Xpath Injection in Echo Go with Basic Auth

Xpath Injection occurs when untrusted input is concatenated into an XPath expression without proper escaping or parameterization, allowing an attacker to alter the query logic. In Echo Go, this typically arises when an API handler builds an XPath string to query an XML or in-memory document using user-controlled values such as query parameters, headers, or JSON fields mapped into XPath variables.

Combining this with HTTP Basic Auth introduces a subtle but important risk vector. Basic Auth sends credentials as a base64-encoded string in the Authorization header; while this is not a secret transport mechanism without TLS, the presence of authentication can lead developers to assume the request is trusted. Consequently, they may apply weaker validation on authenticated requests or log credentials in ways that aid an attacker during reconnaissance. In an Echo Go service, an authenticated endpoint might extract the username from the Basic Auth header and directly embed it into an XPath expression—for example, to select a user-specific node in an XML configuration or to enforce per-user data scopes. If the username is attacker-controlled and not escaped, the attacker can inject additional predicates or path segments, potentially bypassing intended access controls, reading other users’ data, or manipulating the XML structure.

An illustrative vulnerable pattern in Echo Go constructs an XPath expression by string concatenation:

username := c.Request().Header.Get("Authorization")
// naive decoding for illustrative purposes; do not use in production
// attacker can provide: ' or 1=1 or 'admin'='a
xpathQuery := "/users/user[name='" + username + "']/settings"
node, err := selectSingleNode(xpathQuery)
if err != nil {
    c.Error(err)
    return
}

Here, the username extracted from the Authorization header is concatenated directly into the XPath. An authenticated attacker can supply a crafted value such as ' or 1=1 or 'admin'='a, which modifies the predicate to always true, potentially exposing data belonging to other users or administrative entries. Even when the service validates that the user is authenticated, the lack of input sanitization or use of parameterized XPath methods allows the attacker to traverse the document unexpectedly. This becomes particularly impactful in configurations where the XML document contains sensitive settings, role mappings, or configuration flags. The combination of Basic Auth (which may give a false sense of security) and unchecked XPath construction therefore exposes an unauthenticated attack surface despite the presence of credentials, enabling privilege escalation or information disclosure in line with the BOLA/IDOR and Authentication checks performed by middleBrick.

Remediation focuses on never constructing XPath expressions by string concatenation with untrusted input. Instead, use parameterized XPath APIs where available, or apply strict allowlisting and encoding. For Echo Go, prefer compile-time XPath expressions with explicit variable binding, or—if your XML library does not support it—manually escape single quotes and normalize input.

Secure approach using explicit path segments and allowlisting:

username := c.Request().Header.Get("Authorization")
// Validate format: only alphanumeric and limited special characters
if !regexp.MustCompile(`^[a-zA-Z0-9._-]+$`).MatchString(username) {
    c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid username"})
    return
}
// Use a map or lookup instead of building an XPath from raw input
userSettings, exists := settingsByUsername[username]
if !exists {
    c.JSON(http.StatusNotFound, map[string]string{"error": "user not found"})
    return
}
// Process userSettings safely without XPath

If you must use an XPath-capable library that supports variable binding (e.g., via a library that exposes compiled expressions or namespace context), structure your code to bind values rather than interpolate:

// Pseudo-API illustrating parameterized usage; adapt to your library
type XPathExecutable interface {
    WithVariable(name, value string) XPathExecutable
    Execute(ctx context.Context) (Node, error)
}
compiled, _ := compileXPath("/users/user[name]/settings")
exec := compiled.WithVariable("name", username)
node, err := exec.Execute(r.Context())
if err != nil {
    c.Error(err)
    return
}

When a library does not support binding, escape single quotes by doubling them and enforce strict length and character policies:

username := c.Request().Header.Get("Authorization")
username = strings.ReplaceAll(username, "'", "''")
if len(username) > 64 {
    c.JSON(http.StatusBadRequest, map[string]string{"error": "username too long"})
    return
}
xpathQuery := "/users/user[name='" + username + "']/settings"
node, err := selectSingleNode(xpathQuery)
if err != nil {
    c.Error(err)
    return
}

Additionally, enforce transport security (TLS) to protect Basic Auth credentials in transit, and avoid logging Authorization headers. Combine these measures with the 12 parallel checks of middleBrick—such as Input Validation, Authentication, and BOLA/IDOR—to detect misconfigurations early. Using the CLI (middlebrick scan <url>) or the GitHub Action to fail builds on risky scores helps prevent vulnerable patterns like this from reaching production.