Xpath Injection in Chi with Api Keys

XPath Injection in the Chi framework when API keys are handled in route handlers or middleware can occur when user-controlled input is concatenated into XPath expressions without sanitization or parameterized queries. In Chi, API keys are often passed via headers (e.g., Authorization) or query parameters and then forwarded to backend services or used to construct dynamic XPath expressions for XML data processing. If an attacker can influence the key-derived value used in the XPath, they may escape the intended context and manipulate the query logic.

For example, suppose a Chi handler extracts an api_key from a header and uses it to build an XPath to select user-specific configuration nodes from an XML document. A naive implementation might concatenate the key directly into the expression:

let apiKey = req.Header("X-API-Key")
let expr = fmt.Sprintf("//config/user[key='%s']", apiKey)
node, err := selectSingleNode(doc, expr)

If apiKey contains malicious payload such as ' or '1'='1, the resulting XPath becomes:

//config/user[key='' or '1'='1']

This can return unintended nodes, bypass authorization checks, or disclose other users’ data. In Chi, this risk is amplified when APIs are unauthenticated or when API keys are treated as low-trust input during XML processing, as the framework does not enforce schema-bound parameterization for XPath. The vulnerability maps to the broader BOLA/IDOR and Input Validation checks in middleBrick, and to the OWASP API Top 10 category Broken Object Level Authorization when key-derived data influences access control decisions.

Additionally, if the same Chi service exposes an endpoint that returns XML and incorporates API key values into XPath without output encoding, attackers might achieve XML External Entity (XXE) injection or data exfiltration depending on the backend parser configuration. middleBrick’s XML/Input Validation checks would flag such unsafe construction patterns, emphasizing the need to treat API key usage as untrusted input even when sourced from trusted headers.

Remediation centers on avoiding string concatenation for XPath construction and using parameterized or compiled expressions. In Chi, prefer structured data (e.g., JSON) over XML where possible; if XML is required, use an XPath implementation that supports bound parameters or a safe selector API. When API keys must be used in queries, treat them as opaque identifiers and map them to predefined filters rather than embedding them directly in expressions.

Safe approach using a key-to-user mapping instead of direct XPath injection:

// Assume a validated key-to-UserID map obtained from a secure store
userMap, err := validateApiKey(req.Header("X-API-Key"))
if err != nil {
    http.Error(w, "Unauthorized", http.StatusUnauthorized)
    return
}
// Use the mapped user ID in a parameterized context or structured query
userID := userMap["userID"]
// Example with a safe XML library that supports compiled queries (pseudocode)
expr := compileXPath("//config/user[@id=$uid]")
node, err := expr.SelectNode(doc, map[string]interface{}{"uid": userID})

If you must construct XPath from key-derived values, enforce strict allow-lists or regex patterns for key format and use a dedicated XML library that supports parameterized evaluation. For example, with a library that supports placeholders:

// Using placeholders to avoid injection
expr := compileXPath("//config/user[key=$key]")
result, err := expr.SelectNode(doc, map[string]interface{}{"key": apiKey})

In a Chi middleware context, you can integrate key validation before routing:

func ApiKeyValidator(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        apiKey := r.Header.Get("X-API-Key")
        if !isValidKeyFormat(apiKey) { // e.g., regex ^[A-Za-z0-9\-_]+$
            http.Error(w, "Bad Request", http.StatusBadRequest)
            return
        }
        // Optionally enrich context with key metadata instead of raw key
        ctx := context.WithValue(r.Context(), "apiKeyMeta", lookupKeyMetadata(apiKey))
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

These patterns ensure API keys influence access through controlled mappings rather than direct XPath composition, mitigating injection risks while preserving the security checks that middleBrick evaluates under BOLA/IDOR, Input Validation, and Data Exposure.