Regex Dos in Echo Go with Hmac Signatures

A Regex DoS (ReDoS) in an Echo Go service that uses Hmac Signatures can occur when a route parameter or header value is validated with a non-anchored, repetitive regular expression and is also covered by an Hmac Signature check. In this combination, an attacker can send a carefully crafted, long, or pathological input that causes the regex engine to backtrack exponentially while the server still computes the Hmac Signature for the request. Because signature verification typically occurs before or in parallel with input validation, the server expends significant CPU cycles on the regex even after the signature is verified, leading to disproportionate resource consumption for a single request.

For example, consider an Echo route that uses a query parameter for a filter value and validates it with a vulnerable pattern such as (a+)+. If the same route also requires an Hmac Signature in a header to prove request integrity, an unauthenticated attacker can send a long string of a characters. The regex causes catastrophic backtracking, consuming CPU time, while the server continues to perform Hmac computations and other request processing. This pairing means the denial-of-service condition is not mitigated by the integrity check; the signature may be valid, but the validation logic creates a bottleneck that can exhaust server capacity and degrade availability.

In an API security scan, such as those performed by middleBrick, this issue would appear under Input Validation and Authentication/BOLA checks, with the scanner noting that a valid Hmac Signature does not prevent resource exhaustion from malformed input. The scanner tests unauthenticated attack surfaces, so it can detect whether long, repetitive inputs trigger excessive processing time even when signatures are present. This is important because the vulnerability is about the interaction between validation and integrity checks, not about signature forgery.

To remediate the interaction between Regex Dos and Hmac Signatures in Echo Go, you should avoid vulnerable patterns and structure validation so that signature verification and input checks fail fast without performing expensive operations on untrusted input. Use anchored, non-backtracking regular expressions, and perform lightweight format checks before computing or verifying Hmac Signatures.

Below are concrete code examples for a secure Echo route.

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "fmt"
    "net/http"
    "regexp"
    "strings"

    "github.com/labstack/echo/v4"
)

// securePattern is anchored and avoids repetitive capture groups.
var securePattern = regexp.MustCompile(`^[A-Za-z0-9_-]{1,64}$`)

// computeHmac returns a hex-encoded Hmac for testing; use constant-time comparison in production.
func computeHmac(message, secret string) string {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(message))
    return fmt.Sprintf("%x", mac.Sum(nil))
}

func main() {
    e := echo.New()
    sharedSecret := []byte("very-secure-shared-secret")

    e.GET("/resource/:id", func(c echo.Context) error {
        id := c.Param("id")
        receivedSig := c.Request().Header.Get("X-API-Signature")

        // Fail fast: reject obviously malformed IDs before Hmac work.
        if !securePattern.MatchString(id) {
            return echo.NewHTTPError(http.StatusBadRequest, "invalid id format")
        }

        // Compute expected signature using a canonical representation.
        expectedSig := computeHmac(id, string(sharedSecret))

        // Use constant-time comparison to avoid timing leaks.
        if !hmac.Equal([]byte(expectedSig), []byte(receivedSig)) {
            return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
        }

        // At this point the ID format is valid and the signature matches.
        return c.String(http.StatusOK, "ok")
    })

    // Start server on :8080
    e.Logger.Fatal(e.Start(":8080"))
}

Key remediation points:

• Use anchored regex with limited length and no nested quantifiers (e.g., ^[A-Za-z0-9_-]{1,64}$) to prevent backtracking.
• Perform format validation before any Hmac computation to avoid CPU-intensive work on invalid input.
• Compute and compare Hmac signatures using constant-time functions to prevent timing side-channels, even though the primary risk here is resource exhaustion.
• Consider moving signature verification earlier in the middleware chain or using lightweight integrity checks for very high-rate endpoints, while ensuring that validation failures do not trigger expensive operations.

These changes ensure that Regex Dos vectors are eliminated and that Hmac Signature checks do not amplify the impact of malformed input, aligning with findings that middleBrick would report under Authentication and Input Validation checks.