Symlink Attack in Chi with Dynamodb

A symlink attack in the context of a Chi HTTP routing application that stores data in DynamoDB occurs when an attacker manipulates file-system-level symbolic links to redirect or overwrite files that the application reads or writes, and the application uses DynamoDB in a way that reflects or trusts file paths provided by the client. Chi is a lightweight router for Go, and while it does not directly interact with the file system, applications using Chi often serve uploaded files, read configuration or templates from disk, or log request details to files. If any of those operations incorporate user-supplied identifiers (such as a user ID or document key) into file paths and then store or retrieve related metadata in DynamoDB, an attacker can supply a path that traverses directories or points to a symlink, causing the application to read sensitive files or overwrite critical data.

DynamoDB itself is a managed NoSQL service and does not provide traditional POSIX file semantics; it stores items and attributes. However, if the application stores file paths, S3 object keys, or directory markers in DynamoDB and later uses those values in file-system operations, the database becomes a source of trusted references that an attacker can poison. For example, an item might store a user_profile_path attribute derived from user input without normalization or validation. If the application later resolves that path to a file on disk (for reading or writing), an attacker who can inject a symlink under a predictable location can redirect the operation. A concrete scenario: a Chi endpoint accepts a document_id, looks up item attributes in DynamoDB to locate a file on an attached volume, and streams the file back to the client. If the item’s stored path points to a symlink the attacker can control, the stream may expose arbitrary files or be hijacked into writing to sensitive destinations.

The risk is amplified when the application uses assumed-trust attributes from DynamoDB to build absolute paths without canonicalization. An attacker can leverage directory traversal sequences or symlink placement (e.g., by compromising a writable temporary location or a shared volume) so that the path resolves outside the intended directory. This can lead to information disclosure (reading arbitrary files), data corruption (writing to system or application files), or privilege escalation if application processes run with elevated permissions. Mitigations include strict input validation, path normalization, avoiding direct mapping from stored paths to filesystem locations, storing only safe identifiers in DynamoDB, and enforcing chroot-like boundaries for file operations, ensuring that any lookup via DynamoDB is verified against an allowlist and never directly concatenated into a filesystem path.

To remediate symlink risks when using Chi and DynamoDB, ensure that any path derived from DynamoDB attributes is sanitized, validated, and isolated from filesystem operations. Do not directly use user-influenced keys stored in DynamoDB to build filesystem paths. Instead, use an internal mapping (e.g., a lookup table or a deterministic key scheme) and apply strict checks before any I/O operation.

Example safe lookup and file read in Chi

Below is a minimal, secure pattern for a Chi route that retrieves an item from DynamoDB and reads a related file safely. The code avoids using raw paths from DynamoDB and instead uses a controlled base directory and a filename derived from a validated identifier.

// Chi route with safe DynamoDB-driven file access
package main

import (
	"context"
	"fmt"
	"net/http"
	"path/filepath"
	"strings"

	"github.com/go-chi/chi/v5"
	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb"
	"github.com/aws/aws-sdk-go-v2/service/dynamodb/types"
)

const (
	tableName = "MyAppData"
	baseDir   = "/safe/app/data" // restricted directory, no symlinks or traversal outside
)

func main() {
	r := chi.NewRouter()
	r.Get("/documents/{docID}", documentHandler)
	http.ListenAndServe(":8080", r)
}

func documentHandler(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	docID := chi.URLParam(r, "docID")
	if !isValidDocID(docID) {
		http.Error(w, "invalid document identifier", http.StatusBadRequest)
		return
	}

	// Construct a safe filename; avoid using any path from DynamoDB
	filename := fmt.Sprintf("doc_%s.dat", docID)
	fullPath := filepath.Join(baseDir, filename)

	// Ensure the resolved path remains within baseDir
	if !strings.HasPrefix(fullPath, filepath.Clean(baseDir)+string(filepath.Separator)) && fullPath != filepath.Clean(baseDir) {
		http.Error(w, "forbidden path", http.StatusForbidden)
		return
	}

	// Retrieve item metadata from DynamoDB (store only safe identifiers)
	cfg, err := config.LoadDefaultConfig(ctx)
	if err != nil {
		http.Error(w, "internal error", http.StatusInternalServerError)
		return
	}
	client := dynamodb.NewFromConfig(cfg)

	out, err := client.GetItem(ctx, &dynamodb.GetItemInput{
		TableName: aws.String(tableName),
		Key: map[string]types.AttributeValue{
			"doc_id": &types.AttributeValueMemberS{Value: docID},
		},
	})
	if err != nil {
		http.Error(w, "service error", http.StatusInternalServerError)
		return
	}
	if out.Item == nil {
		http.Error(w, "not found", http.StatusNotFound)
		return
	}

	// At this point, do not use any attribute that may contain a path; use only validated fullPath
	data, err := osReadFile(fullPath) // implement osReadFile with os.ReadFile and proper error handling
	if err != nil {
		http.Error(w, "file unavailable", http.StatusNotFound)
		return
	}
	w.Header().Set("Content-Type", "application/octet-stream")
	w.Write(data)
}

func isValidDocID(id string) bool {
	// Allow only alphanumeric and limited safe characters
	for _, r := range id {
		if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9' || r == '-' || r == '_') {
			return false
		}
	}
	return id != "" && len(id) <= 128
}

Key points:

• Do not store or use file-system paths in DynamoDB; store opaque identifiers and map them to safe locations in code.
• Normalize and validate any identifiers used to construct filesystem paths; reject traversal sequences and enforce a strict prefix check.
• Use a controlled base directory and avoid symlinks, shared temporary locations, or writable directories that an attacker can influence.
• If you must store paths in DynamoDB for legacy reasons, canonicalize and validate them against an allowlist of permitted directories before any I/O, and avoid passing them directly to filesystem operations.