Symlink Attack in Buffalo with Hmac Signatures

A symlink attack in Buffalo occurs when an application creates files or directories based on user-supplied paths without ensuring those paths resolve to intended locations. When Hmac Signatures are used to verify the integrity of requests or configuration data, the presence of symlink-based path traversal can cause the application to operate on files the developer believes are isolated. If a request’s parameters include a path that traverses directories via symbolic links, the Hmac signature may still validate because the attacker-controlled path is included in the signed payload. The signature ensures the data has not been altered in transit, but it does not guarantee that the underlying filesystem references are safe. This mismatch allows an attacker to leverage predictable signature verification while directing the application to read or write files outside the intended directory tree.

In Buffalo, routes that accept file identifiers or resource paths can become entry points for this class of issue when those identifiers are resolved through filesystem operations without canonicalization. For example, if an endpoint uses a user-provided filename to construct a path and then verifies an Hmac signature on the request, the signature may cover the filename but not the resolved absolute path. An attacker can supply a filename like ../../../etc/passwd or a symlink that points to sensitive system files. If the application does not resolve symlinks and canonicalize paths before performing I/O, the Hmac verification passes while the actual accessed file differs from what the developer intended. This specific combination of Hmac Signatures and unresolved symlinks undermines the trust model: integrity checks are honored, but authorization and path confinement are bypassed.

Real-world attack patterns mirror known issues such as CVE-2021-41133, where path traversal and symlink resolution allowed unauthorized file access. The OWASP API Top 10 category Broken Object Level Authorization (BOLA) and the broader class of Insecure Direct Object References (IDOR) are relevant because insufficient path validation can allow attackers to reach resources they should not access. Input validation checks that reject sequences like ../ or fail to resolve symlinks are crucial. Without resolving symlinks to their true absolute paths and verifying that the final location remains within the allowed directory, Hmac Signatures alone cannot prevent an attacker from leveraging filesystem tricks to compromise confidentiality or integrity.

To remediate symlink risks while using Hmac Signatures in Buffalo, always canonicalize and validate paths before performing filesystem operations. This includes resolving symlinks, ensuring paths remain within an allowed base directory, and including the resolved path in the data covered by the Hmac signature. Below are concrete examples demonstrating secure handling in a Buffalo application.

Example 1: Secure path resolution with Hmac signing

// Ensure the resolved path stays within the allowed base directory
package actions

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

    "github.com/gobuffalo/buffalo"
)

const baseDir = "/safe/uploads"

func verifyAndResolve(path, signature, secret string) (string, bool) {
    // Resolve symlinks and clean the path
    resolved, err := filepath.EvalSymlinks(path)
    if err != nil {
        return "", false
    }
    resolved = filepath.Clean(resolved)

    // Ensure the resolved path is within the allowed base directory
    absBase, _ := filepath.Abs(baseDir)
    absResolved, _ := filepath.Abs(resolved)
    if !strings.HasPrefix(absResolved+string(fs.Separator), absBase+string(fs.Separator)) && absResolved != absBase {
        return "", false
    }

    // Verify Hmac signature over the resolved path
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(resolved))
    expected := hex.EncodeToString(mac.Sum(nil))
    return resolved, hmac.Equal([]byte(expected), []byte(signature))
}

func SecureFileHandler(c buffalo.Context) error {
    requested := c.Param("file")
    sig := c.Param("sig")
    resolved, ok := verifyAndResolve(requested, sig, "your-secret")
    if !ok {
        return c.Error(http.StatusForbidden, errors.New("invalid path or signature"))
    }
    // Use resolved path for filesystem operations
    data, err := os.ReadFile(resolved)
    if err != nil {
        return c.Error(http.StatusInternalServerError, err)
    }
    return c.Render(200, r.String(string(data)))
}

Example 2: Hmac signing including canonicalized path in payload

package actions

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

    "github.com/gobuffalo/buffalo"
)

func GenerateSignedPath(path, secret string) string {
    resolved, _ := filepath.EvalSymlinks(path)
    resolved = filepath.Clean(resolved)
    // Ensure path is relative to base before signing
    rel, _ := filepath.Rel("/safe/uploads", resolved)
    rel = filepath.ToSlash(rel)
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(rel))
    return hex.EncodeToString(mac.Sum(nil))
}

func ValidateSignedPath(path, signature, secret string) bool {
    resolved, _ := filepath.EvalSymlinks(path)
    resolved = filepath.Clean(resolved)
    rel, _ := filepath.Rel("/safe/uploads", resolved)
    rel = filepath.ToSlash(rel)
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(rel))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(signature))
}

These examples emphasize that the data subject to Hmac verification must be the canonical, resolved path rather than the raw user input. By evaluating symlinks with filepath.EvalSymlinks and enforcing a base directory check, you prevent attackers from using symlinks to escape intended boundaries. Combining this with strict Hmac validation on the normalized path ensures both integrity and confinement. Additional measures such as rejecting path traversal sequences during input validation and enforcing strict Content Security Policies further reduce risk. Note that middleBrick scans can help identify exposed endpoints that may be vulnerable to symlink-based issues by correlating runtime behavior with OpenAPI specifications and security checks.