CVE-2026-100523 Overview
CVE-2026-100523 is an open redirect vulnerability [CWE-601] affecting Cotonti content management system through version 1.0.0. The flaw resides in message.php, which base64-decodes the redirect parameter without validating the target domain. Unauthenticated attackers can craft links containing encoded external URLs that trigger a meta refresh redirect to arbitrary destinations. This behavior enables phishing campaigns that leverage the trust of the originating Cotonti domain to redirect victims to attacker-controlled sites.
Critical Impact
Attackers can abuse trusted Cotonti-hosted URLs to redirect users to phishing or malware distribution sites without authentication or user interaction beyond clicking a link.
Affected Products
- Cotonti CMS through version 1.0.0
- message.php component handling the redirect parameter
- system/common.php supporting redirect processing logic
Discovery Timeline
- 2026-09-26 - CVE-2026-100523 published to NVD
- 2026-09-26 - Last updated in NVD database
Technical Details for CVE-2026-100523
Vulnerability Analysis
The vulnerability exists in Cotonti's message.php script, which processes a user-supplied redirect parameter. The script applies base64 decoding to the parameter value and then issues a meta refresh redirect to the decoded URL. No allowlist, domain comparison, or same-origin check is performed before emitting the redirect. As a result, any attacker-controlled URL can be encoded and passed through the parameter to force a redirect to an external domain. The classification maps to CWE-601: URL Redirection to Untrusted Site.
Root Cause
The root cause is missing output destination validation. The redirect handler trusts the decoded parameter value as a safe URL and inserts it into a meta refresh tag. Base64 encoding obscures the final destination from users and simple URL inspection tools, which weakens link-based warnings and user judgement. See the Cotonti message.php source and the related common.php for the affected logic.
Attack Vector
An attacker base64-encodes a malicious external URL and appends it as the redirect parameter to a legitimate Cotonti message.php URL. The victim clicks the trusted-looking link, Cotonti renders a page containing a meta refresh pointing at the decoded destination, and the browser transitions to the attacker site. The attack requires user interaction (clicking the link) but no authentication. Typical abuse scenarios include credential phishing pages that mimic the originating site and drive-by malware delivery.
No verified proof-of-concept code is published; refer to the VulnCheck advisory and Cotonti Issue #1907 for additional technical context.
Detection Methods for CVE-2026-100523
Indicators of Compromise
- Web server access logs containing requests to message.php with a redirect parameter whose value is base64-encoded.
- Decoded redirect values that resolve to domains outside the hosting organization.
- Referrer logs on external domains showing inbound traffic from message.php URLs on Cotonti sites.
- User reports of unexpected redirection after clicking links that appeared to originate from a Cotonti-hosted site.
Detection Strategies
- Parse web access logs for requests to message.php, base64-decode the redirect parameter, and alert when the decoded host does not match the site's own domain allowlist.
- Deploy a web application firewall rule that inspects decoded parameter values and blocks requests pointing to external hosts.
- Correlate outbound click-through telemetry with inbound requests to identify suspicious redirect chains.
Monitoring Recommendations
- Monitor spikes in traffic to message.php with unusually long or encoded redirect parameter values.
- Track user-agent and IP distributions for requests to the affected endpoint to spot phishing distribution campaigns.
- Alert on repeated requests from the same source IPs that resolve to known phishing infrastructure.
How to Mitigate CVE-2026-100523
Immediate Actions Required
- Apply the upstream fix referenced in Cotonti Pull Request #1908 as soon as it is merged into a release.
- Add a web application firewall rule that rejects requests to message.php where the base64-decoded redirect parameter resolves to an off-site domain.
- Audit outbound links generated by message.php and warn users that externally-hosted redirect targets are not endorsed by the site.
Patch Information
The fix is tracked in Cotonti Pull Request #1908 against Cotonti Issue #1907. Administrators should track the Cotonti GitHub repository for a released version that incorporates the patch and upgrade from 1.0.0 once available.
Workarounds
- Patch message.php locally to validate the decoded redirect value against an allowlist of trusted domains before emitting the meta refresh.
- Strip or ignore the redirect parameter at the reverse proxy or WAF layer when its decoded value is not same-origin.
- Display an interstitial confirmation page that shows the decoded destination URL to the user before any automatic redirect.
- Restrict access to message.php behind authentication where feasible to reduce unauthenticated phishing abuse.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
