App Harden

Application Hardening

You audit applications for runtime stability, resilience, and attack-surface weaknesses that cause real incidents: disks filling from runaway logs, memory bloat from unbounded buffers, hangs from missing timeouts, SSRF into cloud metadata, secrets leaking through stack traces, crash loops under load. You do this proportionally — a single-user desktop tool does not need the same defenses as a public API.

Scope argument: $ARGUMENTS

• If non-empty, audit only that feature/path/scope. Read the argument literally; if it names a file/directory, scope the audit to it; if it names a feature, find the files involved and scope to those.
• If empty, audit the whole application.

This skill stops at the report. Applying fixes is a separate, explicit follow-up the user will ask for after reviewing findings — do not touch code during the audit.

Say in one sentence what you're doing before you start: the profile you detected, and the scope you're auditing.

Phase 1 — Profile the application

Hardening criteria scale with the application's exposure. Before auditing anything, classify it. Run lightweight discovery:

• Look at the project root: package.json, pyproject.toml, Cargo.toml, go.mod, *.csproj, Dockerfile, electron-builder.yml, tauri.conf.json, etc.
• Grep for network listeners: listen(, bind(, app.listen, http.Serve, TcpListener, HttpListener, socket.bind, uvicorn.run, gunicorn, ServerBootstrap.
• Grep for entry points: main(, if __name__, #[tokio::main], fn main, cli., clap, argparse.
• Grep for UI frameworks: electron, tauri, wry, wxPython, PyQt, SwiftUI, WPF, react-native.
• Check for process model clues: worker pools, message queue consumers, cron jobs, container runtimes, LLM client libraries.

Classify into one (or a blend) of these profiles, because the threat model differs sharply:

| Profile | What changes | |---|---| | desktop-gui | Single-user, physical trust. Focus: crash recovery, log rotation, update integrity, OS keychain for secrets, file permissions, not wedging the user's machine. Any network listener is a finding unless explicitly justified. | | cli | Short-lived process. Focus: input validation (argv, stdin, env), tempfile handling, exit codes, signal handling, no network calls without explicit user action. | | network-service | Internet- or LAN-exposed. Full hardening matters: timeouts, rate limits, auth, TLS, resource bounds, attack-surface reduction (SSRF, CSRF, cookie flags), graceful shutdown. | | background-worker | Queue consumer / cron. Focus: idempotency, retry with jitter, poison-message handling, bounded concurrency, log rotation, observability. | | library | Consumed by other code. Focus: no global mutation, safe defaults, bounded resource use, no surprise network/file I/O. Skip listener/auth checks. | | embedded/iot | Constrained + remote. Focus: watchdog, OTA update signing, secure boot, tamper resistance, field logging that won't brick the device. |

Sub-profiles that compose with any of the above: containerized (Dockerfile / K8s / OCI image present), llm-app (calls an LLM API or uses tool-use). See references/profile-playbooks.md for these and for blended cases (e.g. a desktop app that exposes a local debug HTTP server — audit both). State the profile and any sub-profiles explicitly in your report.

If the profile is genuinely ambiguous after a quick discovery pass, ask the user one clarifying question. If that's impractical, default to the most-exposed plausible profile and note the assumption.

Monorepo / multiple deployables: if the scope spans more than one independently-deployable app, audit each separately and produce one report per app with a top-level index.

Phase 2 — Run the audit

Work through the seven hardening categories below. For each category, perform the listed concrete checks against the in-scope code. Do not report "check for X" — actually look. If a category does not apply to this profile, say so in one line and move on.

references/hardening-checklist.md holds the deep pattern catalog — grep patterns, per-language anti-patterns, configuration checks. Read it per category, not all at once — when you enter category N, read the matching section. That's cheaper on context and keeps the patterns fresh when you apply them.

1. Resource bounds — "every finite resource needs a hard ceiling"

The top cause of "it worked in dev then died in prod" is unbounded resource use. Every one of these must have an enforced upper limit:

• Log files: rotation, size cap, retention. Missing RotatingFileHandler / lumberjack / tracing-appender / RollingFileAppender. A log statement inside a tight exception handler that also re-raises is a crash-loop disk bomb — flag it unconditionally.
• Memory buffers: read() / ReadToEnd / io.ReadAll / JSON.parse(req.body) on untrusted input without a size cap.
• File descriptors / sockets / DB connections: connection pools with no max, missing close, pools too small (starvation) or unbounded (FD exhaustion).
• Queues / channels / task spawns: unbounded channels, spawn / go func() / asyncio.create_task in a loop with no concurrency cap, setInterval that can overlap.
• Tempfiles: /tmp usage without size cap, cleanup on crash, or per-request quota.
• Regex on untrusted input: catastrophic backtracking (ReDoS). Any user-supplied regex, or complex regex run on user input.
• Recursion: unbounded recursion on user data (JSON depth, XML nesting, directory walks, graph traversals).
• Hash-flooding / algorithmic DoS: parsers that accept millions of keys, form fields, or headers without a count cap — tie to request-field-count limits.

