Out Of Bounds Write in Gorilla Mux with Jwt Tokens

An Out Of Bounds Write occurs when an application writes data past the boundaries of a fixed-length buffer or data structure. In a Gorilla Mux routing context combined with JWT token handling, this typically arises when unbounded or maliciously crafted claims (e.g., a large "sub" or custom claim) are copied into a fixed-size buffer during token parsing, session storage, or request context assignment without proper length checks.

Gorilla Mux is a URL router and dispatcher for Go. When JWT tokens are used for authentication, developers commonly parse the token, extract claims, and store them in request contexts or maps that feed into Mux route handlers. If the token payload contains an oversized claim and the application uses fixed-size arrays or improperly sized slices to hold claim values, an Out Of Bounds Write can occur in the application’s own data structures (not in JWT libraries themselves), potentially corrupting memory or causing crashes.

Consider a handler that copies a custom claim into a fixed-size array:

// WARNING: Unsafe example demonstrating a potential out-of-bounds write risk
var userID [16]byte // fixed-size buffer
claims := token.Claims.(jwt.MapClaims)
if uid, ok := claims["user_id"].(string); ok {
    copy(userID[:], uid) // risk: if len(uid) > 16, copy writes beyond userID boundary
}

An attacker could supply a long user_id claim (e.g., 64 bytes) in the JWT. The copy call would write past the 16-byte array, corrupting adjacent memory. In a server running multiple requests, this could lead to unpredictable behavior or crashes, and in some environments, it might be leveraged for further exploitation.

Even when using maps or slices, a lack of validation on claim sizes can lead to excessive memory allocation patterns that indirectly stress service stability, but the classic Out Of Bounds Write is about fixed buffers. Gorilla Mux routes then process these corrupted contexts, and the compromised request may behave erratically.

Additionally, if JWT tokens are stored server-side in fixed-size caches or session stores (e.g., a fixed-length byte array keyed by token ID), an oversized token or claim can overflow those stores when written. This is a server-side storage issue, but it originates from the same lack of boundary checks on data originating from the JWT.

Because middleBrick scans test unauthenticated attack surfaces and include JWT-related checks among its 12 parallel security checks, it can surface risky patterns in how JWT claims are handled before they interact with routing logic. The scanner does not fix the code, but its findings highlight where input validation and size constraints are missing.

To summarize, the combination of Gorilla Mux routing and JWT token processing exposes Out Of Bounds Write risks when developer code copies unbounded JWT claim values into fixed-size buffers or structures without validating lengths. This violates basic memory safety and can corrupt state or crash the service.

Remediation focuses on validating and bounding JWT claim data before using it in any fixed-size structures or contexts that feed into Gorilla Mux handlers. Always treat JWT claims as untrusted input.

1. Use slices with length checks instead of fixed arrays

Replace fixed-size arrays with slices and enforce maximum lengths:

// Safe approach: validate length before using
const maxUserIDLen = 128
claims := token.Claims.(jwt.MapClaims)
var userID string
if uid, ok := claims["user_id"].(string); ok {
    if len(uid) > maxUserIDLen {
        // reject token or handle error
        http.Error(w, "invalid token", http.StatusUnauthorized)
        return
    }
    userID = uid
}
// Use userID as a string; no fixed buffer overflow risk

2. Use bounded copy with explicit size checks

If you must use a fixed-size buffer, ensure the source length is bounded:

var userID [16]byte
claims := token.Claims.(jwt.MapClaims)
if uid, ok := claims["user_id"].(string); ok {
    if len(uid) >= len(userID) {
        http.Error(w, "invalid token claim", http.StatusUnauthorized)
        return
    }
    copy(userID[:], uid) // now safe: len(uid) < len(userID)
}

3. Validate all incoming claims used in routing

Gorilla Mux handlers often read claims from context. Validate length and type before usage:

func myMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        claims := GetClaimsFromContext(r) // your helper
        if sub, ok := claims["sub"].(string); ok {
            if len(sub) == 0 || len(sub) > 255 {
                http.Error(w, "invalid subject claim", http.StatusUnauthorized)
                return
            }
            // safe to use sub in routing logic or Mux vars
            ctx := context.WithValue(r.Context(), "sub", sub)
            next.ServeHTTP(w, r.WithContext(ctx))
        } else {
            http.Error(w, "missing claims", http.StatusUnauthorized)
        }
    })
}

4. Avoid storing JWTs in fixed-size server-side caches

If you cache tokens or claims server-side, use structures that grow safely (e.g., maps with size limits or eviction policies), not fixed-size byte arrays.

5. Use a JWT library that enforces claims validation

Configure your parser to enforce expected types and lengths where possible, and reject tokens with unexpected or oversized claims early.

By applying these patterns, you remove the conditions that enable Out Of Bounds Writes when JWT claims interact with Gorilla Mux handlers or any downstream processing. middleBrick’s scans can help you identify locations where JWT claims are used without adequate bounds checks.