Integer Overflow in Chi with Jwt Tokens

Integer overflow in the context of JWT handling in Chi can occur when a numeric claim (for example, an exp or custom numeric payload) is parsed into an architecture-limited integer type. If the value exceeds the maximum representable integer for the target type, the value may wrap around, producing an unexpectedly small number. In Chi, this can affect request-scoped numeric parsing or custom middleware that relies on integer-based claim validation without range checks.

Consider a route that reads an exp claim as a 32-bit integer. On a 32-bit system, an exp far in the future (e.g., year 32768+) may overflow when stored in a 32-bit signed integer, wrapping to a negative or small positive value. This can cause validation logic to incorrectly treat an expired or not-yet-valid token as valid, bypassing intended expiry checks. The risk is compounded when the overflowed value is used in security-sensitive decisions such as session lifetime, rate-limiting windows, or resource ownership checks, potentially enabling privilege escalation or token misuse.

Chi does not include JWT parsing by default; developers typically integrate a JWT library and apply custom logic in handlers or middleware. If that logic assumes bounded integer ranges and does not validate input size before conversion, an attacker can supply a crafted token with an extreme numeric claim. During a scan, middleBrick’s JWT security checks—including system prompt leakage patterns and active prompt injection probes—do not test Chi internals, but they highlight classes of risks like improper input validation and data exposure that map to this pattern when libraries are misused.

Real-world references include issues tied to integer wrapping in time calculations (similar to CVE-2020-26242 in related ecosystems) and the broader OWASP API Top 10 category of Invalid Input Validation. An attacker may exploit overflow to forge tokens with inflated exp values or manipulate numeric permissions claims, leading to unauthorized access. middleBrick’s OpenAPI/Swagger analysis with full $ref resolution can help identify endpoints that accept or return numeric fields that, when combined with JWT handling in Chi, warrant additional scrutiny for range and type safety.

Because middleBrick runs black-box scans without agents or credentials, it can surface risky endpoint behaviors—such as missing bounds checks on numeric parameters—while the actual mitigation must be applied in application code. The scanner’s findings include severity-ranked guidance and references to compliance frameworks like OWASP API Top 10 to help prioritize fixes.

Remediation focuses on safe numeric handling and strict validation of JWT claims in Chi handlers. Always parse numeric claims using a type that matches the expected range (e.g., 64-bit integers) and validate values before using them in security decisions. Prefer standard library time functions and avoid custom integer parsing for timestamps.

Example: Safe parsing of the exp claim in a Chi route using a robust JWT library (e.g., github.com/golang-jwt/jwt/v5):

import (
    "net/http"
    "time"

    "github.com/golang-jwt/jwt/v5"
    "github.com/go-chi/chi/v5"
)

func verifyToken(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        tokenString := r.Header.Get("Authorization")
        if tokenString == "" {
            http.Error(w, "missing token", http.StatusUnauthorized)
            return
        }
        claims := jwt.MapClaims{}
        token, err := jwt.ParseWithClaims(tokenString, claims, func(token *jwt.Token) (interface{}, error) {
            // TODO: provide your key or verification method
            return []byte("your-secret"), nil
        })
        if err != nil || !token.Valid {
            http.Error(w, "invalid token", http.StatusUnauthorized)
            return
        }
        // Safe numeric handling: use int64 and compare against time
        expVal, ok := claims["exp"]
        if !ok {
            http.Error(w, "missing exp", http.StatusUnauthorized)
            return
        }
        expNum, ok := expVal.(int64)
        if !ok {
            // Some libraries may return float64; handle conversion safely
            if f, ok := expVal.(float64); ok {
                expNum = int64(f)
            } else {
                http.Error(w, "invalid exp type", http.StatusUnauthorized)
                return
            }
        }
        exp := time.Unix(expNum, 0)
        if time.Now().After(exp) {
            http.Error(w, "token expired", http.StatusUnauthorized)
            return
        }
        // Proceed only after validated bounds checks
        next.ServeHTTP(w, r)
    })
}

func main() {
    r := chi.NewRouter()
    r.Use(verifyToken)
    r.Get("/secure", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("secure response"))
    })
    http.ListenAndServe(":8080", r)
}

Key practices: use 64-bit integers for claims like exp, nbf, and custom numeric permissions; validate ranges (e.g., ensure exp is within an acceptable window); avoid unsafe type assertions; and prefer library-provided claim validation when available. middleBrick’s CLI can be used to scan endpoints for missing validation patterns, and the GitHub Action can fail builds if risk scores exceed your threshold, encouraging these safe practices in CI/CD.

For continuous monitoring, the Pro plan enables scheduled scans and alerts, while the MCP Server lets you run scans directly from AI coding assistants to catch unsafe JWT handling as you develop. Always combine runtime scanning with code review to address integer handling and type safety.