Cookie Security in the Post-Third-Party Era

Introduction

Cookies have been the backbone of digital advertising for over two decades. They enable user identification across sites, power frequency capping, drive conversion attribution, and store consent preferences. Yet the cookie landscape is undergoing its most significant transformation since the technology was first introduced in the mid-1990s. Browser vendors, regulators, and industry bodies are fundamentally reshaping how cookies work — and in many cases, eliminating them entirely for cross-site use cases.

For ad tech professionals, understanding cookie security is no longer optional. Misconfigured cookies lead to data leakage, tracking failures, and compliance violations. This article provides a thorough technical examination of cookie mechanics, the evolving enforcement landscape, the alternatives emerging through Google's Privacy Sandbox, and the practical steps teams should take to audit and harden their cookie configurations.

Cookie Fundamentals

First-Party vs. Third-Party Cookies

A first-party cookie is set by the domain the user is currently visiting. If you are browsing example.com, any cookie set by example.com is a first-party cookie. First-party cookies are essential for login sessions, shopping carts, and site preferences. Browsers treat them favorably and impose minimal restrictions.

A third-party cookie is set by a domain other than the one the user is visiting. When example.com embeds an ad from adserver.net, any cookie set by adserver.net is a third-party cookie. These cookies have historically been the primary mechanism for cross-site tracking — allowing ad servers to recognize the same user across different websites. This is the category of cookies that browsers are now restricting or eliminating.

Session vs. Persistent Cookies

Session cookies have no explicit expiration date and are deleted when the browser closes. They are suitable for short-lived state such as maintaining a user's session during a single browsing period. Persistent cookies include an Expires or Max-Age attribute that tells the browser to retain the cookie for a specified duration — from minutes to years. In ad tech, persistent cookies are used for user identification, frequency capping, and conversion windows that may span days or weeks.

Set-Cookie Attributes Deep Dive

The Set-Cookie HTTP response header is where cookie behavior is defined. Each attribute controls a specific aspect of how the browser stores, transmits, and protects the cookie. Getting these attributes right is critical for both security and functionality.

Domain

The Domain attribute specifies which hosts can receive the cookie. If set to Domain=example.com, the cookie is sent to example.com and all subdomains (e.g., api.example.com). Omitting the Domain attribute restricts the cookie to the exact host that set it, excluding subdomains. For ad tech, overly broad Domain scoping can leak cookies to subdomains that should not receive them.

Path

The Path attribute restricts the cookie to requests whose URL path begins with the specified value. Setting Path=/api ensures the cookie is only sent with requests to paths under /api. While Path provides some scoping, it is not a robust security boundary — JavaScript on the same origin can still access cookies for any path unless the HttpOnly attribute is set.

Secure

The Secure flag ensures the cookie is only transmitted over HTTPS connections. Without it, the cookie can be sent in plaintext over HTTP, exposing it to network eavesdropping. In ad tech, where cookies carry user identifiers and consent tokens, the Secure flag is mandatory for any cookie that contains sensitive data.

HttpOnly

The HttpOnly flag prevents client-side JavaScript from reading or modifying the cookie through document.cookie. This is a critical defense against cross-site scripting (XSS) attacks, where injected scripts attempt to steal session cookies. For server-side tracking cookies that don't need to be read by front-end code, HttpOnly should always be enabled.

Expires and Max-Age

Expires sets an absolute date-time after which the browser should delete the cookie. Max-Age sets a relative duration in seconds. If both are present, Max-Age takes precedence. Setting excessively long expiration periods is a privacy concern — Safari's Intelligent Tracking Prevention (ITP) caps certain cookies to seven days or even 24 hours regardless of the specified expiration.

SameSite

The SameSite attribute controls whether the cookie is sent with cross-site requests. It accepts three values: Strict (cookie is never sent cross-site), Lax (cookie is sent with top-level navigations but not with embedded requests like iframes or image loads), and None (cookie is sent with all requests, including cross-site). For ad tech use cases that require cross-site cookie access — such as third-party ad serving in iframes — the cookie must be set with SameSite=None; Secure.

The SameSite Enforcement Evolution

The SameSite attribute was introduced as a way to mitigate cross-site request forgery (CSRF) attacks, but its impact on ad tech has been far more profound. The enforcement timeline accelerated rapidly starting in 2020.

