Use After Free in Chi with Jwt Tokens

A Use After Free (UAF) in Chi involving JWT tokens occurs when a token object is deallocated while a reference to it remains active and is subsequently accessed during request handling. In Chi, this can happen when middleware or handlers share token references across goroutines without ensuring the underlying memory remains valid for the full lifetime of the operation.

Consider a Chi route that decodes a JWT token and passes the resulting claims object to a background worker or goroutine for further processing. If the handler returns and the claims object is freed (e.g., due to scope exit or garbage collection timing in a non-GC-safe context), the background worker may attempt to read from that memory. This race condition can lead to undefined behavior, including reading sensitive claim data or causing crashes that expose internal state.

During a black-box scan, middleBrick tests for such instability by sending concurrent requests with varying token lifetimes and inspecting responses for signs of memory safety violations. For example, it may detect scenarios where token validation logic relies on stale pointers or where token payloads are reused across requests, increasing the risk of information leakage or erratic application behavior.

Real-world attack patterns related to this issue include token handling in high-concurrency environments where request context lifetimes are not explicitly managed. Although Chi does not manage memory manually, improper sharing of parsed token data between handlers and asynchronous processes can create conditions similar to UAF-like races. middleBrick’s checks for unsafe consumption and property authorization help surface these risks by correlating token usage patterns with observable side effects across parallel requests.

An example of a vulnerable pattern in Chi might involve passing a pointer to a claims struct to a goroutine without synchronization:

claims := parseToken(tokenString)
go func() {
    processClaims(claims)
}()
// claims may be freed here while the goroutine uses it

middleBrick’s LLM/AI Security checks also evaluate whether token handling logic could be influenced by adversarial inputs, such as prompt injection attempts targeting logging or error paths that include token metadata. This is particularly relevant when tokens are embedded in debug output or telemetry, where exposure could aid further exploitation.

To mitigate Use After Free risks with JWT tokens in Chi, ensure that all token-derived data is fully copied and isolated before being used outside the immediate request context. Avoid passing pointers to token claims to asynchronous routines unless the data is explicitly deep-copied or otherwise guaranteed to remain valid.

A safer approach is to extract necessary fields from the token into local values or immutable structures before spawning goroutines. This prevents any reliance on memory that may be reclaimed prematurely.

Here is a secure code example using the golang-jwt/jwt library with Chi:

import (
    "github.com/golang-jwt/jwt/v5"
    "net/http"
)

func handler(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        tokenString := r.Header.Get("Authorization")
        claims := jwt.MapClaims{}

        token, err := jwt.ParseWithClaims(tokenString, claims, func(token *jwt.Token) (interface{}, error) {
            return []byte("secret"), nil
        })
        if err != nil || !token.Valid {
            http.Error(w, "invalid token", http.StatusUnauthorized)
            return
        }

        // Copy claims into a local struct to avoid pointer sharing
        type localClaims struct {
            UserID string
            Role   string
        }
        safeClaims := localClaims{
            UserID: claims["user_id"].(string),
            Role:   claims["role"].(string),
        }

        // Safe to use safeClaims in goroutines
        go func(c localClaims) {
            processClaims(c)
        }(safeClaims)

        next.ServeHTTP(w, r)
    })
}

Additionally, configure middleware to enforce token validation early in the request lifecycle and ensure that any downstream handlers operate on copies of the claims. middleBrick’s authentication and property authorization checks validate that token scopes and permissions are consistently enforced without relying on shared mutable state.

For applications using the Pro plan, continuous monitoring can detect anomalous token usage patterns that may indicate race conditions or unsafe consumption. The CLI tool can be integrated into development workflows to scan Chi-based APIs and flag insecure token handling during code review.