Out Of Bounds Write in Gin with Hmac Signatures

An Out Of Bounds Write in a Gin application using Hmac Signatures typically arises when unchecked or loosely validated input influences memory operations or byte slice assembly, and the Hmac Signature is either missing, incorrectly verified, or applied after unsafe mutation. This combination can expose the vulnerability because the integrity check does not protect the stage at which the out-of-bounds write occurs, allowing an attacker to manipulate payloads before signature validation or to forge requests that bypass length checks.

Consider a handler that reads a JSON body containing a data field and a signature field. If the server copies the data into a fixed-size buffer or a preallocated slice using the length from the request without validating bounds, and then verifies the Hmac Signature only on the raw JSON bytes, the signature remains valid while the write operates outside intended memory limits. Attackers can supply crafted lengths or chunked content to trigger writes beyond allocated regions, potentially corrupting adjacent memory or influencing control flow. Because the Hmac Signature is computed on the original payload, the server may trust the request as authentic and proceed to process the maliciously shaped data, turning a logic flaw into a security-relevant incident.

In Gin, this often maps to common patterns such as using c.ShouldBindJSON or manually decoding into structs where slice fields are set from untrusted input. If the application later uses those slices in operations like copy(dst, src) or appends in a loop without verifying each element’s index against the destination capacity, the Hmac Signature on the request does not mitigate the out-of-bounds behavior. The signature ensures the payload has not been altered in transit, but it does not guarantee that the application’s handling respects memory boundaries. Real-world analogs include issues indexed under CWE-787 and can be observed in API routes that process arrays, image buffers, or serialized formats without strict length validation.

Middleware that computes and verifies Hmac Signatures can inadvertently encourage unsafe coding if developers assume authenticated requests are safe from structural misuse. For example, a route protected by Hmac may still call functions that iterate over user-provided counts to populate a fixed-size array, leading to writes beyond the array’s limits. The presence of a valid signature gives a false sense of security while the underlying memory safety issue remains. This is why the OWASP API Security Top 10 emphasizes robust input validation and secure handling regardless of authentication or integrity mechanisms.

To understand the intersection concretely, imagine a Gin endpoint that accepts a custom header X-Payload-Digest containing an Hmac of the body. If the handler first reads the body into a byte slice, uses json.Unmarshal into a struct containing a Data []byte field, and then copies Data into a fixed buffer based on a length field inside Data without checking bounds, the Hmac verification that follows does not prevent the out-of-bounds write. An attacker can send a small JSON with a valid Hmac but a length that points outside the buffer, and Gin will process the request as authorized while the copy operation overruns memory.

Mitigation at this intersection requires validating all length and index values before any write, independent of signature status, and ensuring that Hmac verification occurs before any mutation that could affect memory safety. The signature should cover the exact bytes that will be used in bounds-sensitive operations, and the application must enforce strict limits on slice growth and buffer copies. This aligns with the principle that API security checks, such as those in the LLM/AI and Input Validation categories, complement but do not replace careful memory handling.

Remediation focuses on validating input bounds before processing and ensuring Hmac verification covers the relevant payload before any mutation. In Gin, implement a structured approach where the signature is extracted, verified, and only then is the request body parsed and used in memory operations.

Example 1: Verify Hmac before binding, with strict length checks on slices.

// computeHmac returns the hex-encoded Hmac for a given body and secret.
func computeHmac(body []byte, secret string) string {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write(body)
    return hex.EncodeToString(mac.Sum(nil))
}

func ProtectedHandler(c *gin.Context) {
    const secret = "super-secret-key"
    payload, err := c.GetRawData()
    if err != nil {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "failed to read body"})
        return
    }
    receivedSig := c.GetHeader("X-Payload-Digest")
    if receivedSig == "" {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "missing signature"})
        return
    }
    expectedSig := computeHmac(payload, secret)
    if !hmac.Equal([]byte(expectedSig), []byte(receivedSig)) {
        c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
        return
    }
    // Now parse and validate bounds before using data.
    var req struct {
        Data []byte `json:"data"`
        Len  int    `json:"len"`
    }
    if err := json.Unmarshal(payload, &req); err != nil {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "invalid JSON"})
        return
    }
    if req.Len < 0 || req.Len > len(req.Data) {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "invalid length"})
        return
    }
    // Safe bounded copy into a fixed-size buffer.
    const bufferSize = 1024
    var buf [bufferSize]byte
    n := copy(buf[:], req.Data[:req.Len])
    _ = n // use n as needed
    c.JSON(http.StatusOK, gin.H{"status": "processed"})
}

Example 2: Use a custom binding that validates slice capacity before copying.
type SafePayload struct {
    Data []byte `json:"data" validate:"required,max=1024"`
    Index int   `json:"index" validate:"required,min=0,max=1023"`
}

func SafeBind(c *gin.Context) error {
    var sp SafePayload
    if err := c.ShouldBindJSON(&sp); err != nil {
        return err
    }
    // Additional bounds check before any write.
    if sp.Index < 0 || sp.Index >= len(sp.Data) {
        return fmt.Errorf("index out of bounds")
    }
    // Example of a bounded write into application memory.
    var fixed [256]byte
    copy(fixed[:], sp.Data[sp.Index:sp.Index+1])
    return nil
}

func HandlerWithBinding(c *gin.Context) {
    if err := SafeBind(c); err != nil {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": err.Error()})
        return
    }
    c.JSON(http.StatusOK, gin.H{"ok": true})
}

These examples show that Hmac Signatures are applied early to the raw body, and subsequent operations enforce strict bounds on slice indexing and lengths. The signature ensures integrity of the payload, while explicit validation prevents out-of-bounds writes regardless of the authenticated request.