Xpath Injection in Buffalo with Basic Auth

XPath Injection is a server-side injection that manipulates XML traversal logic by injecting crafted data into XPath expressions. When an API built with Buffalo uses Basic Authentication, authentication state is typically validated via HTTP headers rather than influencing authorization decisions for downstream XML processing. This separation can create a scenario where an authenticated request reaches functionality that dynamically constructs XPath queries from user-controlled input, such as query parameters, headers, or form fields.

Buffalo does not provide built-in XML binding or XPath utilities, so developers often add dependencies to handle XML payloads. If input used in XPath expressions is concatenated directly into query strings without escaping or parameterization, attackers can alter the logic of document selection. For example, an XPath expression intended to select a user node by ID can be modified to always return the first node or to bypass identity checks entirely. Consider an endpoint that retrieves user profile data from an XML document based on an identifier taken from the request:

// user[id = '{{.Identifier}}']

An attacker providing identifier value ' or 1=1 or ' can change the predicate to effectively bypass intended scoping. Because Basic Authentication confirms identity at the transport layer, developers might mistakenly assume that the request context is trustworthy and skip rigorous input validation for downstream XML operations.

The combination of authenticated sessions and unchecked XPath usage can also expose sensitive data or enable privilege escalation if role or permission information is encoded in the XML. In a scenario where authorization relies on in-memory data derived from the XML, an injected XPath that selects an administrative node can cause the application to treat a lower-privilege identity as elevated. Because Buffalo applications often integrate with middleware that parses headers and cookies, the risk surface expands when XML processing logic does not treat authenticated input as untrusted.

Real-world exploitation patterns mirror classic injection techniques: attackers probe for boolean-based blind conditions, error-based extraction, and union-based data exfiltration. Detection requires correlating runtime findings from security scans with code paths that handle XML deserialization and XPath evaluation. The scanner’s checks for Input Validation and Property Authorization are valuable here, as they highlight points where user data reaches query construction without canonicalization or strict schema validation.

Remediation centers on treating all external data as untrusted, regardless of authentication status, and avoiding dynamic string assembly for XPath. Use parameterized or compiled XPath expressions where the runtime library supports it, or switch to selective filtering in application logic instead of raw XPath over user-influenced data.

Example 1: Unsafe usage with Basic Auth header parsing

Consider a route that extracts a user ID from an XML document after validating a Basic Auth header:

// In a Buffalo action
import (
    "encoding/base64"
    "net/http"
)

func GetProfile(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if len(auth) < 6 || auth[:6] != "Basic " {
        return c.Error(http.StatusUnauthorized, errors.New("missing basic auth"))
    }
    payload, err := base64.StdEncoding.DecodeString(auth[6:])
    if err != nil {
        return c.Error(http.StatusUnauthorized, err)
    }
    pair := strings.SplitN(string(payload), ":", 2)
    if len(pair) != 2 {
        return c.Error(http.StatusUnauthorized, errors.New("invalid credentials"))
    }
    username, password := pair[0], pair[1]
    // Authentication logic omitted for brevity

    // Unsafe: building XPath from user-controlled identifier
    identifier := c.Param("id")
    query := fmt.Sprintf("//user[id = '%s']", identifier)
    result := evaluateXPath(doc, query)
    return c.Render(200, r.JSON(result))
}

The identifier is taken directly from the request and interpolated into the XPath string. An authenticated request can still exploit this via crafted input.

Example 2: Safe remediation using explicit filtering

Replace dynamic XPath construction with controlled selection logic. For example, load the user by ID using XML traversal functions that do not rely on string concatenation:

// Safe approach: avoid XPath for identity-based selection
func GetProfileSafe(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if len(auth) < 6 || auth[:6] != "Basic " {
        return c.Error(http.StatusUnauthorized, errors.New("missing basic auth"))
    }
    payload, err := base64.StdEncoding.DecodeString(auth[6:])
    if err != nil {
        return c.Error(http.StatusUnauthorized, err)
    }
    pair := strings.SplitN(string(payload), ":", 2)
    if len(pair) != 2 {
        return c.Error(http.StatusUnauthorized, errors.New("invalid credentials"))
    }
    username, _ := pair[0], pair[1]
    // Validate and sanitize identifier as integer
    idStr := c.Param("id")
    id, err := strconv.Atoi(idStr)
    if err != nil || id <= 0 {
        return c.Error(http.StatusBadRequest, errors.New("invalid id"))
    }

    // Use index-based access or structured iteration instead of XPath
    var target *UserXML
    for _, u := range doc.Users {
        if u.ID == id {
            target = &u
            break
        }
    }
    if target == nil {
        return c.NotFound()
    }
    return c.Render(200, r.JSON(target))
}

This approach eliminates XPath injection by removing string-based query construction entirely. If XPath is required for complex document navigation, use compiled expressions or whitelisted paths, and treat the identifier as a numeric key rather than a string predicate.

Additional hardening includes enforcing strict Content-Type checks for XML payloads, applying XML external entity (XXE) protections at the parser level, and ensuring that role or permission data is not derived from unvalidated XML nodes. The scanner’s checks for Input Validation, Data Exposure, and Encryption help surface areas where these mitigations are incomplete.