CVE-2022-24838 is a medium-severity injection vulnerability affecting Nextcloud Calendar versions prior to 3.2.2. Nextcloud Calendar is a widely used calendar application integrated within the Nextcloud framework, which is popular for self-hosted cloud services. The vulnerability arises from improper neutralization of special elements in the email field of appointment-related JSON requests. Specifically, the application fails to sanitize newline characters and other special characters in the email address provided in the JSON payload. This flaw allows an attacker to inject newline characters that break out of the intended SMTP command context, specifically the 'RCPT TO:<BOOKING USER'S EMAIL>' command. By doing so, the attacker can append arbitrary SMTP commands to the email transaction. This SMTP command injection can be exploited to manipulate the mail server behavior, potentially allowing unauthorized email sending, spamming, or other malicious SMTP interactions. The vulnerability is categorized under CWE-74, which involves improper neutralization of special elements used by downstream components, in this case, the SMTP server. No known exploits have been reported in the wild, and no workarounds exist other than upgrading to Nextcloud Calendar version 3.2.2 or later, where the input sanitization has been properly implemented to prevent injection. The vulnerability does not require user interaction beyond sending a crafted JSON request, and it can be triggered remotely if the attacker has access to the calendar API or interface that accepts appointment emails. This makes it a significant risk for environments where Nextcloud Calendar is exposed or accessible by untrusted users or systems.

For European organizations using Nextcloud Calendar, this vulnerability could lead to unauthorized manipulation of SMTP transactions, enabling attackers to send arbitrary emails through the organization's mail server. This could result in spam campaigns originating from trusted infrastructure, phishing attacks, or email-based malware distribution, damaging organizational reputation and potentially leading to data breaches if phishing is successful. Additionally, the injection could disrupt normal mail operations, impacting availability and reliability of email communications. Since Nextcloud is popular among privacy-conscious and self-hosted cloud users, including governmental, educational, and enterprise sectors in Europe, exploitation could undermine trust in internal communication systems. The impact on confidentiality is indirect but significant if phishing or social engineering attacks succeed. Integrity of email communications could be compromised, and availability of mail services might be affected if the SMTP server is overwhelmed or misused. The medium severity rating reflects the need for timely patching to prevent these risks.

The primary mitigation is to upgrade Nextcloud Calendar to version 3.2.2 or later, where the vulnerability is patched. Organizations should prioritize this update in their patch management cycles. Beyond upgrading, administrators should restrict access to the calendar API and interfaces to trusted users only, employing network segmentation and firewall rules to limit exposure. Implementing SMTP server-side restrictions and monitoring can help detect and block anomalous SMTP commands or unusual email sending patterns that may indicate exploitation attempts. Logging and alerting on SMTP command anomalies should be enabled. Additionally, organizations should review email sending policies and consider rate limiting or authentication mechanisms such as SPF, DKIM, and DMARC to reduce the impact of potential abuse. User awareness training on phishing risks remains important to mitigate downstream impacts of any email-based attacks leveraging this vulnerability.