Xpath Injection in Buffalo with Dynamodb

XPath Injection becomes relevant in Buffalo when an application builds XPath expressions dynamically using user-controlled data before evaluating them against an XML document. Although DynamoDB is a NoSQL database and does not natively store or query XML, a Buffalo application can still encounter XPath Injection if it deserializes or transforms DynamoDB-stored data into XML and then evaluates XPath expressions constructed from that data. For example, if DynamoDB items contain XML fragments or if your service layer produces XML representations of DynamoDB records, unsanitized input used in XPath construction can lead to injection.

Consider a scenario where a Buffalo handler reads a DynamoDB item containing an XML document and uses a query parameter to select a node. If the parameter is concatenated into the XPath without proper escaping or validation, an attacker can alter the XPath logic. A typical vulnerable pattern in a Buffalo controller might look like this:

node := xmlDoc.SelectSingleNode("//user[id=" + userSuppliedID + "]/name")

An attacker supplying 1 or true() or as userSuppliedID could change the predicate to always match, exposing other users’ data. Even though DynamoDB does not execute XPath, the risk arises when DynamoDB data is used as input to XML processing in Buffalo. The scan’s XPath Injection checks can flag unsafe string concatenation in XPath construction within Buffalo handlers and helpers.

Because middleBrick tests the unauthenticated attack surface, it can detect endpoints in a Buffalo app where user input influences XPath selection. Findings include the specific location in the Buffalo codebase where concatenation occurs, mapped to relevant OWASP API Top 10 categories such as Injection and Broken Object Level Authorization when combined with BOLA risks. Remediation focuses on avoiding dynamic XPath construction and using parameterized queries or a safe XML library.

To prevent XPath Injection in a Buffalo app that works with DynamoDB data, avoid building XPath expressions via string concatenation. Instead, use a dedicated XML library that supports parameterized XPath or safely escapes user input. Below are concrete remediation examples using Go with DynamoDB and XML handling.

First, retrieve an item from DynamoDB without involving XPath. Then, if you must work with XML, parse it with a library that does not allow expression injection, and avoid constructing XPath from external input.

Safe DynamoDB retrieval and XML handling in Buffalo:

import (
    "context"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/config"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
    "github.com/gobuffalo/buffalo"
    "encoding/xml"
)

func GetUserData(c buffalo.Context) error {
    cfg, err := config.LoadDefaultConfig(context.TODO())
    if err != nil {
        return c.Render(500, r.String("failed to load AWS config"))
    }
    client := dynamodb.NewFromConfig(cfg)

    key := map[string]types.AttributeValue{
        "ID": &types.AttributeValueMemberS{Value: c.Param("id")},
    }
    out, err := client.GetItem(context.TODO(), &dynamodb.GetItemInput{
        TableName: aws.String("Users"),
        Key:       key,
    })
    if err != nil {
        return c.Render(500, r.String("failed to get item"))
    }

    // Suppose DynamoDB returns an XML string attribute; parse safely without XPath injection
    var xmlData struct {
        Name string `xml:"name"`
    }
    if err := xml.Unmarshal([]byte(aws.ToString(out.Item["XmlData"].(*types.AttributeValueMemberS).Value)), &xmlData); err != nil {
        return c.Render(500, r.String("failed to parse XML"))
    }

    // Use xmlData.Name directly; do not construct XPath from user input
    c.Response().Header().Set("Content-Type", "application/json")
    return c.Render(200, r.JSON(map[string]string{"name": xmlData.Name}))
}

If you must evaluate XPath, use a safe library and treat all external input as data, not expression syntax. For example, with a hypothetical XML library that supports compiled expressions:

// Compile an XPath once with placeholders; do not concatenate user input
expr, err := xpath.Compile("//user[id='{id}']/name")
if err != nil {
    // handle error
}
// Bind the user input as a variable or value, never as raw XPath text
result := expr.Evaluate(xmlDoc, xpath.WithVariable("id", userSuppliedID))

In Buffalo, ensure that any XML processing is isolated from user-controlled XPath construction. The middleBrick CLI can be used to verify that no unsafe XPath patterns remain in your codebase by scanning your Buffalo endpoints.

Additional recommendations:

• Do not construct XPath expressions by interpolating request parameters, headers, or DynamoDB-derived strings.
• Validate and sanitize all inputs before using them in any query or selection logic, even if the source is DynamoDB.
• If you store XML in DynamoDB, deserialize it with strict schema validation and avoid re-serializing user input into XPath predicates.