CVE-2022-39369
Service Hostname Discovery Exploitation in phpCAS
In short
phpCAS uses HTTP headers to determine service URLs without proper validation, allowing attackers to redirect valid CAS authentication tickets to unauthorized services. An attacker can trick logged-in users into visiting their site to gain access to victim accounts on other services in the same SSO realm.
Technical detail
The vulnerability exists in phpCAS versions prior to 1.6.0, which derive the service URL from HTTP headers (Host header) without sufficient validation. An attacker can perform a host header injection attack, leveraging a valid CAS ticket obtained for one service to authenticate to a different service within the same CAS federation. Impact ranges from limited (if the CAS server restricts allowed URLs) to critical (if wildcard URL patterns like '^(https)://.*' are configured), potentially enabling unauthorized account access across multiple services in the SSO realm.
Summary generated and translated by AI from the official description.
phpCAS is an authentication library that allows PHP applications to easily authenticate users via a Central Authentication Service (CAS) server. 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. 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. 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. 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 either. If your CAS server service registry is configured to only allow known and trusted service URLs the severity of the vulnerability is reduced substantially in its severity since an attacker must be in control of another authorized service. Otherwise, you should upgrade the library to get the safe service discovery behavior.
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H