1 → 7.4.3
Duende.IdentityServer.EntityFramework: 7.2.1 → 7.4.3
Amazon.AspNetCore.DataProtection.SSM: 3.5.0 → 4.0.1
Microsoft.AspNetCore.Identity.EntityFrameworkCore: 8.0.15 -> 8.0.22

Added
Duende.AccessTokenManagement.OpenIdConnect: 7.4.3

The application is deployed on Kubernetes and runs on the aspnet:8.0-noble-chiseled-extra base image. The base image itself was not changed as part of this update.

[jimeshmandaliyamarutitech]

@wcabus We have identified the root cause, and it is not related to the Identity service.
In certain scenarios, NextAuth.js was reloading the page, which caused the same code verifier to be reused for different authorization code challenges. As a result, one of the requests failed due to a code verifier mismatch.

[skoruba]

Hi @jimeshmandaliyamarutitech , which version of NextAuth.js are you using?

[rohanvachheta]

Hi @skoruba,
Apologies for the confusion earlier. After deeper investigation, we’ve confirmed this was not related to NextAuth.js.

The root cause was actually in our own implementation. In certain scenarios, a method responsible for initiating the authorization request was being called twice on the same page load. As a result:

Two different code_challenge values were generated

Two different code_verifier values were generated

The requests overlapped and eventually conflicted at the token exchange stage

Because of this race/duplication, the code_verifier sent during the token request did not always match the originally associated code_challenge, which caused the intermittent:

"Transformed code verifier does not match code challenge."

Once we fixed the duplicate method invocation and ensured a single PKCE pair is generated per authorization request, the issue was resolved.