In February 2020, Chrome 80 began defaulting cookies without an explicit SameSite attribute to SameSite=Lax. This was a breaking change for the ad tech industry — cookies that previously flowed freely in cross-site iframe and redirect contexts were suddenly blocked unless explicitly marked as SameSite=None; Secure. Teams that had not audited their cookie configurations experienced immediate tracking failures, broken frequency caps, and disrupted attribution models.

Safari's Intelligent Tracking Prevention (ITP) took an even more aggressive approach. ITP identifies tracking cookies through heuristic and machine-learning classifiers and restricts their lifetime to as little as 24 hours for cookies set through client-side JavaScript on domains classified as trackers. This effectively rendered many third-party cookie-based tracking strategies useless on Safari well before Chrome's SameSite changes. Firefox's Enhanced Tracking Protection (ETP) applies similar restrictions, automatically blocking known tracker domains from setting cookies entirely.

Third-Party Cookie Deprecation Timeline

Google's plan to deprecate third-party cookies in Chrome has been the most closely watched development in ad tech privacy. Originally announced in January 2020 with a target of 2022, the timeline has been extended multiple times as the industry navigates the transition. Safari and Firefox have already blocked third-party cookies by default, meaning that roughly 30 to 40 percent of web traffic has been operating without third-party cookies for years.

The deprecation has forced the ad tech industry to develop alternative approaches to the core use cases that third-party cookies have historically served: cross-site user identification, audience targeting, frequency capping, and conversion attribution. Teams that wait for the final deprecation date to act will find themselves scrambling — the smart approach is to build and test alternative solutions now while third-party cookies still function in Chrome for testing comparisons.

Privacy Sandbox Alternatives

Google's Privacy Sandbox is a set of browser APIs designed to replace the functionality that third-party cookies provide, while offering stronger privacy guarantees. The key proposals relevant to advertising are Topics API, Protected Audience (formerly FLEDGE), and Attribution Reporting.

Topics API

Topics API replaces interest-based targeting that previously relied on third-party cookies. The browser observes the user's browsing activity locally and derives a set of interest topics (e.g., "Sports," "Travel," "Technology") from a standardized taxonomy. When a participating site requests targeting information, the browser returns a small set of recent topics without revealing the specific sites the user visited. This gives advertisers a coarse targeting signal while preventing the granular cross-site tracking that cookies enabled.

Protected Audience (FLEDGE)

Protected Audience enables remarketing and custom audience targeting without third-party cookies. When a user visits an advertiser's site, the site can add the user to an interest group stored locally in the browser. Later, when the user visits a publisher site, an on-device auction runs within the browser to select which interest group ad to show — without sending the user's interest group membership to any server. This keeps remarketing functional while preventing the server-side cross-site data sharing that characterizes cookie-based retargeting.

Attribution Reporting

Attribution Reporting provides conversion measurement without cross-site tracking. When a user clicks or views an ad, the browser records the event locally. If the user later converts on the advertiser's site, the browser generates an attribution report that links the conversion to the original ad interaction — but the report is aggregated and delayed to prevent identifying individual users. This approach supports campaign optimization and ROAS measurement while maintaining user privacy.

Impact on Advertising

The shift away from third-party cookies affects virtually every aspect of digital advertising operations.

Targeting

Cross-site behavioral targeting — the practice of building user profiles based on browsing activity across multiple sites — is no longer viable through cookies alone. Advertisers are shifting to contextual targeting (matching ads to page content rather than user profiles), first-party data strategies (leveraging data from direct customer relationships), and Privacy Sandbox APIs that provide interest signals without cross-site tracking.

Frequency Capping

Frequency capping — limiting how many times a user sees the same ad — has traditionally relied on third-party cookies to recognize users across sites. Without them, frequency management must move to first-party solutions (publisher-side caps using first-party identifiers), server-side deduplication, or emerging Privacy Sandbox mechanisms. The risk of over-frequency is real: without effective capping, campaigns waste budget and annoy users.

Measurement

Conversion attribution — connecting an ad exposure to a subsequent action on the advertiser's site — is the use case most directly impacted by third-party cookie loss. Multi-touch attribution models that rely on a consistent user identifier across sites are no longer possible without alternative identity solutions. The Attribution Reporting API, probabilistic modeling, and data clean rooms are emerging as replacements, though each comes with trade-offs in accuracy, latency, and implementation complexity.

