Symlink Attack in Axum with Api Keys

A symlink attack in an Axum service that uses API keys can occur when file system operations are influenced by attacker-controlled data, such as request paths or API key-derived filenames, without proper validation. In this context, API keys are often used for routing, tenant isolation, or logging, and if these keys directly or indirectly map to file paths, an attacker may leverage path traversal sequences (e.g., ../../) or symbolic links to escape intended directories.

Consider an Axum handler that writes a per-API-key log file. If the API key is included in a filename without normalization, and the runtime resolves symbolic links on the filesystem, a crafted API key containing path segments (e.g., mykey/../../../etc/passwd) can redirect writes to a sensitive location. Even though API keys typically should not contain path separators, a lack of input validation allows an attacker to manipulate the resolved path through symlinks. This becomes a symlink attack when the application creates a file at a user-influenced path that the attacker can predict or partially control, and the filesystem resolves the symlink before the write occurs.

In a black-box scan, middleBrick tests input validation and file system interaction patterns. For an Axum endpoint that incorporates API keys into file operations, the scanner can detect whether user input traverses outside the intended directory scope. The presence of symlink resolution on the server side amplifies the risk: an attacker does not need to compromise the server’s filesystem directly; they can trick the application into following a symlink that points to a sensitive resource. This can lead to unauthorized log injection, overwriting critical files, or information disclosure, depending on the file operations implemented.

Real-world examples align with common vulnerability patterns such as CWE-62 (Path Traversal via Symlink) and aspects of OWASP API Top 10’s Broken Object Level Authorization, where indirect object references are not safely constrained. In Axum, even without authentication, if an endpoint accepts a path parameter that is concatenated with an API key to form a filesystem path, and the server follows symlinks, the attack surface is widened. middleBrick’s checks for Input Validation and Property Authorization help surface these weaknesses by correlating spec definitions with runtime behavior, ensuring that path construction logic does not inadvertently trust user-controlled identifiers like API keys.

To mitigate symlink risks when using API keys in Axum, ensure that API keys are never used directly in filesystem paths. Instead, use a one-way mapping (e.g., hash) to generate safe directory or filenames, and enforce strict path confinement. Below are concrete Axum code examples demonstrating secure handling.

Insecure Example (Vulnerable)

use axum::{routing::get, Router};
use std::path::PathBuf;

async fn handle_log(api_key: String) {
    // Vulnerable: API key used directly in path
    let path = PathBuf::from(format!("/var/logs/{}", api_key));
    // If api_key = "mykey/../../../etc/passwd", path traversal occurs
    // If a symlink exists at /var/logs/mykey -> /sensitive, writes escape intended dir
}

Secure Example (Remediated)

use axum::{routing::get, Json};
use std::path::{Path, PathBuf};
use sha2::{Sha256, Digest};

async fn get_safe_path(api_key: &str, suffix: &str) -> PathBuf {
    // Use a hash of the API key to avoid path traversal and symlink issues
    let mut hasher = Sha256::new();
    hasher.update(api_key.as_bytes());
    let hash = format!("{:x}", hasher.finalize());
    // Ensure suffix is alphanumeric to prevent path injection
    let safe_suffix = suffix.chars().filter(|c| c.is_alphanumeric()).collect::();
    PathBuf::from(format!("/var/logs/{}/{}", hash, safe_suffix))
}

async fn handle_log_secure(
    Json(payload): Json,
) -> String {
    let base_dir = Path::new("/var/logs");
    let file_path = get_safe_path(&payload.api_key, &payload.filename);
    let full_path = base_dir.join(file_path);
    // Write to full_path safely; directory structure is controlled and predictable
    "logged".to_string()
}

#[derive(serde::Deserialize)]
struct LogRequest {
    api_key: String,
    filename: String,
}

Key practices:

• Never concatenate raw API keys into filesystem paths.
• Hash API keys (e.g., SHA-256) to produce deterministic but non-guessable directory segments.
• Restrict filename suffixes to alphanumeric characters, rejecting any path separators.
• Use Path::join and avoid manual string assembly to prevent accidental traversal.
• If configuration requires per-tenant directories, create them at initialization time using trusted identifiers, not runtime input.

By combining these patterns with middleBrick’s continuous monitoring (available in the Pro plan) and CI/CD integration via the GitHub Action, you can automatically detect regressions that reintroduce path traversal or symlink risks before deployment.