Summary
Possible Open Redirect Vulnerability in Action Pack
There is a possible Open Redirect Vulnerability in Action Pack.
Versions Affected: >= v6.1.0.rc2
Not affected: < v6.1.0.rc2
Fixed Versions: 6.1.3.2
Impact
This is similar to CVE-2021-22881. Specially crafted Host headers in combination with certain "allowed host" formats can cause the Host Authorization middleware in Action Pack to redirect users to a malicious
website.
Since rails/rails@9bc7ea5, strings in config.hosts that do not have a leading dot are converted to regular expressions without proper escaping. This causes, for example, config.hosts << "sub.example.com" to permit a request with a Host header value of sub-example.com.
Releases
The fixed releases are available at the normal locations.
Workarounds
The following monkey patch put in an initializer can be used as a workaround.
class ActionDispatch::HostAuthorization::Permissions
def sanitize_string(host)
if host.start_with?(".")
/\A(.+\.)?#{Regexp.escape(host[1..-1])}\z/i
else
/\A#{Regexp.escape host}\z/i
end
end
end
Patches
To aid users who aren't able to upgrade immediately we have provided patches for the two supported release series. They are in git-am format and consist of a single changeset.
- 6-1-open-redirect.patch - Patch for 6.1 series
Please note that only the 6.1.Z, 6.0.Z, and 5.2.Z series are supported at present. Users of earlier unsupported releases are advised to upgrade as soon as possible as we cannot guarantee the continued availability of security fixes for unsupported releases.
Credits
Thanks Jonathan Hefner (https://hackerone.com/jonathanhefner) for reporting this bug!
Impact
Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.
CVE-2021-22903 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.1.3.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2021-22903? CVE-2021-22903 is a medium-severity open redirect vulnerability in actionpack (rubygems), affecting versions >= 6.1.0.rc2, < 6.1.3.2. It is fixed in 6.1.3.2. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
- How severe is CVE-2021-22903? CVE-2021-22903 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of actionpack are affected by CVE-2021-22903? actionpack (rubygems) versions >= 6.1.0.rc2, < 6.1.3.2 is affected.
- Is there a fix for CVE-2021-22903? Yes. CVE-2021-22903 is fixed in 6.1.3.2. Upgrade to this version or later.
- Is CVE-2021-22903 exploitable, and should I be worried? Whether CVE-2021-22903 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-22903 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-22903? Upgrade
actionpackto 6.1.3.2 or later.