Regex Dos in Echo Go with Api Keys

A Regex DoS (ReDoS) condition arises when a regular expression has overlapping valid matches that cause catastrophic backtracking on certain inputs. In an Echo Go service that uses API keys exposed in URL or header paths, an attacker can craft a path or parameter that triggers exponential backtracking in the regex used to validate or capture the key. Even when the API key is expected to be a short alphanumeric string, route patterns like ^\/v1\/key\/([A-Za-z0-9]+)$ can become unsafe if the character class is too permissive or if the pattern is combined with quantifiers that allow many ways to match the same input.

Consider an Echo route that extracts an API key from the path and then validates it with a regex that includes optional segments or repetitions. For example, a pattern intended to match keys with optional hyphens (e.g., [A-Z0-9-]{20,40}) can cause the engine to explore many permutations when presented with a long string of hyphens and alphanumeric characters. Because Echo uses the standard library’s regexp package, these patterns are not inherently non-backtracking; a maliciously crafted request can force the server to spend substantial CPU time in the regex engine, leading to a high-severity denial-of-service condition.

When such a regex is applied to API key validation in Echo Go, the impact is compounded by the fact that API keys often appear in logs, metrics, and error messages, increasing exposure surface. If the route also captures the key into a group and passes it to downstream handlers without early rejection of malformed inputs, an attacker can repeatedly send paths like /v1/key/aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa-aaaa to keep the regex engine busy. Because the scan dimensions of middleBrick include Input Validation and Authentication, it would flag an unsafe regex pattern in API key handling as a high-severity finding tied to both authentication robustness and input validation resilience.

To mitigate Regex DoS in Echo Go when working with API keys, avoid open-ended quantifiers and prefer non-overlapping, bounded patterns. Use fixed-length or tightly constrained expressions, and leverage Go’s atomic groups or the (?P>...) recursion where supported. Prefer exact length checks or simple prefix/suffix validation on the key, and reject input early if it contains ambiguous repetitions.

Instead of a permissive pattern like:

keyPattern := regexp.MustCompile(`^\/v1\/key\/([A-Za-z0-9-]{20,40})$`)

Use a more constrained approach. If API keys are known to be exactly 32 hex characters (with or without hyphens at fixed positions), express this precisely:

// Example: 32 hex chars, no variable-length quantifiers that cause backtracking
keyPattern := regexp.MustCompile(`^\/v1\/key\/([0-9a-f]{32})$`)

If hyphens are allowed only at specific positions (e.g., UUID-like keys), validate structure with explicit segments rather than character class repetition:

// UUID-like key: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
keyPattern := regexp.MustCompile(`^\/v1\/key\/([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})$`)

In Echo, integrate this safely into a route handler and fail fast on malformed keys:

func validateAPIKey(c echo.Context) error {
    key := c.Param("key")
    if !keyPattern.MatchString(key) {
        return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid key format"})
    }
    // proceed with authentication logic
    return c.Next()
}

For continuous assurance, you can run a middleBrick scan against your service to detect unsafe regex patterns in API key handling. The CLI offers a quick check with middlebrick scan <url>, while the GitHub Action can enforce that new routes with API key parameters do not introduce high-severity input validation or authentication issues. These steps complement remediation by ensuring that patterns remain non-backtracking and that findings are tracked in the Dashboard over time.