Description

Unauthenticated Cross Site Scripting (XSS) in MailChimp Subscribe Forms <= 4.3.3 versions.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.1",
  "pubDate": "2026-08-13T14:16:56.250Z",
  "pubdate": "2026-08-13T14:16:56.250Z",
  "executiveSummary": "An unauthenticated Cross-Site Scripting (XSS) vulnerability exists in the MailChimp Subscribe Forms plugin for versions 4.3.3 and below. This security flaw enables remote, unauthenticated threat actors to inject malicious scripts, typically JavaScript, into vulnerable input fields or parameters processed by the application. When an unsuspecting user or administrator views the rendered output, the injected payload executes within the context of their browser session. The primary impact of this vulnerability includes session hijacking, credential theft, unauthorized actions performed on behalf of the victim, and potential defacement of the affected web application. The risk implications are severe, as exploitation requires no prior authentication or privileged access, lowering the barrier to entry for malicious actors. Attackers can leverage this flaw via crafted URLs or form submissions, targeting public-facing web pages where the subscribe forms are integrated. Remediation requires updating the affected software to a patched version beyond 4.3.3 once available, alongside implementing robust input sanitization and output encoding mechanisms to neutralize untrusted data streams and prevent arbitrary script execution across all form parameters.",
  "technicalDetails": "The vulnerability is an unauthenticated Cross-Site Scripting (XSS) flaw localized within the MailChimp Subscribe Forms plugin, specifically affecting versions 4.3.3 and below. The root cause stems from insufficient input sanitization and inadequate output encoding of user-supplied data handled by the plugin's form submission or rendering components. Because the application fails to properly neutralize special characters, such as angle brackets (< and >), malicious payloads containing arbitrary JavaScript can be submitted and subsequently reflected or stored without validation.\nThe attack flow begins with a remote, unauthenticated attacker crafting a malicious payload encapsulating executable JavaScript. The attacker targets the vulnerable component by injecting this payload into parameters processed by the MailChimp Subscribe Forms functionality. Due to the lack of proper authentication requirements and privilege checks, any remote network-connected user can initiate this request. When the application processes the input and subsequently renders the output within the Document Object Model (DOM) of a victim's browser, the payload is parsed as executable code rather than inert text.\nDuring execution, the malicious script operates with the privileges of the victim's session. Depending on the target, this could affect standard site visitors or authenticated administrators. Post-exploitation impact includes the potential extraction of sensitive session cookies, DOM manipulation, redirection to malicious external sites, or the execution of unauthorized administrative actions via asynchronous requests (CSRF-like behavior). Network exposure is external and directly accessible via any web browser capable of reaching the vulnerable web application endpoints hosting the subscribe forms. Mitigation of this technical flaw demands strict contextual output encoding and robust server-side input validation frameworks to ensure all user-supplied data is treated as untrusted before rendering."
}