Overview
The CVE-2026-34890 entry describes a DOM-based XSS vulnerability in Mark O’Donnell MSTW League Manager where user-controlled input can be injected into the DOM without proper neutralization during web page generation. In Go (Gin) applications, this class of issue often arises when server-side templates or handlers bypass the framework’s escaping guarantees (for example by passing user input through a trusted HTML wrapper or by directly inserting unsanitized data into a script or DOM manipulation code). In MSTW League Manager, attackers could supply crafted input that, when rendered in the browser, executes arbitrary JavaScript in the context of the victim’s session. This is particularly dangerous because DOM-based XSS depends on the client’s DOM and can be triggered entirely in the user’s browser, potentially exfiltrating session data or performing actions on behalf of the user. The vulnerability is associated with CWE-79 (Cross-Site Scripting). The real-world impact includes user credential theft, session hijacking, and unauthorized actions, especially when the app renders untrusted data into HTML or JavaScript without proper escaping, as seen in versions affected up to 2.10 for MSTW League Manager (CVE-2026-34890).
In Go with the Gin framework, the risk manifests when developers render user input as HTML or inject it directly into JavaScript without using safe encodings. Go’s html/template escapes content by default, but if a vulnerable pattern uses template.HTML (or otherwise bypasses escaping) and injects user data into the DOM, an attacker can craft payloads that execute in the browser. Mitigations include avoiding any bypass of escaping, validating and sanitizing input server-side, encoding data when embedding into JavaScript, and applying a strict Content Security Policy (CSP). This guide shows concrete code fixes in Go (Gin) to prevent DOM-based XSS and explains how to apply these practices to maintain safer, standards-compliant web apps.
The remediation patterns here focus on avoiding trusting user input as HTML, using Go’s templating escaping for all dynamic content, and encoding values when embedding into JavaScript. They also cover server-side validation and client-side hardening (CSP). By following these patterns, Go (Gin) applications can prevent DOM-based XSS like that described in CVE-2026-34890 and prevent similar issues in MSTW League Manager-style deployments.
Affected Versions
n/a through 2.10
Code Fix Example
Go (Gin) API Security Remediation
Vulnerable pattern (unsafe by bypassing escaping):
package main
import (
"html/template"
"net/http"
"github.com/gin-gonic/gin"
)
func main() {
r := gin.Default()
// Single template set with two views: vuln (vulnerable) and fix (safe)
tmpl := template.Must(template.New("root").Parse(`{{define "vuln"}}<!DOCTYPE html><html><body><h3>Vulnerable</h3><div id="output">{{ .Content }}</div></body></html>{{end}}{{define "fix"}}<!DOCTYPE html><html><body><h3>Fixed</h3><div id="output">{{ .Content }}</div></body></html>{{end}}`))
r.SetHTMLTemplate(tmpl)
// Vulnerable: user-provided input is inserted into the DOM as HTML (template escaping bypassed)
r.GET("/vuln", func(c *gin.Context) {
user := c.Query("name")
// Intentionally unsafe: template.HTML bypasses escaping
c.HTML(http.StatusOK, "vuln", gin.H{"Content": template.HTML(user)})
})
// Fixed: user input is treated as text by the template (escaping happens automatically)
r.GET("/fix", func(c *gin.Context) {
user := c.Query("name")
c.HTML(http.StatusOK, "fix", gin.H{"Content": user})
})
r.Run(":8080")
}
Notes:
- The vulnerable route renders user input as HTML (via template.HTML) and can execute injected scripts if attacker supplies HTML/JS payloads.
- The fixed route relies on the template escaping by default (Content is a string, not trusted HTML), preventing execution of injected code.
To prevent DOM-based XSS, always avoid using template.HTML for untrusted input, encode data properly when embedding into JavaScript (for example using JSON encoding), and consider a strict CSP with nonces.