Formula Injection in Chi with Dynamodb

Formula injection occurs when untrusted input is concatenated into expressions that are later evaluated, often in spreadsheet or templating contexts. When using Chi (a lightweight HTTP router for Go) together with DynamoDB, the combination can expose applications to formula injection if user-controlled data is written into items that are later exported or rendered in spreadsheets, CSV exports, or downstream systems that interpret formulas.

DynamoDB itself does not evaluate formulas, but if your application stores user input directly into a DynamoDB attribute that is later exported to a CSV or spreadsheet format (for example, during reporting or analytics), and that output is opened in a tool like Excel, formulas embedded in the data can execute. A common pattern in Chi-based services is to store or query data from DynamoDB and then generate downloadable reports. If an attacker can inject strings like =HYPERLINK("https://evil.com", "Click") or =7*7 into a field stored in DynamoDB, and that field is later rendered in a CSV without proper escaping, Excel may evaluate the injected formula when the file is opened.

Consider a Chi route that accepts a name and stores it in DynamoDB:

import (
	"github.com/go-chi/chi/v5"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb"
	"github.com/aws/aws-sdk-go-vendor/aws-sdk-go-v2/feature/dynamodb/attributevalue"
)

func storeNameHandler(db *dynamodb.Client) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		name := r.FormValue("name") // user-controlled
		item := map[string]any{
			"id":   "report-001",
			"name": name,
		}
		av, _ := attributevalue.MarshalMap(item)
		_, err := db.PutItem(r.Context(), &dynamodb.PutItemInput{
			TableName: aws.String("UserReports"),
			Item:      av,
		})
		if err != nil {
			http.Error(w, err.Error(), 500)
			return
		}
		w.Write([]byte("stored"))
	}
}

If the stored name is later exported to CSV without sanitization, a value such as =HYPERLINK("https://evil.com") will be interpreted as a formula when opened in Excel. This can lead to unintended network calls or phishing behavior triggered by the victim’s own tools, not by DynamoDB or Chi themselves.

The risk is amplified when DynamoDB data is streamed into BI tools or spreadsheet exports that do not enforce strict input sanitization. Because Chi does not perform output encoding, it is the application’s responsibility to validate and sanitize data at the boundaries — especially when data leaves the database and enters contexts like spreadsheets or HTML templates.

An attacker might probe for this vulnerability by submitting names containing formula-like strings during registration or data entry. A security scan using middleBrick would check for improper output encoding and flag findings related to input validation and data exposure, aligning with OWASP API Top 10 and relevant compliance mappings.

Remediation focuses on preventing untrusted data from being interpreted as executable content when exported or rendered. For DynamoDB data consumed by Chi services, apply validation and escaping at the point of output, not at storage. This ensures flexibility while protecting downstream consumers.

1. Input validation and allowlisting

Validate user input against an allowlist where possible. For names, restrict to alphanumeric characters and safe punctuation:

import "regexp"

var safeName = regexp.MustCompile(`^[A-Za-z\-\s]{1,100}$`)

func isValidName(name string) bool {
	return safeName.MatchString(name)
}

Reject or sanitize input that does not conform before storing in DynamoDB.

2. Output sanitization for CSV/spreadsheet exports

When exporting data that originated from DynamoDB, escape leading formula characters. For CSV generation, prefix values that start with =, +, -, or @ with a single quote (') or wrap the field in double quotes with proper escaping:

import "encoding/csv"

func sanitizeForCSV(value string) string {
	if len(value) > 0 && strings.IndexAny(value, "=+-@") == 0 {
		return "'" + value // Excel treats prefixed ' as plain text
	}
	return value
}

// When writing CSV records:
record := []string{sanitizeForCSV(name)}

Alternatively, wrap the field in double quotes and escape internal quotes:

import "strconv"

func sanitizeForCSV(value string) string {
	return strconv.Quote(value)
}

3. Use structured reporting with safe templates

If generating reports with Go templates, use the html/template package which auto-escapes content, or define custom sanitization functions:

import (
	"html/template"
	"strings"
)

func escapeFormula(s string) template.HTML {
	if strings.HasPrefix(s, "=") {
		s = "'" + s
	}
	return template.HTML(s)
}

// In template:
{{ .Name | customEscape }}

These patterns ensure that data retrieved from DynamoDB and rendered in any downstream system — including spreadsheets — cannot trigger unintended formula evaluation.

middleBrick scans can validate that your API outputs properly sanitized data and flags cases where formula-injection-prone patterns reach export endpoints, helping you align with secure coding practices and compliance requirements.