Regex Dos in Echo Go with Bearer Tokens

A Regex DoS (ReDoS) occurs when a regular expression has overlapping or ambiguous quantifiers that cause catastrophic backtracking on certain inputs. In Echo Go, this risk is amplified when Bearer token validation is performed using complex or non-anchored regex patterns on user-controlled data such as Authorization headers. Because Bearer tokens are often long, opaque strings containing characters that can interact with regex metacharacters, an attacker can craft tokens that trigger exponential path exploration in the regex engine.

For example, if an Echo Go route uses a middleware pattern like regexp.MustCompile("^Bearer ([A-Za-z0-9-_]+)$") and the token contains repeated substrings or characters that cause the character class to backtrack extensively, the server can consume high CPU for each request. This becomes a vector for resource exhaustion when the regex is applied to every incoming request in a high-traffic API endpoint. The vulnerability is not in Echo itself but in how the developer writes the token validation pattern; an unanchored or overly permissive regex on a high-frequency path can turn a simple authentication check into a denial-of-service vector.

When OpenAPI/Swagger specs are analyzed, definitions describing security schemes with type apiKey and in: header may map to Bearer token usage in code. If the implementation uses a non-optimized regex for validation, scanning can flag the pattern as a potential ReDoS finding. Since middleBrick tests unauthenticated attack surfaces, it can detect risky regex usage in API specifications and runtime behavior without requiring credentials. This is particularly relevant when token handling intersects with authorization logic, where repeated or nested quantifiers in route patterns can amplify latency.

In practice, a vulnerable Echo Go handler might look like this:

import ( "net/http" "regexp" "github.com/labstack/echo/v4" ) var tokenPattern = regexp.MustCompile(`^Bearer ([A-Za-z0-9\-._~+/=]+)$`) func ValidateToken(next echo.HandlerFunc) echo.HandlerFunc { return func(c echo.Context) error { auth := c.Request().Header.Get("Authorization") if !tokenPattern.MatchString(auth) { return c.String(http.StatusUnauthorized, "invalid") } return next(c) } }

An attacker sending Authorization: Bearer aaaaa...aaaa with a crafted sequence can force the regex engine into extensive backtracking. Because the pattern includes a large character class and a greedy quantifier, the computational cost grows quickly. This risk is compounded when multiple routes share similar token validation logic, increasing the overall attack surface. middleBrick’s checks for Input Validation and Property Authorization help surface such patterns by correlating spec definitions with observed runtime behavior, ensuring that authentication checks do not introduce unintended denial-of-service paths.

Remediation focuses on simplifying and anchoring regex patterns, avoiding repetition and ambiguous quantifiers, and moving validation away from complex per-request regex matching where possible. For Bearer tokens, prefer exact prefix checks and constant-time substring operations instead of character-class-heavy regexes. If regex is necessary, ensure it is linear in complexity by using non-overlapping groups, avoiding nested quantifiers, and anchoring patterns tightly.

Below is a secure version of the earlier handler that avoids ReDoS by using strings.HasPrefix and minimal parsing:

import ( "net/http" "strings" "github.com/labstack/echo/v4" ) func ValidateTokenSecure(next echo.HandlerFunc) echo.HandlerFunc { return func(c echo.Context) error { auth := c.Request().Header.Get("Authorization") if !strings.HasPrefix(auth, "Bearer ") { return c.String(http.StatusUnauthorized, "invalid") } token := auth[7:] // len("Bearer ") if token == "" { return c.String(http.StatusUnauthorized, "missing") } // Optional: additional token format checks without heavy regex return next(c) } }

If you must validate token structure (e.g., UUID or base64-like format), use a compiled regex that is linear and anchored, and avoid character classes that can cause backtracking:

var simpleTokenRegex = regexp.MustCompile(`^Bearer [A-Za-z0-9\-]{22,}$`) func ValidateTokenLinear(c echo.Context) error { auth := c.Request().Header.Get("Authorization") if !simpleTokenRegex.MatchString(auth) { return c.String(http.StatusUnauthorized, "invalid") } return c.Next() }

For API definitions, describe Bearer token security schemes using type: apiKey and in: header in your OpenAPI spec without embedding validation logic complexity. middleBrick’s OpenAPI/Swagger analysis resolves $ref definitions and cross-references them with runtime findings, so you can see whether your spec implies a pattern that may lead to insecure implementations. With the Pro plan, continuous monitoring can alert you if a new endpoint introduces a high-risk regex pattern during development, and the GitHub Action can fail builds if risk scores exceed your configured thresholds.

Finally, consider moving token validation to a lightweight prefix check and a dedicated authentication service rather than embedding complex regex in handlers. This reduces per-request CPU load and aligns with secure-by-default practices. The MCP Server allows you to scan APIs directly from your IDE, catching problematic patterns before code is committed, while the Dashboard helps track security scores over time to ensure remediation reduces long-term risk.