Symlink Attack in Echo Go with Hmac Signatures

A symlink attack in the Echo Go ecosystem when Hmac Signatures are used occurs when an attacker can influence which file path is signed and which file is ultimately accessed. In Go services built with Echo, Hmac Signatures are commonly applied to verify integrity of URLs, file paths, or API parameters. If the application signs a user-supplied path or identifier and later uses that same user input to open a file, an attacker can supply a path that traverses directories or includes symbolic links. The signature validates the attacker-controlled input, the server follows the symlink, and sensitive files are read or overwritten outside the intended directory.

For example, an endpoint that generates a signed token for a document download might concatenate a user-provided document ID into a path like /data/documents/{id}, sign that path with Hmac Signatures, and later serve the file by reading the same path after signature verification. If the document ID is user-controlled and not canonicalized, an attacker can set id to something like ../../../etc/passwd or a symlink such as ../../../var/log/app-secret. The signature remains valid because it covers the attacker-supplied string, but the filesystem operation escapes the document root due to the symlink, leading to unauthorized information disclosure or modification.

This pattern becomes especially risky when the signed value is used in a redirect or when constructing file paths for storage. An attacker can place a symlink at a predictable location that the application will later follow after verifying the Hmac Signatures. Because the signature confirms the value has not been tampered with, the server trusts the path and performs an operation on the target of the symlink. The vulnerability is not in Hmac Signatures itself, which ensures integrity, but in the application logic that maps signed user input directly to filesystem operations without canonicalizing paths, resolving symlinks, or enforcing a strict allowlist of accessible resources.

In the context of Echo Go, routes that bind path parameters and then use those parameters in file operations should treat symlink resolution as a precondition of signature verification. If the server resolves symlinks before signing or before verification, the signed value no longer matches the resolved path and the signature fails. Conversely, if the server signs the raw input and later resolves symlinks when accessing files, the security boundary is bypassed. Real-world parallels include cases where file download endpoints expose sensitive system files via directory traversal and symlink manipulation, a pattern often seen in misconfigured document management services.

Middleware that inspects or modifies request paths before routing can inadvertently create opportunities for symlink abuse if it does not normalize paths consistently. Developers using Echo must ensure that any value covered by Hmac Signatures is canonicalized, validated against a strict prefix, and verified to not resolve outside the intended directory before any filesystem access occurs. Security checks should include ensuring that the signed identifier maps to a single, expected file within a controlled base path, with no possibility for directory traversal or symlink redirection.

To remediate symlink risks when using Hmac Signatures in Echo Go, canonicalize and validate all paths before signing or verification, and never directly use user input in filesystem operations. Use Go’s filepath.Clean and filepath.EvalSymlinks to resolve any potential traversal or symlink indirection, and enforce that resolved paths remain within an allowed base directory.

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

    "github.com/labstack/echo/v4"
)

var secretKey = []byte("super-secret-key")

func signPath(path string) string {
    mac := hmac.New(sha256.New, secretKey)
    mac.Write([]byte(path))
    return hex.EncodeToString(mac.Sum(nil))
}

func isValidPath(base, input string) bool {
    // Clean the input and resolve symlinks
    cleaned := filepath.Clean(input)
    resolved, err := filepath.EvalSymlinks(cleaned)
    if err != nil {
        return false
    }
    // Ensure the resolved path stays within the allowed base directory
    rel, err := filepath.Rel(base, resolved)
    if err != nil || strings.HasPrefix(rel, "..") {
        return false
    }
    return true
}

func downloadHandler(c echo.Context) error {
    userID := c.Param("id")
    baseDir := "/data/documents"
    candidatePath := filepath.Join(baseDir, userID)

    if !isValidPath(baseDir, candidatePath) {
        return c.String(http.StatusBadRequest, "invalid path")
    }

    // Resolve after validation to ensure symlink safety
    finalPath, err := filepath.EvalSymlinks(candidatePath)
    if err != nil {
        return c.String(http.StatusInternalServerError, "unable to resolve path")
    }

    // Double-check base directory containment after resolution
    if !strings.HasPrefix(finalPath, baseDir) {
        return c.String(http.StatusForbidden, "access denied")
    }

    // Proceed to serve the file at finalPath
    signature := signPath(userID)
    _ = signature // use signature as needed, e.g., logging or audit

    return c.File(finalPath)
}


Key points in this remediation:

Canonicalization and symlink resolution happen before the signature is verified or compared; this prevents an attacker from relying on a signed path that resolves elsewhere.
Validation ensures that the resolved path is within the allowed base directory, blocking traversal and symlink redirection.
The signature is computed over the original, controlled identifier (e.g., a document ID), not the filesystem path, if the path is derived server-side. If signing a path, ensure the path input is already canonical and trusted.


For APIs that serve files, prefer storing files under a strict base directory and use an internal mapping (e.g., a database or key-value store) between opaque identifiers and filesystem locations. Sign the opaque identifier and map it to a resolved, validated path server-side. This approach keeps Hmac Signatures tied to trusted values and avoids exposing raw filesystem logic to the client.