Regex Dos in Buffalo with Basic Auth

Regex Denial-of-Service (Regex DoS) occurs when a regular expression exhibits catastrophic backtracking on certain inputs, causing CPU usage to spike and response times to increase dramatically. In Buffalo, this risk is amplified when Basic Authentication is handled in application code using regular expressions to parse or validate the Authorization header, especially if the regex is non-anchored or uses nested quantifiers.

Consider a typical approach where developers use a regex to extract credentials from the Authorization header:

// Avoid: vulnerable to Regex DoS via crafted input
const authHeader = req.Header.Get("Authorization")
re := regexp.MustCompile(`^Basic\s+(\w+)=(\w+)$`)
matches := re.FindStringSubmatch(authHeader)

If the regex is more complex—for example, to allow optional whitespace or to validate token formats—and uses patterns like (\w+)* or unanchored repetitions, an attacker can supply a long, specially crafted string that causes exponential backtracking. Because Buffalo applications often process requests synchronously in handlers, this can degrade performance for all requests served by that process.

When Basic Auth is validated with such a regex, the header value is directly exposed to this risk. An attacker can send many requests with strings like Basic aaaaa|baaaa|... (depending on the regex structure) to trigger the worst-case behavior. Since the scan profile for middleBrick includes Input Validation checks, such patterns would be surfaced as findings, highlighting the regex path as an exposed attack surface. In a black-box scan, the scanner submits crafted headers and measures response behavior, identifying latency anomalies tied to regex evaluation without needing authentication.

Even with simpler patterns, edge cases in header formatting (e.g., extra spaces, unusual encodings) can lead to inefficient matching. Because middleBrick tests unauthenticated endpoints and includes Input Validation and Unsafe Consumption checks, it can detect endpoints where regex-based parsing of Basic Auth headers shows signs of instability. Remediation focuses on simplifying or avoiding regex for parsing structured credentials and using standard library functions instead.

To mitigate Regex DoS in Buffalo when using Basic Authentication, avoid parsing the Authorization header with custom regular expressions. Instead, rely on standard library functions that parse Base64-encoded credentials deterministically and safely.

The recommended pattern decodes the header value without regex:

// Safe: no regex, using standard library
import (
    "encoding/base64"
    "strings"
)

func parseBasicAuth(authHeader string) (username, password string, err error) {
    const prefix = "Basic "
    if !strings.HasPrefix(authHeader, prefix) {
        return "", "", fmt.Errorf("invalid authorization header")
    }
    encoded := authHeader[len(prefix):]
    decoded, err := base64.StdEncoding.DecodeString(encoded)
    if err != nil {
        return "", "", err
    }
    // Credentials are in the format "username:password"
    parts := strings.SplitN(string(decoded), ":", 2)
    if len(parts) != 2 {
        return "", "", fmt.Errorf("invalid basic auth format")
    }
    return parts[0], parts[1], nil
}

func authRequired(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        header := c.Request().Header.Get("Authorization")
        user, pass, err := parseBasicAuth(header)
        if err != nil || !isValidUserPass(user, pass) {
            c.Response().Header().Set("WWW-Authenticate", `Basic realm="restricted"`)
            return c.Error(401, errors.New("unauthorized"))
        }
        return next(c)
    }
}

This approach eliminates regex entirely, removing the risk of catastrophic backtracking. It also aligns with security best practices by avoiding custom validation of structured credentials. If your project uses middleware for authentication, ensure it uses this decoding pattern rather than inline regex checks.

For projects that integrate with external identity providers or need more advanced handling, consider using dedicated libraries that are well-tested and maintained, but still avoid introducing regex-based parsing into request-handling paths. With middleBrick’s CLI, you can verify that endpoints no longer exhibit risky regex patterns by running middlebrick scan <url> and reviewing the Input Validation findings. Teams on the Pro plan can enable continuous monitoring to catch regressions early via GitHub Action or CI/CD pipeline gates.