Regex Dos in Buffalo with Hmac Signatures

A Regex DoS (also known as `ReDoS`) occurs when a regular expression has patterns that can cause catastrophic backtracking on certain inputs. When such patterns are used to validate or parse Hmac Signatures in the Buffalo web framework, an attacker can send carefully crafted HTTP requests that make the server spend excessive CPU time evaluating the regex, leading to severe latency or denial of service.

In Buffalo, Hmac Signatures are commonly used to verify the integrity of webhook payloads or API requests. A typical pattern might look like a long alphanumeric string, often encoded in hex or base64. If the developer uses a permissive or complex regex to "be flexible" with the signature format, the engine can enter exponential backtracking when presented with specially crafted long strings that partially match the pattern. For example, a naive regex like ^([a-fA-F0-9]+)$ applied to a very long string of the same character (e.g., ffffffffffffffff…) can cause the backtracking engine to explore an enormous number of paths, consuming disproportionate resources.

Because Buffalo applications often handle high-volume webhook endpoints, a single malicious request can stall a worker, increasing latency for legitimate traffic. The vulnerability is not in Buffalo itself but in the regex logic applied to the Hmac Signature. The signature is usually extracted from a header or a form parameter and then compared to a computed value; if the extraction or validation uses a vulnerable pattern, the attack surface is exposed.

An example vulnerable handler might look like this, where a broad character class is used:

// Vulnerable: permissive regex for Hmac Signature validation
func VerifySignature(c buffalo.Context) error {
    sig := c.Param("signature")
    matched, _ := regexp.MatchString(`^[a-zA-Z0-9+/=\s]{10,200}$`, sig)
    if !matched {
        return c.Error(http.StatusBadRequest, errors.New("invalid signature"))
    }
    // proceed with Hmac verification
    return nil
}

The character class [a-zA-Z0-9+/=\s] combined with a large range {10,200} can lead to excessive branching. An attacker can send a 200-character string that keeps the regex engine busy for seconds, especially if the underlying regex implementation does not optimize for such cases.

To mitigate this within the constraints of using regex for Hmac Signature validation, it is best to keep patterns as strict and simple as possible, avoid large unbounded ranges, and prefer exact-length checks or fixed character sets. Because Buffalo does not have built-in regex optimization guards, developers must design patterns defensively.

middleBrick’s scanning checks for risky regex patterns in API specifications and runtime behavior, flagging patterns that can lead to excessive backtracking. If your API specifications include endpoints that validate Hmac Signatures with complex regexes, a scan can surface these findings before they are exploited, complementing runtime caution.

Remediation focuses on simplifying and tightening the regular expression used to validate Hmac Signatures, ensuring it cannot cause catastrophic backtracking. For Hmac Signatures, which are typically hex strings or base64url-encoded values, a strict, non-overlapping pattern is sufficient and safe.

1. Use a strict hex-only pattern with a fixed or reasonable length range.

Hexadecimal Hmac Signatures (e.g., SHA256) are often 64 characters long. If you expect a fixed length, enforce it strictly:

// Safe: strict hex pattern with exact length for SHA256 Hmac
func VerifySignature(c buffalo.Context) error {
    sig := c.Param("signature")
    // Exactly 64 hex characters, no ambiguity
    matched, _ := regexp.MatchString(`^[a-fA-F0-9]{64}$`, sig)
    if !matched {
        return c.Error(http.StatusBadRequest, errors.New("invalid signature"))
    }
    // proceed with Hmac verification
    return nil
}

If lengths vary, use a bounded range that is small and non-exponential:

// Safe: bounded length for variable-length hex signatures (e.g., 32–64)
func VerifySignature(c buffalo.Context) error {
    sig := c.Param("signature")
    matched, _ := regexp.MatchString(`^[a-fA-F0-9]{32,64}$`, sig)
    if !matched {
        return c.Error(http.StatusBadRequest, errors.New("invalid signature"))
    }
    // proceed
    return nil
}

2. For base64url Hmac Signatures, avoid classes that cause backtracking.

Base64url characters are A-Za-z0-9_-. Use a fixed length when possible, or a modest bounded range:

// Safe: base64url pattern with a reasonable max length to prevent runaway backtracking
func VerifySignature(c buffalo.Context) error {
    sig := c.Param("signature")
    // Base64url: A-Za-z0-9_- with length cap
    matched, _ := regexp.MatchString(`^[A-Za-z0-9_-]{43,44}$`, sig)
    if !matched {
        return c.Error(http.StatusBadRequest, errors.New("invalid signature"))
    }
    // proceed
    return nil
}

3. Consider moving validation out of regex entirely.

Regex is not always necessary for Hmac Signature validation. If the signature is a fixed-length hex string, you can use simple string operations to check length and character set before applying a constant-time comparison:

// Non-regex safe check for hex Hmac Signature
func VerifySignature(c buffalo.Context) error {
    sig := c.Param("signature")
    if len(sig) != 64 {
        return c.Error(http.StatusBadRequest, errors.New("invalid signature"))
    }
    for _, ch := range sig {
        if !(('0' <= ch && ch <= '9') || ('a' <= ch && ch <= 'f') || ('A' <= ch && ch <= 'F')) {
            return c.Error(http.StatusBadRequest, errors.New("invalid signature"))
        }
    }
    // proceed with cryptographic comparison
    return nil
}

These fixes ensure that validation of Hmac Signatures in Buffalo does not introduce catastrophic backtracking while still enforcing the expected format. Because the scans complete in 5–15 seconds, you can quickly validate that your patterns are safe. For ongoing safety, use the middleBrick CLI to scan your API definitions and the GitHub Action to fail builds if risky regex patterns are introduced.