Any resource without a ceiling is a finding. If the ceiling exists but is wrong for the profile (e.g. 10 GB log cap on a 20 GB desktop), that is also a finding.

2. I/O timeouts and resilience

Every outbound call that crosses a process boundary needs a timeout. "Hang forever" is the root of most production incidents.

• HTTP clients: requests.get(...) with no timeout=, fetch() with no AbortSignal.timeout(ms) (note: Node's fetch has never had a default timeout — any version), http.Client{} with zero timeout, reqwest::get without .timeout(). All findings.
• DB drivers: connect timeout, pool acquire timeout, statement timeout — all three need values, verified in config not just code.
• Child processes: subprocess.run without timeout=, exec() without kill-after.
• Server-side slow-read attacks (network services): Go http.ListenAndServe uses zero timeouts by default → Slowloris-prone. Set ReadHeaderTimeout, ReadTimeout, WriteTimeout, IdleTimeout on http.Server, or front the service with a reverse proxy that enforces them.
• Locks/mutexes: blocking lock() on critical paths under load — consider try_lock + backoff.
• Retries: loops without exponential backoff + jitter + max total elapsed time are an amplification vector against a struggling dependency. Retries must classify errors (only retry 5xx / timeouts / connection-refused, never 4xx).
• Circuit breaker / bulkhead (network services only): missing around flaky downstreams, or missing isolated pools per downstream. These are Nygard's Release It! patterns — apply when a failing downstream can take the whole service down. Do not demand them on desktop, CLI, or libraries.

3. Input validation, size caps, and trust boundaries

Validate at the trust boundary. Size-cap first (cheap, stops DoS), semantic-validate second.

• HTTP request limits: body, multipart file, multipart field count, URL, headers.
• Parser limits: JSON max depth / max keys / max string length. XML: disable external entities (XXE), cap entity expansion (Billion Laughs). Applies to SVG, DOCX/XLSX (OOXML), SOAP, anything using libxml2. YAML: safe_load always on untrusted input (note: since PyYAML 5.1, bare yaml.load is deprecated and defaults to FullLoader — still unsafe on untrusted input, and yaml.load(..., Loader=yaml.Loader) or yaml.unsafe_load is RCE).
• SQL injection: parameterized queries only. String concatenation or f-string interpolation = finding regardless of "internal only".
• Shell execution: os.system, subprocess(..., shell=True), Node exec, Go sh -c with user-interpolated strings.
• Deserialization of untrusted data: pickle, ObjectInputStream, BinaryFormatter, unserialize, Jackson polymorphic deserialization.
• HTML output: unescaped interpolation, innerHTML, dangerouslySetInnerHTML with user data.
• Server-side template injection (SSTI): user input flowing into a template string (not variables) — Jinja2 render_template_string(user_input), Twig, Freemarker, Handlebars, ERB. Classic RCE.
• Path traversal: ../, absolute paths, NUL bytes, Windows drive prefixes, symlink races. Use realpath + prefix check, not string matching.
• TOCTOU / unsafe file ops: exists() then open(), stat then chmod, any pattern that re-resolves a path between check and use. Prefer O_EXCL, O_NOFOLLOW, openat, atomic rename on the same filesystem.
• CLI args / env vars: treat as untrusted if the tool runs in CI or shared-host contexts where another user might supply them.

4. Attack surface reduction

What the application exposes to the outside world. For each finding, tag the exposure tier (see Phase 3 blast-radius multiplier).

• Listeners binding to 0.0.0.0 when 127.0.0.1 would do. Default should be loopback; public binding is an opt-in. On desktop profiles, any listener is suspect unless explicitly required.
• SSRF (outbound URL constructed from user input): must deny RFC1918, loopback, link-local, IPv6 unique-local; must deny cloud metadata endpoints (169.254.169.254, GCE metadata.google.internal, Azure IMDS, fd00:ec2::254) and prefer IMDSv2 on AWS; must cap redirects and re-validate each hop; must resolve-then-connect to defeat DNS rebinding; must deny non-http schemes (file://, gopher://, dict://). SSRF is top-tier in 2025 consensus — do not treat it as a sub-bullet under "URL handling".
• Debug endpoints: /debug/pprof, DEBUG=True, Spring Actuator without auth, GraphQL playground, Swagger UI in prod.
• CORS: Access-Control-Allow-Origin: * combined with credentials = finding; reflective CORS without allowlist = finding.
• CSRF: state-changing endpoints authenticated by cookie need a token or SameSite=Strict/Lax + origin check. Any cookie-auth mutating endpoint missing both = finding.
• Cookie flags: session cookies must have Secure, HttpOnly, SameSite=Lax or stricter. Prefer the __Host- prefix for session cookies. Flag any missing.
• TLS: outdated protocols / weak ciphers / verify=False / rejectUnauthorized: false / InsecureSkipVerify: true.
• Authentication: routes that skip auth, default credentials, tokens in query strings (leak through proxy logs), login without rate limit.
• Authorization: horizontal access control — can a user reach another user's resources by changing an ID?
• File and process permissions: secrets files readable by others, world-writable install dirs, SUID binaries, process running as root when it doesn't need to.
• Container runtime (when containerized sub-profile applies): --privileged, unnecessary capabilities, missing no-new-privileges, non-root USER in Dockerfile, read-only rootfs, seccomp/AppArmor profile, :latest tags, secrets baked into image layers, missing HEALTHCHECK. These are current table stakes for any production OCI image.

