CVE-2026-40732: High-Severity Unauthenticated XSS in Notification for Telegram ≤3.5

Technical summary

CVE-2026-40732 is a reflected or stored XSS vulnerability (CWE-79) in the Notification for Telegram application affecting versions 3.5 and below. The vulnerability exists because user-supplied input is not properly sanitized before being rendered in the DOM. The attack vector is network-based with low attack complexity, requiring no privilege escalation but necessitating user interaction (clicking a link or viewing crafted content). The CVSS 3.1 score of 7.1 (HIGH) reflects the combination of low confidentiality, integrity, and availability impact with changed scope, indicating potential cross-domain effects.

Business impact

Compromised user accounts could lead to unauthorized access to Telegram notifications, interception of sensitive communications, or use of legitimate user sessions for further attacks. If the application handles sensitive notification metadata or integrates with enterprise messaging workflows, the impact extends to data exposure and compliance violations. Reputation damage may occur if users report credential theft or unauthorized activity tied to the application.

Affected systems

Notification for Telegram version 3.5 and all earlier versions are affected. Users running version 3.6 or later are not impacted by this specific vulnerability. Verify your installed version in the application settings or package manager before assuming protection.

Exploitability

This vulnerability is exploitable without authentication, making it accessible to any remote attacker who can craft a malicious URL or inject content into a page the victim visits. Exploitation requires user interaction—the victim must click a link or view specific content—but the barrier is low. No known public exploit code is currently documented in the KEV catalog, though the attack pattern is straightforward and requires only basic web development knowledge to weaponize.

Remediation

Immediately upgrade Notification for Telegram to version 3.6 or later. If immediate patching is not feasible, implement input validation and output encoding at the application level, and consider restricting access to the notification interface to trusted networks pending patch deployment. Web application firewalls (WAF) may help detect XSS payloads, though they should not be relied upon as a primary defense.

Patch guidance

Check the official Notification for Telegram release notes and repository for version 3.6 or the latest available patch. Update through your standard package manager or direct download from the vendor's trusted source. Verify the new version by checking the application settings after installation. For organizations with multiple instances, prioritize environments handling sensitive or user-facing notifications.

Detection guidance

Monitor web server and application logs for unusual patterns in notification parameters, such as script tags (<script>), event handlers (onclick, onerror), or encoded payloads. Search for requests containing common XSS patterns (e.g., 'javascript:', '&#x', '%3c'). Track user reports of unexpected behavior in the notification interface, such as pop-ups or redirects. Endpoint detection and response (EDR) tools should flag suspicious JavaScript execution in the context of the application's process.

Why prioritize this

Although not yet listed in the CISA KEV catalog, this HIGH-severity XSS vulnerability warrants rapid patching because it requires no authentication, affects a notification system (often trusted for real-time alerts), and can be exploited with minimal user friction. Organizations should treat this as a near-term remediation priority, especially if the application is internet-facing or used by remote workers.

Risk score, explained

The CVSS 3.1 score of 7.1 reflects a network-exploitable vulnerability with no privilege requirement and low attack complexity, balanced by the requirement for user interaction and limited individual impact (low C, I, A). The 'changed scope' designation indicates the attacker can affect resources beyond the vulnerable component, such as user sessions or other domains in the same browser context. This places the risk solidly in the HIGH category for most organizations.

Frequently asked questions