Race Condition in Fiber with Hmac Signatures

A race condition in the Fiber web framework involving Hmac Signatures typically arises when signature verification and request processing are not performed as a single, atomic operation. In an API endpoint that relies on an Hmac Signature for request authentication, the application may first read a resource (e.g., a record identifier or a version/timestamp) and then compute and verify the Hmac. An attacker can exploit the time gap between reading the resource and verifying the signature to mutate the resource in a way that the earlier read does not reflect, bypassing intended integrity checks.

Consider an endpoint that accepts a JSON payload with a resource ID and an Hmac covering the payload and an entity version. A vulnerable pattern in Fiber might look like this:

// Vulnerable: read then verify creates a window
app.Post("/update", func(c *fiber.Ctx) error {
    var req UpdateRequest
    if err := c.BodyParser(&req); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid body"})
    }
    // Read current entity state (e.g., from cache or DB)
    current, err := getEntityVersion(req.ID)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "server error"})
    }
    // Build the signed string using the read version
    toSign := fmt.Sprintf("%s|%d|%s", req.ID, current.Version, req.Data)
    expected, err := hex.DecodeString(req.Signature)
    if err != nil || !hmac.Equal([]byte(expected), []byte(computeHmac(toSign, secret))) {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
    }
    // Proceed to apply update, possibly using stale version context
    return applyUpdate(req.ID, req.Data)
})

The race condition occurs because current.Version is read before the Hmac verification. An attacker can send two concurrent requests: the first reads version N and prepares a valid Hmac for N; the second updates the entity from N to N+1 and commits before the first request proceeds to verify. The first request then uses the now-stale version N to construct its signed string, but the server recomputes the Hmac with version N (still in memory from the earlier read) and may incorrectly accept it, or the logic around version usage becomes inconsistent, leading to authorization bypass or unintended state changes.

Another scenario involves timing and caching layers. If the entity version is cached and the cache is updated asynchronously after a write, a read in one goroutine may see a version that lags behind the committed state. An Hmac computed over the cached version does not match what the server expects after the cache syncs, but depending on implementation details this may either cause false rejections or, worse, allow an attacker to replay a request with a valid Hmac that was computed over an older state that has since changed in ways the server does not enforce as part of the signature context.

LLM/AI Security does not directly mitigate this class of issue, but the scanner’s checks (e.g., Input Validation and BOLA/IDOR) can flag endpoints that perform sensitive operations without strong integrity guarantees around versioning and signatures. Consistent handling—treating signature verification and state mutation as a single, atomic check—is essential to prevent exploitation of the window between read and verify.

To eliminate the race condition, ensure that the data used to compute the Hmac and the validation logic encompass all mutable state relevant to the request, and perform verification before any state changes. Treat the signature as a binding over the exact resource version and payload that will be acted upon. Compute the Hmac over a canonical representation that includes the entity version obtained within the same critical section as the verification, and reject the request if the version used for signing does not match the current version unless you implement strict replay protections.

A secure Fiber pattern moves reading and verification together and applies updates only after a successful, version-bound validation:

// Secure: verify with the current version in a single flow
app.Post("/update", func(c *fiber.Ctx) error {
    var req UpdateRequest
    if err := c.BodyParser(&req); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid body"})
    }
    // Lock or transaction scope conceptually: read and verify together
    current, err := getEntityVersion(req.ID)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "server error"})
    }
    // Ensure the request includes the version it believes it is updating
    if req.ExpectedVersion != current.Version {
        return c.Status(fiber.StatusConflict).JSON(fiber.Map{"error": "version mismatch, please refresh"})
    }
    toSign := fmt.Sprintf("%s|%d|%s", req.ID, req.ExpectedVersion, req.Data)
    expected, err := hex.DecodeString(req.Signature)
    if err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid signature format"})
    }
    computed := computeHmac(toSign, secret)
    if !hmac.Equal(expected, []byte(computed)) {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
    }
    // Only now apply the update with the confirmed version
    return applyUpdate(req.ID, req.ExpectedVersion, req.Data)
})

Key remediation points:

• Include the entity version explicitly in the signed string and require the client to provide the version it intends to update. This prevents the server from silently accepting a request bound to an outdated version.
• Validate the version before using it in Hmac computation. If the current version differs, reject with a 409 Conflict to force the client to re-fetch and re-sign.
• Perform Hmac verification before any write, ensuring that the verified context (version + payload) matches the context used for the update. This makes the operation effectively atomic with respect to the signature check.
• Use hmac.Equal for constant-time comparison to avoid timing attacks on the signature itself.

Example Hmac helper for Fiber in Go:

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

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

By binding the signature to the exact version used in the verification step and rejecting mismatches, you close the race window. This approach aligns with guidance reflected in security checks that emphasize integrity controls and proper handling of concurrent modifications, and it complements broader API security practices mapped to standards such as OWASP API Top 10.