CVE-2026-100263 in YouTrack
Summary
by MITRE • 09/30/2026
In JetBrains YouTrack before 2026.2.18991 stored HTML injection via the User-Agent header was possible
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified in JetBrains YouTrack prior to version 2026.2.18991 represents a significant security flaw rooted in improper input validation and sanitization mechanisms within the application's HTTP request handling logic. Specifically, this issue manifests as stored HTML injection facilitated through the User-Agent header field of incoming HTTP requests. In typical web architecture, the User-Agent string is used by servers to identify the client software, operating system, or device making a request, often for logging purposes, analytics, or compatibility adjustments. However, in this specific instance, the application failed to adequately sanitize or encode user-supplied data contained within this header before storing it in its backend database and subsequently rendering it back to users without proper context-aware escaping. This failure allows an attacker to inject malicious HTML markup into the system's persistent storage rather than merely executing code temporarily during a single request session, which distinguishes it from reflected variants of cross-site scripting attacks.
The technical mechanism behind this vulnerability relies on the application's tendency to trust and store data received in standard HTTP headers without applying rigorous security filters. When an attacker crafts a malicious User-Agent string containing HTML tags or JavaScript event handlers, such as script injection payloads designed to execute within the context of other users' browsers, YouTrack stores this content directly into its database records associated with user activity logs or request metadata. Because the data is stored rather than just reflected, it persists in the system until manually removed by an administrator. This persistence transforms what might otherwise be a transient session-based attack into a long-term threat vector that can affect multiple users over time without requiring further interaction from the attacker after the initial payload submission.
The operational impact of this vulnerability is substantial, primarily centering on the compromise of user confidentiality and integrity through stored cross-site scripting attacks. When other authenticated users view pages where the tainted User-Agent data is rendered—such as audit logs, activity feeds, or administrative dashboards—the embedded HTML executes within their browser context. This execution allows attackers to perform a variety of malicious actions including stealing session cookies, hijacking user sessions, performing actions on behalf of victims such as modifying project settings or creating new users, and phishing for sensitive credentials by displaying fake login prompts that mimic the legitimate YouTrack interface. The severity is amplified because it targets authenticated enterprise environments where trust in internal tools is high, potentially leading to broader network compromise if session tokens are exfiltrated and used to access other connected systems.
From a classification perspective, this vulnerability aligns with CWE-79: Improper Neutralization of Input During Web Page Generation known as Cross-site Scripting, specifically the stored variant where input data is saved for later use by other users. It also relates to CWE-601: URL Redirection to Untrusted Site via User-controlled Key if the injected HTML facilitates redirection attacks. In terms of offensive security frameworks, this technique maps to MITRE ATT&CK tactic TA0005: Defense Evasion and specifically sub-techniques involving client-side script injection for credential harvesting or persistence within web applications. The exploitation does not require complex network-level bypasses but relies on social engineering or automated scanning tools that can send crafted headers to the target endpoint, making it a relatively low-effort attack vector with high potential impact in unpatched environments.
Mitigation strategies must prioritize immediate patching and defensive configuration changes. JetBrains has addressed this issue in version 2026.2.18991 and subsequent releases by implementing robust input validation and output encoding mechanisms that ensure any data derived from the User-Agent header is properly escaped before being stored or rendered. Organizations running affected versions should upgrade to the patched release immediately as part of their standard vulnerability management lifecycle. In addition to patching, administrators can implement Web Application Firewall rules designed to detect and block anomalous patterns in HTTP headers, such as strings containing HTML tags like script, img, or iframe within the User-Agent field. Furthermore, enabling Content Security Policy directives that restrict inline script execution can provide a layer of defense-in-depth, reducing the impact even if an injection attempt succeeds by preventing the browser from executing untrusted scripts embedded in the page content. Regular security audits and code reviews focusing on how external inputs are handled before database storage remain critical for preventing similar stored injection vulnerabilities in future development cycles.