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MEDIUM severity

CVE-2021-23435 — clearance

MEDIUMFix: thoughtbot/clearance#945

CVE-2021-23435 is a medium-severity (CVSS 6.1) Open Redirect vulnerability in clearance. A fix is available for clearance — see the affected versions and patch details below.

Clearance Gem Open Redirect Vulnerability

Also known asGHSA-4hpq-rjcx-7vj9SNYK-RUBY-CLEARANCE-1577284
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Sep 30, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.7%probability of exploitation in next 30 days
Lower Risk+0.01%
Lower risk than most CVEs51th percentile — riskier than 51% of all scored CVEsHighest risk
0.00%0.40%0.80%1.20%0.3%0.7%Apr 26Aug 26Sep 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-23435 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
💎clearance

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

This affects the package clearance before 2.5.0. The vulnerability can be possible when users are able to set the value of session[:return_to]. If the value used for return_to contains multiple leading slashes (/////example.com) the user ends up being redirected to the external domain that comes after the slashes (http://example.com).

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
💎RubyGemsclearanceall versions2.5.0bundle update clearance --conservative

Affected Products

1 product · 1 configurations
Application
clearancethoughtbot
< 2.5.0
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for clearance, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update clearance to 2.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23435 is resolved across your whole dependency graph.

  3. 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

This affects the package clearance before 2.5.0. The vulnerability can be possible when users are able to set the value of session[:return_to]. If the value used for return_to contains multiple leading slashes (/////example.com) the user ends up being redirected to the external domain that comes after the slashes (http://example.com).
O3 Security · Impact-Aware SCA

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CVE-2021-23435: clearance — Fixed in 2.5.0