Regex Dos in Buffalo with Dynamodb

A Regex Denial-of-Service (ReDoS) pattern occurs when a regular expression accepts input that causes catastrophic backtracking. In Buffalo, if you use a Go regex to validate or extract data from a request and that regex is applied to user-controlled strings, an attacker can craft input that forces exponential time. When a Buffalo application also interacts with Amazon DynamoDB—such as using the AWS SDK to query or batch-get items based on user-supplied values—the combination can amplify impact: the regex runs on every request before any DynamoDB call, and a slow regex can block the event loop or worker threads, leading to resource exhaustion and degraded availability.

Consider a route that extracts an id parameter and uses it to fetch an item from DynamoDB:

import (
    "github.com/gobuffalo/buffalo"
    "github.com/aws/aws-sdk-go/service/dynamodb"
)

func getItemHandler(c buffalo.Context) error {
    id := c.Param("id")
    // Unsafe: regex with potential catastrophic backtracking
    matched := regexp.MustCompile(`^(a+)+$`).MatchString(id)
    if !matched {
        return c.Error(400, errors.New("invalid id"))
    }
    _, err := svc.GetItem(&dynamodb.GetItemInput{
        TableName: aws.String("Items"),
        Key: map[string]*dynamodb.AttributeValue{
            "ID": {S: aws.String(id)},
        },
    })
    if err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(item))
}

The regex ^(a+)+$ is a classic ReDoS pattern. An input like aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa! causes the regex engine to explore many paths, consuming CPU for seconds. In Buffalo, each request is handled in a goroutine, but excessive backtracking can still saturate the runtime, causing request timeouts and high latency. If the DynamoDB call follows the regex check, the attacker can tie up server resources without ever reaching the database. Moreover, if the regex is used to parse or filter query parameters that are later passed to DynamoDB condition expressions, a maliciously crafted payload can degrade performance both at the application layer and indirectly affect database throughput due to increased load.

Another scenario involves parsing OpenAPI specs or scanning API definitions within Buffalo apps. If a regex is used to extract paths or parameters from spec files or runtime routes, an overly complex pattern can be triggered by maliciously large or nested spec documents. Since middleBrick scans include checks for Input Validation and the broader API security posture, it flags such risky regex usage before it reaches production.

To mitigate Regex Dos in Buffalo when working with DynamoDB, replace vulnerable regex patterns with safe, linear-time alternatives and avoid regex for validation when simpler methods suffice. Prefer structured parsing, strict allowlists, and length limits. Below are concrete code examples demonstrating secure patterns.

1. Replace catastrophic backtracking with safe validation

Instead of using a regex like ^(a+)+$, use a simple length check and character set validation, which runs in linear time:

import "unicode"

func isValidID(id string) bool {
    if len(id) > 64 {
        return false
    }
    for _, r := range id {
        if !unicode.IsLetter(r) && !unicode.IsDigit(r) {
            return false
        }
    }
    return true
}

func getItemHandler(c buffalo.Context) error {
    id := c.Param("id")
    if !isValidID(id) {
        return c.Error(400, errors.New("invalid id"))
    }
    // Safe DynamoDB call
    out, err := svc.GetItem(&dynamodb.GetItemInput{
        TableName: aws.String("Items"),
        Key: map[string]*dynamodb.AttributeValue{
            "ID": {S: aws.String(id)},
        },
    })
    if err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(out.Item))
}

2. Use structured parsing for IDs and keys

If IDs are numeric or UUIDs, parse them using strconv or github.com/google/uuid instead of regex:

import (
    "strconv"
    "github.com/google/uuid"
)

func parseID(id string) (string, error) {
    // If expecting numeric IDs
    if _, err := strconv.Atoi(id); err != nil {
        return "", errors.New("invalid numeric id")
    }
    return id, nil
}

// Or for UUIDs
func parseUUID(id string) (string, error) {
    _, err := uuid.Parse(id)
    if err != nil {
        return "", errors.New("invalid uuid")
    }
    return id, nil
}

3. Avoid regex in middleware for route protection

If you use middleware to validate paths or headers, prefer exact matches or prefix checks:

func secureMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        path := c.Request().URL.Path
        if !strings.HasPrefix(path, "/api/v1/items/") {
            return c.Error(403, errors.New("forbidden"))
        }
        return next(c)
    }
}

By combining these practices—linear validation, structured parsing, and avoiding complex regex—you reduce the attack surface for ReDoS while keeping DynamoDB interactions predictable and performant. Tools like middleBrick can help identify risky regex patterns during scans, supporting Input Validation checks before deployment.