CVE-2021-23387 in trailing-slash
Summary
by MITRE • 05/25/2021
The package trailing-slash before 2.0.1 are vulnerable to Open Redirect via the use of trailing double slashes in the URL when accessing the vulnerable endpoint (such as https://example.com//attacker.example/). The vulnerable code is in index.js::createTrailing(), as the web server uses relative URLs instead of absolute URLs.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 05/27/2021
The vulnerability identified as CVE-2021-23387 affects the trailing-slash npm package version 2.0.1 and earlier, presenting a significant open redirect flaw that can be exploited by attackers to manipulate user navigation. This vulnerability stems from improper handling of URL redirection when trailing double slashes are present in web requests, creating a pathway for malicious actors to redirect users to arbitrary destinations. The flaw specifically manifests in the index.js file within the createTrailing() function where the web server processes relative URLs rather than absolute URLs, allowing for unsafe redirection patterns that can be leveraged in phishing attacks or malicious link manipulation.
The technical implementation of this vulnerability relies on the package's failure to properly sanitize or validate URL paths that contain double slashes, which creates a condition where a malicious user can craft a URL such as https://example.com//attacker.example/ to redirect users to attacker-controlled domains. This behavior occurs because the package's URL handling logic does not adequately distinguish between legitimate path separators and potentially malicious redirection sequences. The vulnerability falls under CWE-601 Open Redirect, which is categorized under the broader category of insecure direct object references and represents a well-known weakness in web applications that can be exploited to redirect users to malicious websites. The flaw is particularly concerning because it operates at the URL processing level where relative path resolution can be manipulated to bypass normal security controls.
From an operational perspective, this vulnerability poses a serious risk to web applications that utilize the trailing-slash package for URL management or redirection purposes. Attackers can exploit this weakness to create convincing phishing campaigns by redirecting users from legitimate domains to malicious sites that appear to be extensions of the original domain. The impact extends beyond simple redirection as it can enable more sophisticated attacks such as credential theft, malware distribution, or social engineering campaigns that rely on the trust users place in legitimate domain names. The vulnerability is particularly dangerous in environments where users may not immediately notice the domain change, especially when the malicious domain closely mimics the legitimate one through visual similarities or when the redirection occurs in contexts where users might not carefully inspect URLs. Organizations using this package in production environments face potential reputational damage, user trust erosion, and possible regulatory compliance issues if user data is compromised through these redirect attacks.
The recommended mitigation strategy involves upgrading to version 2.0.1 or later of the trailing-slash package where the vulnerability has been addressed through proper URL sanitization and absolute URL handling. Security teams should also implement comprehensive URL validation at the application level, ensuring that all redirections are properly validated and that relative URLs are not used in contexts where they could be manipulated for malicious purposes. Additional defensive measures include implementing proper input validation for URL parameters, using absolute URLs in all redirection scenarios, and monitoring for suspicious redirection patterns in application logs. Organizations should also consider implementing web application firewalls that can detect and block known patterns of open redirect attacks, and conduct regular security assessments to identify similar vulnerabilities in other components of their web infrastructure. The remediation process should include thorough testing to ensure that the upgrade does not introduce compatibility issues with existing applications while also verifying that the fix properly addresses the specific conditions that led to the vulnerability.