CVE-2026-77838: SOY Calendar XSS Vulnerability

CVE-2026-77838 Overview

CVE-2026-77838 is a stored cross-site scripting (XSS) vulnerability in SOY Calendar, a web-based scheduling application. An authenticated attacker with low privileges can inject arbitrary script content that executes in the browser of any user logged in to the product. The flaw is tracked under CWE-79, improper neutralization of input during web page generation.

The vulnerability was disclosed through JVN Security Advisory JVN04485476 and analyzed in a SaitoDev Security Analysis Article. No public exploit code is currently available, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
Authenticated attackers can execute arbitrary JavaScript in the browsers of logged-in SOY Calendar users, enabling session theft, unauthorized actions, and defacement of calendar content.

Affected Products

SOY Calendar (versions specified in JVN04485476)
Web front-end components that render user-supplied calendar data
Environments where multiple users share a single SOY Calendar instance

Discovery Timeline

2026-08-28 - CVE-2026-77838 published to NVD
2026-08-28 - Last updated in NVD database

Technical Details for CVE-2026-77838

Vulnerability Analysis

CVE-2026-77838 is a cross-site scripting flaw in SOY Calendar. The application accepts user-supplied input in one or more calendar fields and renders that input back into HTML pages without adequate output encoding. When another authenticated user views the affected view, the injected payload executes in their browser session.

The attack requires the attacker to be authenticated with low privileges and requires user interaction from the victim, such as opening a calendar entry or view containing the payload. Because the exploit runs inside an authenticated session, an attacker can perform actions on behalf of the victim, exfiltrate session tokens, or read calendar data visible to the victim. The scope of impact extends beyond the vulnerable component to the victim's browser, affecting confidentiality and integrity in that subsystem.

The EPSS probability is 0.169%, indicating low observed exploitation likelihood at this time.

Root Cause

The root cause is missing or insufficient contextual output encoding when SOY Calendar renders attacker-controlled input into HTML. Input passes from a persistent store (for example, an event title, description, or metadata field) into the rendered document without being escaped for the HTML context. This allows <script> tags or event-handler attributes such as onerror and onload to become executable markup rather than displayed text.

Attack Vector

The attack vector is network-based over HTTP or HTTPS. A low-privileged authenticated attacker submits a crafted payload through a SOY Calendar input field. The payload persists server-side and is later served to any user who loads the affected view. Victim interaction, such as navigating to the calendar entry, triggers execution. Refer to the SaitoDev Security Analysis Article for technical details of the affected input paths.

No verified exploit code is published. The vulnerability mechanism is described in prose only; see the referenced advisories for reproduction details.

Detection Methods for CVE-2026-77838

Indicators of Compromise

Calendar entries containing HTML tags such as <script>, <img onerror=...>, or <svg onload=...> stored in event titles, descriptions, or metadata fields.
Outbound HTTP requests from user browsers to unfamiliar domains immediately after loading SOY Calendar views.
Anomalous session activity, including unexpected calendar modifications or account changes performed under a legitimate user account.

Detection Strategies

Scan the SOY Calendar database and backups for stored payloads matching common XSS patterns in text fields.
Enable and review web application firewall (WAF) logs for requests containing script tags or JavaScript URI schemes directed at SOY Calendar endpoints.
Correlate authentication logs with browser telemetry to identify sessions where injected script executed and issued unexpected API calls.

Monitoring Recommendations

Monitor HTTP response bodies from SOY Calendar for unescaped user input reflected into HTML contexts.
Alert on Content Security Policy (CSP) violation reports if CSP is deployed in front of the application.
Track privileged user sessions accessing calendar entries created by lower-privileged accounts for signs of triggered payloads.

How to Mitigate CVE-2026-77838

Immediate Actions Required

Apply the vendor-supplied fix referenced in JVN04485476 as soon as it is available for your deployment.
Audit existing SOY Calendar records for stored payloads and remove or sanitize any suspicious entries before restoring normal access.
Restrict account creation and reduce the number of low-privileged accounts that can submit calendar content until the patch is deployed.

Patch Information

Refer to JVN Security Advisory JVN04485476 and the SaitoDev Security Analysis Article for fixed version information and upgrade instructions. Verify the installed SOY Calendar version after applying the update and confirm that stored fields are rendered with proper HTML encoding.

Workarounds

Deploy a WAF rule that blocks requests containing <script> tags or JavaScript event-handler attributes on SOY Calendar submission endpoints.
Enforce a strict Content Security Policy that disallows inline scripts and restricts script sources to trusted origins.
Limit SOY Calendar access to trusted internal networks or VPN users until the patch is applied.

bash

# Example nginx Content Security Policy header for SOY Calendar
add_header Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'; base-uri 'self'; frame-ancestors 'self'" always;