Integer Overflow in Echo Go with Hmac Signatures

An integer overflow in an Echo Go application that handles Hmac Signatures can arise when user-supplied numeric values (such as payload length, timestamp, or a message counter) are used in arithmetic that determines buffer sizes or signature inputs. If these values are not validated, an attacker can provide values that wrap around the integer type, producing a much smaller size than expected. This can lead to a buffer overflow or an out-of-bounds read when the application allocates a buffer based on the wrapped value but later processes a full-size message.

In the context of Hmac Signatures, integer overflow can weaken integrity guarantees. For example, if the length of the data to be signed is computed with overflow, the resulting signature may be computed over fewer bytes than intended. An attacker could supply a crafted length that results in signing only a partial message, while the server verifies the signature against a different, larger payload. This mismatch can be exploited to bypass signature verification or to forge requests that appear authentic.

Echo Go, being a minimalist web framework, does not inherently protect against these arithmetic issues. Developers must explicitly validate and sanitize any numeric inputs that influence memory allocation or cryptographic operations. The combination of low-level integer handling and cryptographic signature logic increases the attack surface: an unchecked overflow in a length field used during signature computation can lead to signature malleability or verification bypass, potentially enabling tampering with API requests that rely on Hmac Signatures for authentication.

Consider a scenario where an API uses a timestamp and a message length to compute an Hmac Signature for replay protection. If the length is stored in a 32-bit unsigned integer and an attacker sends a length near the maximum value, the addition used to compute a buffer size may overflow, resulting in a small allocation. The runtime then processes the full message, corrupting memory or causing information disclosure. This could lead to vulnerabilities such as CVE-2023-XXXX-class issues where signature verification is subverted due to incorrect size calculations.

To detect such issues, security scans like those provided by middleBrick analyze the unauthenticated attack surface, checking for missing validation on numeric inputs that affect cryptographic workflows. The scanner correlates findings from the Input Validation and Authentication checks with the presence of Hmac Signature logic, highlighting where integer overflow could undermine signature integrity. By mapping these findings to standards such as OWASP API Top 10 and providing prioritized remediation guidance, tools like middleBrick help teams identify risky patterns before they can be weaponized.

Remediation focuses on strict input validation, safe arithmetic, and canonical handling of lengths used in Hmac Signatures. Always parse and verify numeric inputs against defined ranges before using them in buffer allocations or cryptographic operations. Use Go's built-in overflow checks and explicit length checks to ensure values remain within safe bounds.

Below is a secure example of computing and verifying an Hmac Signature in Echo Go, with protections against integer overflow and misuse of user-supplied lengths.

// Safe Hmac Signature handling in Echo Go
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "errors"
    "fmt"
    "net/http"
    "strconv"
    "math"
)

const MaxMessageLength = 1 << 20 // 1 MB

func verifyHmac(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        // Parse provided length safely
        lengthStr := c.QueryParam("length")
        length, err := strconv.ParseUint(lengthStr, 10, 32)
        if err != nil || length == 0 {
            return echo.NewHTTPError(http.StatusBadRequest, "invalid length parameter")
        }
        // Enforce business-defined maximum to prevent resource exhaustion
        if length > MaxMessageLength {
            return echo.NewHTTPError(http.StatusRequestEntityTooLarge, "length exceeds limit")
        }

        // Read body with a size-limited buffer
        limitedReader := &io.LimitedReader{R: c.Request().Body, N: int64(length)}
        data, err := io.ReadAll(limitedReader)
        if err != nil {
            return echo.NewHTTPError(http.StatusBadRequest, "failed to read message")
        }
        // Ensure we read exactly the expected length to avoid under-read issues
        if uint64(len(data)) != length {
            return echo.NewHTTPError(http.StatusBadRequest, "message length mismatch")
        }

        // Compute Hmac using a server-side key
        key := []byte("server-secret-key")
        mac := hmac.New(sha256.New, key)
        mac.Write(data)
        expectedMac := mac.Sum(nil)

        // Compare with the client-supplied signature
        clientSig := c.Request().Header.Get("X-Signature")
        if !hmac.Equal(expectedMac, []byte(clientSig)) {
            return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
        }

        return next(c)
    }
}

// Example route using the secure handler
func main() {
    e := echo.New()
    e.POST("/sign", verifyHmac(func(c echo.Context) error {
        return c.String(http.StatusOK, "verified")
    }))
    e.Start(":8080")
}

Key practices include: validating length parameters against a maximum, using strconv.ParseUint with explicit bit sizes to prevent overflow, applying io.LimitedReader to cap read sizes, and ensuring the read data length matches the expected length before computing the Hmac. Avoid using unchecked integer arithmetic when deriving buffer sizes or truncation points. These measures reduce the risk of integer overflow and ensure that Hmac Signatures are computed and verified over the intended message scope.