CVE-2018-8899 in IdentityServer4
Summary
by MITRE
IdentityServer IdentityServer4 1.x before 1.5.3 and 2.x before 2.1.3 does not encode the redirect URI on the authorization response page, which might lead to XSS in some configurations.
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Analysis
by VulDB Data Team • 02/22/2023
The vulnerability identified as CVE-2018-8899 affects IdentityServer4 versions prior to 1.5.3 and 2.1.3, representing a critical security flaw in the authentication framework's handling of redirect URIs during authorization responses. This issue stems from insufficient input validation and output encoding mechanisms within the authorization response page, creating a potential cross-site scripting vulnerability that could be exploited by malicious actors. The flaw specifically manifests when the system fails to properly encode redirect URIs before rendering them in the authorization response page, allowing attackers to inject malicious scripts through crafted redirect parameters.
The technical implementation of this vulnerability involves the improper handling of user-supplied redirect URI values within the authorization response mechanism. When IdentityServer4 processes authorization requests and generates responses containing redirect URIs, it does not adequately sanitize or encode these values before displaying them in the HTML output. This creates an environment where an attacker can manipulate the redirect URI parameter to include malicious script content, which then gets executed in the context of the victim's browser when the authorization response page renders. The vulnerability is particularly concerning because it occurs in the core authentication flow where user credentials and session information are typically handled, making it a prime target for exploitation.
The operational impact of this vulnerability extends beyond simple script execution, as it can potentially lead to session hijacking, credential theft, and unauthorized access to protected resources within the affected systems. Attackers exploiting this flaw could redirect users to malicious domains while maintaining the appearance of legitimate authentication flows, enabling them to capture sensitive information or perform unauthorized actions on behalf of authenticated users. The vulnerability's severity is amplified by the fact that it affects widely used IdentityServer4 versions, potentially exposing numerous applications and services that rely on this authentication framework for securing access to their resources.
Organizations affected by CVE-2018-8899 should implement immediate mitigations including upgrading to patched versions of IdentityServer4, specifically versions 1.5.3 or later for the 1.x series and 2.1.3 or later for the 2.x series. Additionally, administrators should review their redirect URI configurations to ensure that only trusted domains are permitted and implement proper input validation and output encoding mechanisms throughout the authorization response handling process. The vulnerability aligns with CWE-79, which specifically addresses cross-site scripting flaws, and represents a common pattern in web application security where insufficient output encoding creates exploitable conditions. From an attack perspective, this vulnerability maps to ATT&CK technique T1566.001, which involves social engineering through malicious links, as attackers could craft malicious redirect URIs to deceive users into executing harmful scripts within the authentication context.