Symlink Attack in Chi with Mutual Tls

A symlink attack in the context of Chi with mutual TLS (mTLS) occurs when an attacker leverages file system race conditions or predictable temporary paths to redirect a file operation that an mTLS-secured service will later read or write. Even though mTLS protects transport integrity and server authentication, it does not change how the application handles file paths, temporary files, or symbolic links on the host. If the application creates temporary files in a shared directory (for example, /tmp) or follows user-supplied paths without canonicalization, an attacker who can write to that directory can create a symbolic link that points to a sensitive file.

In Chi, this often maps to common OWASP API Top 10 risks such as Broken Access Control (BOLA/IDOR) and Improper Neutralization of Special Elements used in an OS Command Injection or Path Traversal context. An mTLS endpoint that authenticates clients with client certificates may still be vulnerable if it accepts a filename or path parameter, resolves it relative to a mutable directory, and later opens the resolved path. An attacker who can race a symlink into place between the path resolution and the open/read/write call can cause the service to operate on an unintended file, potentially reading sensitive configuration or writing malicious content that the mTLS-authenticated logic will later process.

Consider a Chi route that exports user data to a file under /tmp and then returns it. If the route builds the path using path.join(tempDir, userSuppliedName) without cleaning path traversal sequences or symlink components, and an attacker can write to tempDir, the attacker can create a symlink in tempDir that points to /etc/secrets or a sensitive application file. When the route opens the file to stream it back, the mTLS client is authenticated, but the application leaks data because the file descriptor refers to the symlink target. This demonstrates that mTLS secures channel and peer identity, but does not mitigate path manipulation or insecure file handling bugs.

Another scenario involves log or import processing where mTLS clients upload files for processing. If the application extracts archives or follows symbolic links inside the archive, an attacker can include symlinks in a crafted archive that point outside the intended extraction directory (a classic zip slip variant). Even with mTLS ensuring the upload came from a trusted client, the application may inadvertently write sensitive files to privileged locations if it does not sanitize entries. Chi handlers that parse multipart forms and then iterate over file headers are at risk if they do not explicitly reject or sanitize symlinks and absolute paths within the archive.

To detect such issues, scans should correlate mTLS-enabled endpoints that accept file uploads or path parameters with checks for path traversal, symlink following, and insecure temporary file usage. middleBrick’s checks for Input Validation, Property Authorization, and Unsafe Consumption can surface risky patterns in how Chi routes resolve and use file paths, while BFLA/Privilege Escalation and BOLA/IDOR checks help identify cases where authenticated mTLS sessions operate on overly permissive file targets. Because mTLS does not mitigate these logic flaws, the scanner highlights them as high-severity findings that require code-level fixes rather than transport-layer controls.

Remediation focuses on ensuring that mTLS provides transport assurance while the application enforces strict path validation, canonicalization, and isolation. Never rely on mTLS to validate file paths or to prevent path manipulation. Instead, treat client certificates as identity and apply strict authorization checks that are independent of transport security.

First, validate and sanitize all user-supplied path components before using them in filesystem operations. Use path.normalize (in Node.js) or filepath.Clean (in Go) to eliminate traversal sequences and ensure the resolved path remains within an allowed base directory. Reject paths that attempt to traverse outside the base by checking that the normalized path still starts with the base directory. In Chi, you can implement this in middleware or route handlers to enforce a safe working directory for file operations.

Second, avoid following symlinks when reading or writing files that are tied to mTLS-authenticated contexts. When opening files, prefer flags that do not follow symlinks where available (for example, open with O_NOFOLLOW on POSIX systems) or explicitly check the file’s inode to ensure it is not a symlink. If you must handle archives, disable automatic symlink extraction and validate each entry’s resolved path to ensure it remains within the intended extraction directory.

Third, isolate file operations per mTLS client where appropriate by binding temporary directories or file prefixes to the authenticated client identity. Use a per-request unique subdirectory under a controlled base path, and ensure that each request cleans up its own artifacts. This reduces the window for symlink races and prevents cross-client interference. Combine this with strict file permissions and avoid running the service with unnecessary privileges.

Example secure Chi middleware that enforces path safety alongside mTLS authentication:

import { resolve, normalize, sep } from 'path';
import { existsSync, statSync } from 'fs';
import { Router } from 'itty-router';

const BASE_DIR = resolve('/safe/data');

function safeFilePath(userPath) {
  const normalized = normalize(userPath).replace(/^(\/|\.\.)/, '');
  const full = resolve(BASE_DIR, normalized);
  if (!full.startsWith(BASE_DIR + sep)) {
    throw new Error('Path traversal detected');
  }
  return full;
}

const router = Router();

// Assume mTLS has already validated req.client.cert
router.get('/export/:filename', (req) => {
  const filename = req.params.filename;
  const requested = safeFilePath(filename);

  if (!existsSync(requested)) {
    return new Response('Not found', { status: 404 });
  }
  const stats = statSync(requested);
  if (stats.isSymbolicLink()) {
    return new Response('Invalid file type', { status: 400 });
  }

  // Stream file safely
  const stream = new Response(/* implementation-specific streaming */);
  return stream;
});

export default router;

Example secure file extraction that rejects symlinks and enforces isolation:

import { resolve, join } from 'path';
import { createWriteStream, openSync, closeSync, readSync, writeSync } from 'fs';
import { tmpdir } from 'os';
import { randomBytes } from 'crypto';

function processUpload(archiveBuffer, mtlsClientId) {
  const tmpBase = resolve(tmpdir(), `uploads-${mtlsClientId}`);
  // Ensure directory exists and is isolated per client
  // mkdirSync(tmpBase, { recursive: true });

  // Archive extraction library should be configured to:
  // - reject absolute paths
  // - reject paths containing '..' that resolve outside tmpBase
  // - explicitly skip symlink entries
  // For illustration, we show a minimal safe write pattern:
  const safeWrite = (entryName, content) => {
    const safeName = entryName.replace(/^(\.\.|\/)/, '');
    const dest = resolve(tmpBase, safeName);
    if (!dest.startsWith(tmpBase + '/')) throw new Error('Invalid entry');
    // Ensure no symlinks by checking resolved path
    const fd = openSync(dest, 'w');
    writeSync(fd, content, 0, content.length, 0);
    closeSync(fd);
  };

  // process entries...
}

These examples emphasize that mTLS secures the channel, but file handling must be explicitly secured by canonicalizing paths, rejecting symlinks, isolating per-client temporary storage, and validating archives before extraction. middleBrick’s scans can surface risky patterns in how Chi routes handle file paths and uploaded content, helping teams focus remediation on the logic layer rather than assuming transport security prevents file system abuse.