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Service Hostname Discovery Exploitation

High
jfritschi published GHSA-8q72-6qq8-xv64 Oct 31, 2022

Package

composer apereo/phpcas (Composer)

Affected versions

< 1.6.0

Patched versions

1.6.0

Description

Impact

The phpCAS library uses HTTP headers to determine the service URL used to validate tickets. This allows an attacker to control the host header and use a valid ticket granted for any authorized service in the same SSO realm (CAS server) to authenticate to the service protected by phpCAS.
Depending on the settings of the CAS server service registry in worst case this may be any other service URL (if the allowed URLs are configured to "^(https)://.*") or may be strictly limited to known and authorized services in the same SSO federation if proper URL service validation is applied.

This vulnerability may allow an attacker to gain access to a victim's account on a vulnerable CASified service without victim's knowledge, when the victim visits attacker's website while being logged in to the same CAS server.

Patch

phpCAS 1.6.0 is a major version upgrade that starts enforcing service URL discovery validation, because there is unfortunately no 100% safe default config to use in PHP. Starting this version, it is required to pass in an additional service base URL argument when constructing the client class.

For more information, please refer to the upgrading doc.

Workarounds

This vulnerability only impacts the CAS client that the phpCAS library protects against. The problematic service URL discovery behavior in phpCAS < 1.6.0 will only be disabled, and thus you are not impacted from it, if the phpCAS configuration has the following setup:

  1. phpCAS::setUrl() is called (a reminder that you have to pass in the full URL of the current page, rather than your service base URL), and
  2. phpCAS::setCallbackURL() is called, only when the proxy mode is enabled.
  3. Alternatively, if your PHP's HTTP header input X-Forwarded-Host, X-Forwarded-Server, Host, X-Forwarded-Proto, X-Forwarded-Protocol is sanitized before reaching PHP (by a reverse proxy, for example), you will not be impacted by this vulnerability.

Otherwise, you should upgrade the library to get the safe service discovery behavior.

If your CAS server service registry is configured to only allow known and trusted service URLs, the severity of the vulnerability is reduced substantially since an attacker must be in control of another authorized service.

Acknowledgement

We would like to thank Filip Hejsek for discovering this vulnerability, responsibly reporting it to the developers, and helping harden the patch.

Henry Pan and Joachim Fritschi helped with the patch and release effort as phpCAS developers.

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H

CVE ID

CVE-2022-39369