CVE-2020-8166 in Ruby on Rails
Summary
by MITRE
A CSRF forgery vulnerability exists in rails < 5.2.5, rails < 6.0.4 that makes it possible for an attacker to, given a global CSRF token such as the one present in the authenticity_token meta tag, forge a per-form CSRF token.
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Analysis
by VulDB Data Team • 07/03/2020
The vulnerability identified as CVE-2020-8166 represents a critical cross-site request forgery weakness affecting Ruby on Rails applications running versions prior to 5.2.5 and 6.0.4. This flaw specifically targets the application's CSRF protection mechanism, which is designed to prevent unauthorized commands from being executed on behalf of authenticated users. The vulnerability stems from how Rails handles global CSRF tokens, particularly those embedded within the authenticity_token meta tag that is commonly used in web applications to protect against CSRF attacks. When an attacker can obtain a global CSRF token, they can leverage this knowledge to construct valid per-form CSRF tokens, effectively bypassing the security controls that should protect against unauthorized form submissions.
The technical implementation of this vulnerability exploits the predictable nature of CSRF token generation within Rails applications. The flaw occurs because the system generates a global token that can be used as a building block for creating form-specific tokens, rather than maintaining proper isolation between different token types. This weakness allows attackers to craft malicious requests that appear legitimate to the Rails application's security mechanisms, as the forged tokens will pass validation checks that are meant to verify user authorization. The vulnerability specifically affects the way Rails handles token validation for forms, where the global token serves as a foundation for generating form-specific tokens that should normally be unpredictable and unique. According to CWE-352, this represents a classic cross-site request forgery vulnerability where the application fails to validate the origin of requests, allowing attackers to manipulate the request flow.
The operational impact of CVE-2020-8166 is significant for organizations running affected Rails applications, as it could enable attackers to perform unauthorized actions on behalf of authenticated users. This includes but is not limited to changing user passwords, making financial transactions, modifying application data, or executing administrative functions. The vulnerability is particularly dangerous because it operates at the application layer, where attackers can leverage existing authenticated sessions to perform actions that would normally require explicit user consent. The attack vector typically involves tricking users into visiting malicious websites or clicking on compromised links that automatically submit forms to the target application using the forged CSRF tokens. This aligns with ATT&CK technique T1566, which describes the use of social engineering to gain access to systems through various attack vectors including web-based exploits.
Organizations affected by this vulnerability should immediately implement mitigations including upgrading to Rails versions 5.2.5 or 6.0.4, which contain patches addressing the specific CSRF token generation flaw. The upgrade process should include thorough testing to ensure that existing application functionality remains intact while the security vulnerability is resolved. Additional protective measures include implementing proper Content Security Policy headers to limit the sources from which scripts can be loaded, and ensuring that applications properly validate the referer header in addition to CSRF tokens. Security teams should also conduct comprehensive vulnerability assessments to identify any other applications that might be running vulnerable versions of Rails or other frameworks that might be susceptible to similar token manipulation attacks. The fix implemented in the patched versions ensures that global CSRF tokens cannot be used to forge form-specific tokens, thereby restoring the intended security boundaries between different token types and preventing the exploitation pattern described in the vulnerability.