CVE-2026-93869 in Cotonti
Summary
by MITRE • 09/18/2026
Cotonti through 1.0.0 contains an open redirect vulnerability in the cot_url_check() function that validates redirect destinations using a regular expression lacking an end-of-string anchor. Attackers can bypass the redirect guard by supplying hostnames beginning with the site domain to redirect users to attacker-controlled hosts through the ratings plugin or other redirect callers.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in Cotonti versions up to 1.0.0 represents a classic open redirect flaw rooted in insufficient input validation within the cot_url_check() function. This specific routine is designed to sanitize and validate URLs intended for redirection, ensuring that users are not inadvertently or maliciously directed away from trusted domains without warning. The core technical deficiency lies in the regular expression used to verify whether a provided URL points to an allowed host. Specifically, the pattern fails to include an end-of-string anchor, such as $, at its conclusion. In regex syntax, this omission means that the validation logic only checks if the beginning of the input string matches the expected domain structure, rather than verifying that the entire string conforms to it. Consequently, any URL starting with the legitimate site's hostname is considered valid by the function, regardless of what follows after that initial match.
This technical oversight allows attackers to craft malicious payloads that bypass the redirect guard mechanism entirely. By appending arbitrary characters or paths immediately following the trusted domain name in a query parameter, an attacker can construct a link that appears safe during validation but resolves to a completely different destination when processed by the browser. For instance, if the site is example.com, an input of example.com.evil.com would pass the regex check because it starts with example.com, yet browsers interpret this as a request for the external domain evil.com. This bypass technique effectively neutralizes the protective intent of the cot_url_check() function, turning a security control into a vector for phishing and session hijacking attacks.
The operational impact of this vulnerability is significant due to its potential use in social engineering campaigns. Attackers can exploit this flaw through various entry points within the application, notably via the ratings plugin or other components that accept user-supplied redirect URLs. When a victim clicks on a crafted link containing the malicious payload, they are seamlessly redirected from the trusted Cotonti site to an attacker-controlled server. This seamless transition often tricks users into believing they are still interacting with the legitimate service, thereby increasing the likelihood of success for phishing attempts aimed at stealing credentials or installing malware. The lack of visual cues indicating a domain change exacerbates the risk, as modern browser indicators may not always clearly distinguish between subdomains and completely different hosts in all contexts.
From a classification perspective, this issue aligns with CWE-601, which defines URL Redirection to Untrusted Site (Open Redirect). It also maps to MITRE ATT&CK techniques related to phishing and credential harvesting, specifically those involving the abuse of trusted domains to lower user suspicion. The vulnerability highlights the critical importance of strict input validation in web applications that handle redirection logic. To mitigate this risk, developers must update the regular expression used in cot_url_check() to include an end-of-string anchor, ensuring that only URLs exactly matching the allowed domain structure are processed. Alternatively, implementing a whitelist-based approach where only specific paths on trusted domains are permitted would provide stronger protection against such bypasses. Until patched, administrators should monitor for unusual redirect patterns and consider restricting user-facing parameters that control navigation flow to reduce exposure to this attack vector.