CVE-2021-21265 is a vulnerability identified in October CMS, an open-source content management system built on the Laravel PHP framework. The issue affects versions prior to 1.1.2 and stems from improper neutralization of HTTP headers, specifically related to Host Header Poisoning attacks (CWE-644). This vulnerability arises when October CMS is deployed on servers that are misconfigured to route any incoming HTTP request, regardless of the Host header value, to the October CMS instance. Under such conditions, an attacker can manipulate the Host header in HTTP requests to inject malicious scripting syntax or manipulate application behavior. This can lead to integrity violations such as cache poisoning, password reset poisoning, or web cache deception attacks. The vulnerability does not directly impact confidentiality or availability but can compromise the integrity of the application’s responses and potentially lead to further exploitation. The issue was addressed in October CMS version 1.1.2 by introducing a configuration feature allowing administrators to specify a set of trusted hosts, effectively validating incoming Host headers against this whitelist. Additionally, a workaround involves setting the cms.linkPolicy configuration to force, which restricts link generation to trusted hosts. The CVSS v3.1 base score is 6.8 (medium severity), reflecting a network attack vector with high attack complexity, no privileges required, no user interaction, and a scope change due to the potential for integrity impact beyond the vulnerable component. No known exploits have been reported in the wild as of the publication date.

For European organizations using October CMS versions prior to 1.1.2, this vulnerability poses a moderate risk primarily to the integrity of web applications. Successful exploitation could allow attackers to poison HTTP headers, potentially leading to cache poisoning or manipulation of password reset links, which could facilitate unauthorized account access or phishing attacks. While confidentiality and availability are not directly impacted, the integrity compromise can undermine trust in the affected web services and lead to reputational damage. Organizations with publicly accessible October CMS installations on poorly configured servers are particularly at risk. Given the widespread use of CMS platforms in Europe across sectors such as government, education, and small to medium enterprises, the vulnerability could be leveraged to target these entities, especially if they have not applied the patch or implemented the recommended host validation. The lack of known exploits reduces immediate risk but does not eliminate the threat, as attackers may develop exploits targeting misconfigured deployments.

European organizations should take the following specific actions to mitigate this vulnerability: 1) Upgrade all October CMS instances to version 1.1.2 or later, where the trusted hosts feature is implemented. 2) Review and correct server configurations to ensure that HTTP requests are routed only to intended hosts, preventing arbitrary Host header routing. 3) Configure the cms.linkPolicy setting to 'force' as a temporary workaround if immediate upgrading is not feasible, restricting link generation to trusted hosts. 4) Implement strict input validation and sanitization for HTTP headers at the web server or application firewall level to detect and block suspicious Host header values. 5) Monitor web server logs for anomalous Host header values or unusual request patterns that could indicate attempted exploitation. 6) Educate development and operations teams about the risks of Host header manipulation and ensure secure deployment practices that avoid routing all requests indiscriminately. These measures go beyond generic patching by addressing the root cause in server configuration and application behavior.