What Cookies Cannot Be Used For (2025): Limits, Myths, and Legal Rules

• Written by Sep, 8 2025
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You clicked this because you want a straight answer: what cookies cannot be used for? Here’s the short version-cookies are tiny text files. They don’t run, they don’t peek into your files, and they can’t magically identify you. But they can still track you, and that’s where the rules get strict. I’ll give you the fast facts, the legal lines you can’t cross in 2025, and a simple plan to use cookies safely without breaking trust (or the law).

• Cookies can’t run programs, read device files, or install malware.
• Cookies can’t access data set by other websites (same-origin rules apply).
• Cookies can’t store sensitive data safely (payments, health, government IDs).
• Third-party tracking is blocked by most browsers and being phased out in Chrome.
• Consent is required for most non-essential cookies in the EU/UK, and strong opt-out and transparency rules apply in California and Australia.

What cookies cannot be used for (fast answer and myths busted)

Let’s clear the biggest myths first. A cookie is just text that the browser saves and sends back to the same website. It’s not an app, it’s not a file reader, and it can’t jump between sites on its own.

• They cannot execute code. A cookie doesn’t “run.” It cannot carry or execute malware. Security problems with cookies come from how code uses them, not from the cookie itself.
• They cannot read your device or other apps. Cookies don’t have permission to browse your hard drive, photos, emails, or anything outside the browser.
• They cannot read other sites’ cookies. SameSite and same-origin rules prevent cross-site cookie reading. A cookie set by site A isn’t readable by site B.
• They cannot reliably identify a person on their own. A cookie value is an ID the site made up. Without extra account data, it’s not real-world identity. (Privacy law still treats it as personal data if it can single you out.)
• They cannot store large or sensitive data safely. Most browsers cap cookie size to ~4KB and limit how many you can store per site. Sensitive data (credit cards, health, government IDs) should never go into cookies.
• They cannot bypass modern browser privacy features. Safari and Firefox block most third‑party cookies by default. Chrome is phasing them out for tracking. You can’t “make” a third‑party cookie work where it’s blocked.
• They cannot guarantee persistence. Users clear cookies, switch devices, use private browsing, or block them entirely. Any identity or tracking based only on cookies will break.

Translation: cookies are limited, fragile, and regulated. If your plan relies on them doing more than basic session management and simple preferences, you need a rethink.

Laws and browser limits you can’t ignore in 2025

Cookies are heavily regulated because they can track people. Here’s what’s non‑negotiable right now in key regions.

• European Union and UK (GDPR + ePrivacy Directive): You need clear, prior consent for non‑essential cookies (analytics, ads, personalization). Pre‑ticked boxes and dark patterns are not valid consent. Users must be able to withdraw easily. Maximum fines can reach €20 million or 4% of global annual turnover, whichever is higher (GDPR).
• California (CPRA/CCPA): You need to disclose tracking and provide a right to opt out of “selling” or “sharing” personal information (this includes cross‑context behavioral ads using cookies). Penalties generally start at $2,500 per violation or $7,500 for intentional violations and those involving minors.
• Australia (Australian Privacy Act + APPs): No cookie‑specific law, but cookies that identify or reasonably identify an individual are “personal information.” You need transparency, a lawful basis for collection, and opt‑out for direct marketing and targeted ads. Serious or repeated interferences with privacy can trigger big penalties under the 2022 amendments-up to the greater of AUD $50 million, three times the benefit, or 30% of adjusted turnover during the breach period. Here in Australia, the government has also been consulting on broader Privacy Act reforms that tighten consent, targeting, and profiling rules.

Browser changes matter too:

• Third‑party cookies: Safari and Firefox block them by default. Chrome is phasing them out in 2025 under Privacy Sandbox, with regulator oversight. If your analytics or ads stack still relies on third‑party cookies, you need a plan B.
• SameSite, Secure, HttpOnly: Modern browsers enforce stricter defaults. SameSite=Lax is a common default to curb cross‑site tracking. Use Secure for HTTPS‑only transport and HttpOnly to keep cookies away from JavaScript where possible.

Standards and guidance you can cite to your legal or dev team: IETF RFC 6265 (HTTP State Management Mechanism), GDPR and ePrivacy texts, California CPRA regulations, Australian Privacy Principles (APPs) and OAIC guidance.

Do this instead: safe patterns, step-by-step audit, and pro tips

If you’re a site owner, marketer, or developer, the real job is to keep what works (sessions, preferences) and ditch what’s risky (silent tracking, sensitive data in cookies). Use this as your checklist.

