Symlink Attack in Buffalo with Dynamodb

A symlink attack in the Buffalo framework when using Amazon DynamoDB typically arises from insecure handling of user-supplied paths or filenames that later influence how application code or underlying systems reference storage. In a web context, an attacker may supply a filename or key that includes path traversal sequences or a deliberate symbolic link name. If the application constructs a local filesystem path by concatenating user input without canonicalization or validation, the application may resolve the path to a location outside the intended directory, leading to unauthorized read or write operations.

When DynamoDB is used as the persistence layer, the vulnerability surface shifts from traditional file operations to how the application maps user input to DynamoDB keys and to any auxiliary local filesystem operations. For example, an application might use a user-controlled identifier to name a local cache file or a temporary export file, then store a reference to that identifier in a DynamoDB item. If the identifier is not strictly validated, an attacker can craft identifiers that traverse directories or resolve to symlinks. Later, when the application opens that local file path, the symlink redirects writes to a sensitive file, effectively bypassing DynamoDB-based access controls.

The risk is compounded when the application uses AWS SDK calls to DynamoDB without validating the provenance of keys derived from user input. While DynamoDB itself does not follow symlinks (it operates on key-based item access), the surrounding runtime environment does. A typical chain includes: (1) user-supplied data used to derive a local filename; (2) that filename used in filesystem operations; (3) a symlink manipulated by an attacker to redirect those operations; (4) the application performing unintended local file actions while believing it is interacting solely with DynamoDB-stored metadata. This illustrates that the vulnerability is not in DynamoDB, but in the application logic that bridges user-controlled identifiers to local filesystem behavior.

To mitigate symlink risks, treat all user input used for local file naming as untrusted. Use strict allowlists for identifiers, avoid direct concatenation with filesystem paths, and resolve paths with canonicalization before use. Below are concrete code examples for Buffalo applications using DynamoDB, showing safe handling of identifiers and file operations.

First, define a safe helper to construct filesystem paths without allowing directory traversal. In your Buffalo application, create a utility that joins a controlled base directory with a sanitized key:

// app/utils/fs.go
package utils

import (
    "path"
    "strings"
)

// SafeFilePath ensures a filename is confined to baseDir.
// It rejects "..", path separators beyond the base, and empty segments.
func SafeFilePath(baseDir, userKey string) (string, error) {
    // Reject path traversal indicators
    if strings.Contains(userKey, "..") || strings.Contains(userKey, "\x00") {
        return "", ErrInvalidKey
    }
    // Clean and ensure no absolute path or symlink resolution surprises
    cleaned := path.Clean("/" + userKey)
    if cleaned == "/" || strings.HasPrefix(cleaned, "../") {
        return "", ErrInvalidKey
    }
    // Use path.Join to safely combine; result remains within baseDir when baseDir is absolute
    result := path.Join(baseDir, cleaned)
    // Ensure the result still resides inside baseDir after joining
    if !strings.HasPrefix(result, path.Clean(baseDir)+"/") && result != path.Clean(baseDir) {
        return "", ErrInvalidKey
    }
    return result, nil
}

Second, when storing references in DynamoDB, avoid using raw user input as the key without normalization. Use a deterministic, safe key format and store additional metadata about the local file association separately:

// app/models/document.go
package models

import (
    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/service/dynamodb"
    "github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
)

type DocumentMetadata struct {
    ID           string `json:"id"`
    SafeFileName string `json:"safe_file_name"` // derived from sanitized ID
    BucketKey    string `json:"bucket_key"`     // optional S3 reference
}

// BuildItem converts metadata to DynamoDB attribute values safely.
func (m *DocumentMetadata) BuildItem() (map[string]*dynamodb.AttributeValue, error) {
    av, err := dynamodbattribute.MarshalMap(m)
    if err != nil {
        return nil, err
    }
    // Ensure ID is used as partition key, not a filesystem path
    if av["ID"] == nil {
        return nil, ErrMissingID
    }
    return av, nil
}

// Example usage: create a new item with a sanitized key.
// Assume baseDir is an absolute path configured per environment.
func CreateDocument(db *dynamodb.DynamoDB, baseDir, userSupplied string) (string, error) {
    safeName, err := utils.SafeFilePath(baseDir, userSupplied)
    if err != nil {
        return "", err
    }
    meta := DocumentMetadata{
        ID:           userSupplied, // store original for lookup, but validate elsewhere
        SafeFileName: safeName,     // canonicalized local filename
    }
    item, err := meta.BuildItem()
    if err != nil {
        return "", err
    }
    input := &dynamodb.PutItemInput{
        TableName: aws.String("Documents"),
        Item:      item,
    }
    _, err = db.PutItem(input)
    if err != nil {
        return "", err
    }
    return safeName, nil
}

Third, when reading files locally based on DynamoDB items, always resolve the final path and verify it remains within the allowed directory:

// app/handlers/documents.go
package handlers

import (
    "net/http"
    "path/filepath"
    "yourproject/app/utils"
)

func ShowDocument(rw http.ResponseWriter, r *http.Request) {
    // Fetch metadata from DynamoDB by a validated key (e.g., from query or session)
    key := r.URL.Query().Get("id")
    if key == "" {
        rw.WriteHeader(http.StatusBadRequest)
        return
    }
    // Assume GetMetadataFromDynamoDB fetches and unmarshals DocumentMetadata
    meta, err := GetMetadataFromDynamoDB(key)
    if err != nil {
        rw.WriteHeader(http.StatusInternalServerError)
        return
    }
    // Resolve against baseDir and canonicalize
    resolved, err := utils.SafeFilePath(config.BaseDir, meta.SafeFileName)
    if err != nil {
        rw.WriteHeader(http.StatusBadRequest)
        return
    }
    // Ensure the resolved path is still within baseDir
    if !filepath.HasPrefix(resolved, config.BaseDir) {
        rw.WriteHeader(http.StatusBadRequest)
        return
    }
    http.ServeFile(rw, r, resolved)
}

These examples emphasize input validation, canonicalization, and strict path confinement, preventing symlink-based escapes even when DynamoDB is used for metadata storage. The DynamoDB operations themselves remain unchanged in semantics, but local filesystem interactions are hardened against manipulation via controlled identifiers.