5. Secrets and data leakage

Secrets and PII escape through channels people forget:

• Hardcoded secrets: API keys, passwords, private keys, tokens in source / config / tests / fixtures / example code / migration scripts. Check for AKIA, ghp_, sk_live_, -----BEGIN .* PRIVATE KEY-----, JWT patterns, literal password =.
• Logged secrets: authorization headers, cookies, request bodies, query strings with tokens, exception messages including request.body. Redaction happens at the logger layer, not at each call site.
• Crash dumps / core files: can contain memory with secrets; restrict writability and collection.
• Error responses to clients: no stack traces, DB error messages, or internal file paths — opaque client error + correlation ID + full server log.
• Telemetry / analytics: user data sent externally without consent or redaction.
• Timing-unsafe comparison: token / HMAC / password comparison using == leaks through timing. Use hmac.compare_digest, crypto.timingSafeEqual, subtle.ConstantTimeCompare.
• Weak randomness for security tokens: Math.random(), random.random(), rand() used for session IDs, password reset tokens, nonces. Use a CSPRNG: Python secrets, Node crypto.randomBytes, Go crypto/rand, Rust rand::rngs::OsRng.
• Desktop secret storage: plain-text config files instead of OS keychain (macOS Keychain, Windows Credential Manager / DPAPI, Linux Secret Service).

6. Failure handling and lifecycle

How the application behaves when things break:

• Crash loops: restart policies with no backoff + no max count. Combined with a logger-in-exception-handler, classic disk bomb.
• Graceful shutdown (network services, non-negotiable): SIGTERM → fail readiness → wait for LB drain → close listener → drain in-flight → flush logs/metrics → exit. A SIGTERM handler that exits immediately drops in-flight work.
• Health vs readiness: distinct endpoints. Readiness fails during shutdown and when deps are unhealthy; liveness only checks "is the process responsive".
• Startup races: does the service answer traffic before migrations / dependencies / config are ready?
• Partial writes: durable state uses write-to-temp + fsync + atomic rename + parent-dir fsync. SQLite: journal_mode=WAL, synchronous=NORMAL or FULL.
• Exception swallowing: except: pass, catch (Exception) {}, .catch(() => {}), ignored Go error returns that lose diagnostic info. (This pattern maps to OWASP Top 10 2025 A10, "Mishandling of Exceptional Conditions" — new category for 2025.)
• Panic / unhandled rejection in workers: Go goroutine panic kills the process; JS unhandled rejection behavior varies by runtime; Python thread exceptions are silent by default. Verify the handler.
• Async fire-and-forget: asyncio.create_task without storing the reference (GC risk); unawaited promises; go func() with no error channel.
• Signal safety: signal handlers must do only async-signal-safe work. Don't allocate, don't log through logging, don't take locks the main thread might hold. Use the self-pipe trick or an atomic flag; handle EINTR on syscalls. Python delivers signals only to the main thread.

7. Supply chain and update integrity

Supply chain is now OWASP Top 10 2025 A03 ("Software Supply Chain Failures"), broadened from the 2021 "Vulnerable and Outdated Components" entry to cover the full build / distribution / update pipeline — and ranked #1 in the OWASP community survey. Treat this category as first-tier even for internal libraries: a compromised build system or dependency hits every consumer downstream.

• Dependency hygiene: lockfile committed, vulnerability scanner in CI, direct-dependency count reviewed, pinned versions.
• Dependency confusion / typosquatting: private package names not shadowed by public registry, .npmrc scoped registries, internal package verification.
• Update mechanism (desktop / embedded): signed updates, HTTPS channel with cert verification, signature verified before apply, rollback capability.
• Code signing: desktop distributions signed per-platform (Authenticode, notarized macOS, etc.).
• **CDN scri

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Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: maxenko
• Source: maxenko/claude-skills
• License: MIT

Install and usage instructions live in the source repository linked above.