1. Decide what’s essential vs non‑essential
   • Essential: login/session IDs, CSRF tokens, basic site preferences.
   • Non‑essential: analytics beyond strictly necessary, A/B testing identifiers, ad and social pixels, cross‑site tracking, personalization tags.
2. Configure secure cookie attributes
   • Set HttpOnly for session/auth cookies to block JavaScript access.
   • Set Secure so cookies only travel over HTTPS.
   • Set SameSite=Lax or Strict for auth and CSRF safety; use SameSite=None; Secure only when third‑party use is truly needed and lawful.
   • Set short expiries for session/auth; avoid “never expire.”
3. Strip out sensitive content
   • Never put payment card numbers, CVV, bank details, health data, government IDs, or precise location in cookies-ever. (Think PCI DSS and health privacy obligations.)
   • Don’t store raw personal info (name, email, phone). If you must reference a person, store a short‑lived random ID and keep the real data server‑side.
4. Make consent real (not performative)
   • Show a clear choice before setting non‑essential cookies in the EU/UK. Respect “Reject all” as much as “Accept all.”
   • Offer opt‑out in California and Australia for targeted advertising. Honor Global Privacy Control where required.
   • Log consent state (yes/no, date/time, preferences) without tracking people across sites.
5. Minimize and move server‑side when possible
   • Use server‑side sessions with a random token in the cookie. Keep the heavy data in your database, not in the cookie.
   • Prefer first‑party analytics with IP masking and no cross‑site identifiers. Aggregate where you can.
   • Replace third‑party tags with server‑side tagging or API‑based integrations that don’t rely on third‑party cookies.
6. Test against modern browsers and privacy modes
   • Check behavior in Safari, Firefox, and Chrome with third‑party cookies blocked.
   • Test private/incognito windows and cookie clearing.
   • Use developer tools to inspect Set‑Cookie headers and confirm attributes.

Field‑tested rules of thumb:

• If you wouldn’t print it on a postcard, don’t put it in a cookie.
• Assume a cookie is public and temporary. Anything secret or long‑term belongs server‑side.
• Prefer short‑lived tokens you can revoke. Rotate them often.
• Measure value vs risk: if a cookie adds less value than the consent friction it creates, drop it.

Examples, checklist, table, and quick FAQ

Here are concrete cases of what cookies cannot be used for, with what to do instead. And yes, I’m going to be blunt so it’s easy to act on this today.

Short checklist you can run today:

• Inventory: List all cookies your site sets (name, purpose, domain, expiry, attributes).
• Classify: Essential vs non‑essential. Tag anything for ads, analytics, or personalization as non‑essential.
• Harden: Add HttpOnly, Secure, and SameSite to all cookies where possible.
• Reduce: Delete cookies you don’t truly need. Shorten expiries.
• Consent: Make your banner honest and simple. Don’t set non‑essential cookies before consent in the EU/UK.
• Honor: Respect opt‑out signals (CPRA, Australia). Avoid dark patterns.
• Document: Keep a cookie policy that maps purposes to cookies in plain English.

Real‑world example: You run an online store in Melbourne with traffic from Europe and the US. You need a session cookie for login (HttpOnly, Secure, SameSite=Lax), a CSRF token (Strict), and a few preferences (like currency). For analytics, switch to a first‑party tool with IP masking and cookieless mode for EU if you can. For marketing, stop relying on third‑party cookies; build permission‑based email and measure with server‑side events where it’s lawful and consented. Your cookie banner offers Accept/Reject and a manage link. Only essential cookies load until a choice is made for EU/UK visitors.

Quick myth vs fact recap:

• Myth: “Cookies are spyware.” Fact: A cookie is a small text file. Tracking happens when sites use them to follow you. That part is regulated.
• Myth: “A cookie can hack your device.” Fact: It can’t execute. Poor site security is the risk, not the cookie itself.
• Myth: “If it’s first‑party, it’s always fine.” Fact: First‑party tracking can still be personal information and subject to consent/opt‑out rules.
• Myth: “We need year‑long cookies to measure.” Fact: You don’t. Shift to aggregated, short‑lived, first‑party measurement and model the rest.

Here’s the core phrase many people search for: cookies cannot be used for running code, reading your device, storing sensitive data, guaranteeing identity, or bypassing consent and browser limits. If you’re using cookies for any of those, you’re skating on thin ice.

Mini‑FAQ

• Can a cookie identify me by name? No. It’s just an ID string unless the site links it to your account data.
• Are first‑party cookies exempt from laws? No. If they track or profile, they’re still regulated.
• Can I share a login across domains with cookies? Not directly. Use centralized auth, server‑side sessions, or standards that support first‑party sets where allowed.
• Is localStorage safer than cookies? It’s different, not safer. localStorage is readable by JavaScript and not sent with every request. Don’t store secrets there either.
• What’s a safe expiry for auth cookies? Hours to days, not months. Pair with refresh tokens that you can revoke.

Next steps and troubleshooting

• If you’re a small business: Run the checklist, drop non‑essentials, and switch to first‑party analytics. Update your privacy policy and banner copy to plain English.
• If you’re in marketing: Build consented audiences. Stop planning around third‑party cookies. Test contextual ads and clean‑room partnerships that don’t rely on cross‑site IDs.
• If you’re engineering: Add HttpOnly, Secure, and SameSite attributes today. Move state server‑side. Set up automated tests that fail when a cookie is missing these flags.
• If you’re legal/compliance: Map cookies to purposes, regions, and lawful bases. Monitor GDPR, CPRA, and Australian reform updates. Audit consent logs.
• If something breaks after blocking third‑party cookies: Replace cross‑site dependencies with server‑side integrations, or use approved privacy‑preserving APIs with user consent.

Sources worth knowing by name: IETF RFC 6265 (HTTP State Management Mechanism), EU GDPR and the ePrivacy Directive, California Consumer Privacy Act as amended by CPRA, and Australia’s Privacy Act with the Australian Privacy Principles and OAIC guidance. Those are the benchmarks regulators and engineers look to when they decide what’s allowed-and what isn’t.