---
id: CVE-2020-15233
aliases:
  - GHSA-rfq3-w54c-f9q5
title: >-
  OAuth2 Redirect URL validity does not respect query parameters and character
  casing for loopback addresses
summary: >-
  OAuth2 Redirect URL validity does not respect query parameters and character
  casing for loopback addresses
severity: medium
cvss: 6.1
cvssVector: 'CVSS:3.0/AV:N/AC:L/PR:H/UI:R/S:U/C:H/I:H/A:N/E:P/RL:O/RC:C'
vendor: ory
product: github.com/ory/fosite
ecosystem: go
affected:
  - 'github.com/ory/fosite >= 0.30.3, < 0.34.1'
patched:
  - github.com/ory/fosite 0.34.1
published: '2021-05-24'
updated: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-rfq3-w54c-f9q5'
references:
  - url: 'https://github.com/ory/fosite/security/advisories/GHSA-rfq3-w54c-f9q5'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2020-15233'
  - url: 'https://github.com/ory/fosite/pull/400'
  - url: >-
      https://github.com/ory/fosite/commit/cdee51ebe721bfc8acca0fd0b86b030ca70867bf
  - url: 'https://github.com/ory/fosite'
tags:
  - osv
  - go
epss: 0.008
epssPercentile: 0.54641
ingestedAt: '2026-07-09T18:56:36.902Z'
---

## Overview

### Impact

[fosite#400](https://github.com/ory/fosite/pull/400) (released as v0.30.2) introduced a new feature for handling redirect URLs pointing to loopback interfaces ([rfc8252#section-7.3](https://tools.ietf.org/html/rfc8252#section-7.3)). As part of that change new behavior was introduced which failed to respect the redirect URL's (only for loopback interfaces!) query parameters

1. Registering a client with allowed redirect URL `http://127.0.0.1/callback`
2. Performing OAuth2 flow and requesting redirect URL `http://127.0.0.1/callback?bar=foo`
3. Instead of an error, the browser is redirected to `http://127.0.0.1/callback?bar=foo` with a potentially successful OAuth2 response.

as well as the host parameter (as long as the host is a loopback interface):

1. Registering a client with allowed redirect URL `https://example.com/callback`
2. Performing OAuth2 flow and requesting redirect URL `http://127.0.0.1/callback`
3. Instead of an error, the browser is redirected to `http://127.0.0.1/callback` with a potentially successful OAuth2 response.

These bugs are only applicable in scenarios where the attacker has control over the loopback interface (`localhost`, `127.0.0.1`, `[::1]`) where the browser performing the OAuth2 flow is running.

## Affected packages

- `github.com/ory/fosite >= 0.30.3, < 0.34.1`

## Remediation

Upgrade to a patched release:

- `github.com/ory/fosite 0.34.1`