Attack Vectors

Hammas Calendar (slug: hammas-calendar) has a Medium severity vulnerability (CVSS 6.4) identified as CVE-2026-1902. It affects versions up to and including 1.5.11.

The issue can be exploited by an authenticated user with Contributor-level access or higher by placing a malicious payload into the apix attribute of the hp-calendar-manage-redirect shortcode. Because it is a stored cross-site scripting (XSS) issue, the injected content can persist on the site and run when other people view the affected page.

From a business perspective, the most likely real-world scenario is a compromised contributor account (or an overly broad role assignment) being used to embed harmful scripts into pages or posts that appear legitimate to internal teams and external visitors.

Security Weakness

The vulnerability stems from insufficient input sanitization and output escaping for the apix parameter used by the hp-calendar-manage-redirect shortcode in Hammas Calendar. This creates an opportunity for untrusted content to be stored and later displayed in a way that a browser will interpret as active script.

Even though the attacker must be logged in with at least Contributor privileges, that requirement should not be treated as strong protection. Contributor accounts are common in marketing workflows, and they are frequent targets for credential theft and reuse attacks, making “authenticated” issues operationally relevant.

Technical or Business Impacts

Stored XSS can translate quickly into brand and revenue risk. A visitor who loads an injected page may be exposed to content changes, unwanted redirects, or deceptive prompts that appear to come from your organization.

Potential impacts include reputational damage (malicious content displayed on branded pages), lead and campaign disruption (form tampering or traffic diversion), and account and workflow compromise (scripts that attempt to act on behalf of logged-in users). For leadership teams, this is best understood as a risk to digital trust, conversion performance, and governance over who can publish what.

For compliance and risk stakeholders, the presence of a known vulnerability (CVE-2026-1902) can create audit and due diligence concerns if left unaddressed, particularly when the remediation is straightforward.

Remediation

Update Hammas Calendar to version 1.5.12 or a newer patched release. This is the recommended remediation per the published advisory.

As a practical risk-reduction step, review who has Contributor (or higher) access, remove unused accounts, and ensure strong authentication practices are in place. Because the exploit involves content entry, also consider reviewing recently edited pages that use the affected shortcode if you suspect an account was compromised.

Similar Attacks

Stored XSS has been used across the web to harm brands and end users. Notable examples include the MySpace “Samy” worm, which demonstrated how quickly self-propagating script injection can spread through trusted pages, and the widespread cross-site scripting (XSS) attack category that continues to be a common root cause behind session abuse and content manipulation.