Regex Dos in Gin with Hmac Signatures

A Regex DoS (ReDoS) occurs when a regular expression has overlapping or ambiguous quantifiers that cause catastrophic backtracking on certain inputs. In Gin, this risk appears when you use a custom regexp based route pattern or middleware validation that processes user-controlled data in an unbounded way. Combining such regexes with HMAC signature verification can amplify the impact: an attacker may send many crafted requests that force the server to spend excessive CPU time evaluating the regex before the signature check even runs, leading to high latency or service degradation.

Consider a Gin route that validates a token format with a complex pattern and then verifies an HMAC signature in a header. If the regex is poorly constructed (e.g., using nested quantifiers like (a+)+), an attacker can supply a long string that causes exponential backtracking. Because Gin typically processes routes and middleware in sequence, the regex evaluation occurs before your HMAC verification middleware, allowing an attacker to tie up goroutines without ever reaching the signature logic. This becomes a vector for resource exhaustion in high-concurrency scenarios, even when signatures are eventually checked.

An example of a vulnerable Gin handler might use a pattern like ^([a-zA-Z0-9]+){1,30}$ to validate part of a path or query parameter. If an input such as aaaaaaaaaaaaaaaaaaaa! is provided, the regex engine can enter catastrophic backtracking. Even if you later compute an HMAC to verify request integrity, the damage is already done because the CPU cost of the regex dominates. The issue is not the HMAC itself, but the unsafe regex preceding it in the processing pipeline.

In practice, this risk surfaces in three dimensions: the regex pattern design, how Gin applies route matching and middleware ordering, and how HMAC verification is integrated. If the regex is placed in a middleware that runs before signature validation, an attacker can force unnecessary computation. If route definitions use regex-based parameters without constraints, the attack surface grows. And if HMAC verification is implemented in a way that does not short-circuit processing on early validation failures, server resources are wasted on invalid inputs.

To illustrate, a vulnerable Gin route might look like this, where a regex-based parameter is used directly in the path:

r.GET("/api/data/:token{[a-zA-Z0-9]{1,50}", func(c *gin.Context) {
    token := c.Param("token")
    // HMAC verification happens later
    verifyHMAC(c)
    c.JSON(200, gin.H{ "echo": token })
})

An input like a very long alphanumeric string can trigger excessive backtracking in the route matcher’s regex engine, delaying the call to verifyHMAC. A safer approach is to constrain patterns tightly, avoid nested quantifiers, and move validation into explicit Go code after early checks, ensuring HMAC verification is not bypassed or delayed by pathological regex behavior.

To mitigate Regex DoS in Gin while using HMAC signatures, focus on simplifying regex patterns, controlling middleware execution order, and validating input before performing cryptographic checks. Avoid complex, user-influenced regexes for routing; instead, use fixed formats or simple character class patterns. Ensure that HMAC verification runs only after basic format checks pass, and short-circuit processing on malformed input.

Here is a concrete, safe Gin implementation that avoids ReDoS-prone patterns and integrates HMAC verification correctly:

func verifyHMAC(c *gin.Context) {
    expected := os.Getenv("API_SECRET")
    received := c.GetHeader("X-API-Signature")
    if received == "" {
        c.AbortWithStatusJSON(401, gin.H{ "error": "missing signature" })
        return
    }
    // Use a simple, bounded check before crypto work
    if len(received) < 8 || len(received) > 128 {
        c.AbortWithStatusJSON(400, gin.H{ "error": "invalid signature format" })
        return
    }
    mac := hmac.New(sha256.New, []byte(expected))
    mac.Write([]byte(c.Request.URL.Path))
    expectedMAC := hex.EncodeToString(mac.Sum(nil))
    if !hmac.Equal([]byte(received), []byte(expectedMAC)) {
        c.AbortWithStatusJSON(403, gin.H{ "error": "invalid signature" })
        return
    }
    c.Next()
}

func safeRouteHandler(c *gin.Context) {
    token := c.Param("token")
    // Simple, bounded pattern check instead of complex regex
    matched, _ := regexp.MatchString(`^[A-Za-z0-9\-_]{8,64}$`, token)
    if !matched {
        c.AbortWithStatusJSON(400, gin.H{ "error": "invalid token format" })
        return
    }
    verifyHMAC(c)
    c.JSON(200, gin.H{ "token": token })
}

func SetupRoutes(r *gin.Engine) {
    // Use a fixed format route parameter instead of a complex regex
    r.GET("/api/data/:token", safeRouteHandler)
}

Key practices shown:

• Avoid deeply nested quantifiers; use bounded patterns like [A-Za-z0-9\-_]{8,64}.
• Perform lightweight format checks before invoking HMAC computation to avoid unnecessary crypto work on malformed input.
• Abort early with clear error codes so that invalid requests do not proceed to expensive verification steps.
• Keep route parameters simple; if you need complex validation, do it in Go after basic pattern checks.

Additionally, consider moving format constraints into explicit validation functions rather than relying on Gin’s regex-based route patterns. This reduces the risk of ReDoS and ensures that HMAC verification is only reached after input has passed safe, bounded checks.