CVE-2020-15233 is a medium-severity (CVSS 4.8) Improper Input Validation vulnerability in github.com/ory/fosite. A fix is available for github.com/ory/fosite — see the affected versions and patch details below.
OAuth2 Redirect URL validity does not respect query parameters and character casing for loopback addresses
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2020-15233 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
github.com/ory/fositeReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Impact
fosite#400 (released as v0.30.2) introduced a new feature for handling redirect URLs pointing to loopback interfaces (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
- Registering a client with allowed redirect URL
http://127.0.0.1/callback - Performing OAuth2 flow and requesting redirect URL
http://127.0.0.1/callback?bar=foo - Instead of an error, the browser is redirected to
http://127.0.0.1/callback?bar=foowith a potentially successful OAuth2 response.
as well as the host parameter (as long as the host is a loopback interface):
- Registering a client with allowed redirect URL
https://example.com/callback - Performing OAuth2 flow and requesting redirect URL
http://127.0.0.1/callback - Instead of an error, the browser is redirected to
http://127.0.0.1/callbackwith 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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/ory/fosite | ≥ 0.30.3&&< 0.34.1 | 0.34.1go get github.com/ory/fosite@v0.34.1 |
Affected Products
fositeoryDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/ory/fosite, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/ory/fosite to 0.34.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-15233 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.
Frequently Asked Questions
Is CVE-2020-15233 in your dependencies?
Find it across Go, including transitive dependencies.