Use After Free in Gin with Hmac Signatures

Use After Free (UAF) in the context of an API built with the Gin web framework and Hmac Signatures occurs when memory that was previously allocated for request-scenario data (such as parsed headers, signature parameters, or a derived signing key) is released or reused while a subsequent operation still holds a reference to it. In Go, UAF is rare in pure managed code because of garbage collection, but it can manifest through unsafe patterns or when integrating with lower-level libraries and Cgo. More commonly in Gin, this class of bug arises from improper handling of request context values, byte slices, or signature state across concurrent handlers or between middleware and business logic.

Consider a handler that uses Hmac Signatures to verify a request. A typical flow includes reading a timestamp and a signature from headers, constructing a signing string, and computing an Hmac to compare against the provided signature. If the code stores references to buffers or objects in the Gin context to avoid allocations and then reuses or mutates those objects in another concurrent request, the reused memory may overwrite sensitive data. An attacker could trigger the timing or request ordering that causes the freed memory to be repurposed, potentially exposing another request’s partial signature material or keying material. This is especially risky when the handler uses pooled buffers or manually managed byte slices to improve performance but fails to isolate per-request state.

For example, a middleware that copies header values into a context value as a []byte and later reuses that slice for a different request can inadvertently create a UAF-like condition if the slice is mutated after being passed to a verification routine. In systems that interface with external C libraries for cryptographic operations, improper memory management on the C side can compound the issue, making it appear as though Gin itself is vulnerable. The risk is not in Hmac itself but in how state is captured, retained, and possibly shared across requests. The 12 security checks in middleBrick test for such unsafe consumption patterns and data exposure, flagging scenarios where signature-related data might be improperly handled or exposed across concurrent operations.

Another angle is signature validation logic that parses and then retains a reference to request body or header data beyond the immediate verification step. If the handler caches or logs parts of the signing material without copying, and later goroutines access that stale reference, the effective protection provided by Hmac Signatures can be undermined. middleBrick’s checks for Unsafe Consumption, Data Exposure, and Unsafe Consumption are designed to surface these patterns so that developers can ensure each request’s cryptographic state remains isolated and short-lived.

To remediate Use After Free risks and ensure robust Hmac Signature handling in Gin, adopt practices that guarantee per-request isolation, avoid mutable shared state, and use standard library primitives correctly. Below are concrete code examples and guidelines.

1. Isolate per-request signing data

Do not store slices or structs derived from request headers in context as mutable references. Instead, copy values and store immutable copies.

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "strings"

    "github.com/gin-gonic/gin"
)

func verifyHmac(c *gin.Context) {
    // Extract raw values without retaining references to c.Request.Body
    timestamp := c.Request.Header.Get("X-Timestamp")
    receivedSig := c.Request.Header.Get("X-Signature")
    if timestamp == "" || receivedSig == "" {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "missing headers"})
        return
    }

    // Build the signing string from request-scoped data
    method := c.Request.Method
    path := c.Request.URL.Path
    body, _ := c.GetRawData() // consumes body; make a copy if you need it later
    message := method + path + string(body) + timestamp

    // Use a per-request key derivation or retrieval; do not reuse a mutable key variable
    secret := []byte("your-secure-secret")
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(message))
    expected := hex.EncodeToString(mac.Sum(nil))

    if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
        c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
        return
    }

    // Proceed only after successful verification; do not retain body or headers in context
    c.Set("verified", true)
    c.Next()
}

func handler(c *gin.Context) {
    if verified, _ := c.Get("verified"); verified == true {
        c.JSON(http.StatusOK, gin.H{"status": "ok"})
    }
}

func main() {
    r := gin.Default()
    r.Use(verifyHmac)
    r.GET("/resource", handler)
    r.Run()
}

This pattern copies headers and body data into local variables, constructs the message per request, and performs comparison without retaining references to request-scoped memory beyond the handler’s invocation.

2. Avoid reusing buffers or context values across requests

Do not assign slices or objects to gin.Context in a way that allows cross-request contamination. If you must cache data, ensure deep copies and clear them after use.

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"

    "github.com/gin-gonic/gin"
)

type SafeContext struct {
    verified bool
    // other request-scoped fields, no shared slices
}

func verifyHmacSafe(c *gin.Context) {
    // Local variables only; no assignment to c.Keys that retain slices
    timestamp := c.GetHeader("X-Timestamp")
    receivedSig := c.GetHeader("X-Signature")
    if timestamp == "" || receivedSig == "" {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "missing headers"})
        return
    }

    secret := []byte("your-secure-secret")
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(c.Request.Method + c.Request.URL.Path + c.GetRawDataMustRead(timestamp)))
    expected := hex.EncodeToString(mac.Sum(nil))

    if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
        c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
        return
    }

    // Store immutable primitives, not buffers
    c.Set("safe_ctx", SafeContext{verified: true})
    c.Next()
}

func main() {
    r := gin.Default()
    r.Use(verifyHmacSafe)
    r.GET("/secure", func(c *gin.Context) {
        if ctx, exists := c.Get("safe_ctx"); exists && ctx.(SafeContext).verified {
            c.JSON(http.StatusOK, gin.H{"result": "verified"})
        }
    })
    r.Run()
}

3. Use standard library crypto/hmac correctly

Always use hmac.Equal for comparison to prevent timing attacks, and avoid manual concatenation that can introduce encoding issues. Do not truncate or modify the signature before comparison.

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "fmt"
    "net/http"
)

func computeHmac(message, secret string) string {
    key := []byte(secret)
    h := hmac.New(sha256.New, key)
    h.Write([]byte(message))
    return hex.EncodeToString(h.Sum(nil))
}

func verifyRequest(r *http.Request, secret string) bool {
    timestamp := r.Header.Get("X-Timestamp")
    receivedSig := r.Header.Get("X-Signature")
    if timestamp == "" || receivedSig == "" {
        return false
    }
    message := r.Method + r.URL.Path + timestamp
    expected := computeHmac(message, secret)
    return hmac.Equal([]byte(expected), []byte(receivedSig))
}

func example() {
    req, _ := http.NewRequest("GET", "/api/data", nil)
    req.Header.Set("X-Timestamp", "1234567890")
    // signature would be generated similarly on the client
    isValid := verifyRequest(req, "your-secure-secret")
    fmt.Println(isValid)
}

These practices align with security checks that middleBrick performs, such as Unsafe Consumption and Data Exposure, ensuring that Hmac Signatures are handled safely without introducing memory hazards. By isolating per-request data and using immutable copies, you reduce the chance of Use After Free-like conditions even in complex integrations.