CVE-2021-23435 in clearance
Summary
by MITRE • 09/13/2021
This affects the package clearance before 2.5.0. The vulnerability can be possible when users are able to set the value of session[:return_to]. If the value used for return_to contains multiple leading slashes (/////example.com) the user ends up being redirected to the external domain that comes after the slashes (http://example.com).
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Analysis
by VulDB Data Team • 09/16/2021
The vulnerability identified as CVE-2021-23435 represents a critical path traversal issue within the clearance package version prior to 2.5.0, specifically impacting session handling mechanisms that process the return_to parameter. This flaw enables malicious actors to manipulate session state by injecting specially crafted URLs containing multiple leading slashes, creating a dangerous redirection scenario that bypasses normal security controls. The vulnerability stems from inadequate input validation and sanitization of user-supplied session data, particularly when processing redirect URLs that are meant to maintain user context after authentication or authorization flows.
The technical implementation of this vulnerability exploits the way the clearance package handles URL parsing and validation when processing the session[:return_to] parameter. When a user provides a value such as /////example.com, the application fails to properly normalize or validate the URL structure, allowing the system to interpret the multiple leading slashes as part of the URL scheme rather than as malformed input. This parsing error creates a condition where the application treats the portion of the URL following the slashes as a legitimate external domain, resulting in an unintended redirect to the attacker-controlled external site. The flaw essentially allows for open redirect vulnerabilities where the application's own redirect logic becomes compromised due to insufficient sanitization of user input.
From an operational impact perspective, this vulnerability creates significant security risks including potential phishing attacks, credential theft, and unauthorized access to external systems. Attackers can craft malicious links that appear legitimate to users while redirecting them to malicious domains, potentially leading to credential harvesting or malware distribution. The vulnerability also enables bypassing of security controls that rely on proper redirect handling, such as those implemented in authentication flows, single sign-on systems, or application security policies. This type of vulnerability can be particularly dangerous in enterprise environments where users may trust the application's interface and follow redirects without questioning their legitimacy, making it a prime target for social engineering campaigns.
The vulnerability maps directly to CWE-601 Open Redirect and CWE-20 Improper Input Validation, with potential ATT&CK mappings to T1566 Phishing and T1071.004 Application Layer Protocol: DNS. Organizations should immediately upgrade to clearance version 2.5.0 or later to address this vulnerability, as the fix typically includes proper URL normalization and input validation for session redirect parameters. Additionally, implementing strict URL validation routines, sanitizing user input before processing session data, and employing proper redirect whitelisting mechanisms can provide additional defense-in-depth measures. Security teams should also conduct thorough audits of session handling code and implement monitoring for suspicious redirect patterns, particularly those involving multiple leading slashes or unusual URL structures that could indicate exploitation attempts.