Xpath Injection in Echo Go with Hmac Signatures

XPath Injection occurs when untrusted data is concatenated into an XPath expression without proper sanitization or parameterization, allowing an attacker to alter query logic and access or modify unintended nodes. In Echo Go, if you build an XPath expression by string concatenation to filter or authenticate based on user-controlled values, the application can be tricked into injecting additional path segments or predicates. The vulnerability is not introduced by Hmac Signatures itself, but by how you use signatures in combination with XPath-based decisions.

Consider an API endpoint that uses an Hmac Signature to verify request integrity and then uses a parameter from the signed payload to construct an XPath expression. For example, you might sign a userId and later use that userId in an XPath lookup such as /users/user[id='123']. If the userId is taken directly from the request (even if covered by the Hmac), and concatenated into the XPath string, an attacker who can observe or influence parts of the signed payload might inject additional conditions like or id='123' or true()='true, bypassing intended access controls. The Hmac Signature ensures the payload has not been tampered with in transit, but it does not sanitize the content of the payload for safe usage within XPath.

In Echo Go, this can manifest when you bind JSON or form values into a struct, sign selected fields with Hmac, store or forward the signed values, and then use those values to query an XML document via XPath. Because the signature does not restrict how the value is used downstream, an attacker might attempt to exploit XPath injection by providing crafted input that changes the semantics of the query. Real-world attack patterns include attempting to bypass authentication checks, escalate privileges by accessing other users’ data via IDOR-like paths, or exfiltrating XML nodes containing sensitive information.

The combination therefore creates risk when the following occurs: the application derives an XPath expression using string interpolation, the data used is included in the signed payload to ensure integrity, but the integrity check does not enforce safe usage context. Hmac Signatures protect against modification, but they do not enforce safe composition. To detect this during a scan, middleBrick compares the runtime behavior of endpoints using signed parameters with OpenAPI/Swagger definitions and flags usages where signed or authenticated inputs influence unparameterized XPath queries.

Remediation focuses on avoiding direct concatenation of user-influenced values into XPath expressions and using language-native mechanisms for safe query construction. In Echo Go, you should treat any value that participates in XPath as untrusted, even if it is covered by an Hmac Signature, and use parameterized approaches or strict validation instead of string building.

Example of vulnerable code that concatenates a signed userId into an XPath string:

userId := c.FormValue("userId")
// Assume hmacVerified is true after verification
query := "/users/user[id='" + userId + "']"
nodes, err := selectNodes(query)
if err != nil { /* handle */ }

This pattern is unsafe because an attacker-controlled userId can inject additional predicates. Instead, use a method that treats the value as a literal, such as constructing an in-memory XML document and filtering programmatically, or using an XPath implementation that supports variables/bound parameters. Below is a safer approach using Go’s standard library and explicit validation:

import (
    "encoding/xml"
    "net/http"
    "strings"

    "github.com/labstack/echo/v4"
)

type User struct {
    XMLName xml.Name `xml:"user"`
    ID      string   `xml:"id,attr"`
    Name    string   `xml:"name"`
}

func safeXpathLookup(c echo.Context) error {
    userId := c.FormValue("userId")
    // Validate format strictly: only alphanumeric and expected length
    if !isValidUserId(userId) {
        return c.String(http.StatusBadRequest, "invalid user id")
    }
    // Load XML source (could be from a safe internal store)
    data, err := os.ReadFile("/data/users.xml")
    if err != nil {
        return c.String(http.StatusInternalServerError, "server error")
    }
    var users struct {
        Users []User `xml:"users>user"`
    }
    if err := xml.Unmarshal(data, &users); err != nil {
        return c.String(http.StatusInternalServerError, "parse error")
    }
    // Filter in Go instead of XPath injection-prone string building
    for _, u := range users.Users {
        if u.ID == userId {
            return c.JSON(http.StatusOK, u)
        }
    }
    return c.String(http.StatusNotFound, "not found")
}

func isValidUserId(id string) bool {
    // Allow only expected characters and length to prevent injection
    return len(id) > 0 && len(id) <= 64 && strings.Trim(id, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_" ) == "" 
}

If you must use an XPath library, bind values through supported APIs rather than string concatenation. For example, with a library that supports variable substitution, construct the expression with placeholders and supply values separately. This ensures that special characters in the userId cannot change the structure of the query.

Additionally, enforce least privilege on the XML data accessed, avoid using XPath on attacker-influenced documents, and audit any signed values that influence query construction. middleBrick’s scans can surface these risky patterns by correlating authenticated inputs with XPath usage and flagging endpoints where signatures do not prevent unsafe composition.