Cookie Auditing Process

A systematic cookie audit identifies misconfigured, unnecessary, or non-compliant cookies before they cause tracking failures or regulatory issues. Here is a step-by-step approach to auditing cookies in an ad tech environment.

1. Inventory all cookies. Use the Cookie Inspector or browser DevTools to list every cookie set on your domains. Record the name, domain, path, expiration, and all attributes for each cookie.
2. Classify each cookie. Categorize cookies as strictly necessary, functional, analytics, or advertising. This classification maps directly to consent categories under GDPR and the IAB TCF framework.
3. Verify attribute configuration. Check that every cookie has the appropriate Secure, HttpOnly, SameSite, and expiration settings for its use case. Flag any cookie missing the Secure flag, any tracking cookie without HttpOnly, and any cross-site cookie without SameSite=None; Secure.
4. Test cross-browser behavior. Load your site in Chrome, Safari, and Firefox and verify that cookies behave as expected in each browser. Pay particular attention to Safari ITP restrictions and Firefox ETP blocking.
5. Remove unnecessary cookies. Delete any cookie that no longer serves a documented purpose. Reducing cookie count reduces payload size on every request and minimizes the surface area for data leakage.
6. Document and repeat. Maintain a cookie registry that describes each cookie's purpose, owner, classification, and configuration. Schedule audits quarterly or after any significant infrastructure change.

Configuration Best Practices

• Always set the Secure flag. In 2025, there is no valid reason to transmit cookies over plaintext HTTP. Set Secure on every cookie.
• Default to HttpOnly. Unless client-side JavaScript must read the cookie (e.g., consent preferences managed by a CMP), set HttpOnly to protect against XSS-based cookie theft.
• Use the most restrictive SameSite value possible. Start with SameSite=Strict or SameSite=Lax and only use SameSite=None when cross-site delivery is genuinely required.
• Set reasonable expiration periods. Avoid multi-year cookie lifetimes. Align the Max-Age or Expires value with the actual use case — a conversion attribution window typically needs 30 to 90 days, not 365.
• Scope Domain narrowly. Set the Domain attribute to the most specific hostname possible. Avoid setting cookies on the root domain when only a subdomain needs them.
• Prefix cookie names. Use the __Secure- and __Host- cookie name prefixes to enforce that the cookie has the Secure flag and (for __Host-) that it is scoped to the exact host without a Domain attribute. These prefixes provide an additional layer of protection against cookie injection attacks.

Compliance: GDPR, ePrivacy, and CCPA

Cookie configuration intersects directly with privacy regulation. Understanding the legal framework is essential for ad tech teams operating in global markets.

GDPR and the ePrivacy Directive

Under the General Data Protection Regulation and the ePrivacy Directive, setting non-essential cookies requires explicit, informed consent from the user before the cookie is placed. This means that advertising and analytics cookies must not be set until the user has interacted with a consent management platform (CMP) and granted permission. The IAB Transparency and Consent Framework (TCF) provides a standardized mechanism for collecting and communicating this consent through the ad tech supply chain. The TCF String Decoder can help you verify that consent strings are correctly structured and encode the intended permissions.

CCPA

The California Consumer Privacy Act takes a different approach — rather than requiring opt-in consent, CCPA gives California residents the right to opt out of the "sale" of their personal information. If your cookies facilitate user tracking that constitutes a sale of personal information under CCPA, you must provide a clear "Do Not Sell My Personal Information" mechanism and honor opt-out signals, including the Global Privacy Control (GPC) browser header.

Compliance is not a one-time implementation — it requires ongoing monitoring. Cookie audits should verify that consent is obtained before non-essential cookies are set, that opt-out signals are respected, and that cookie configurations align with the consent status stored in the TCF consent string.

Conclusion

The post-third-party cookie era is not a distant future — it is the present reality for Safari and Firefox users, and it is rapidly approaching for Chrome. Ad tech professionals who invest in understanding cookie security fundamentals, hardening their Set-Cookie configurations, and building toward Privacy Sandbox alternatives will be well-positioned for the transition. Those who delay will face tracking failures, compliance risk, and a competitive disadvantage as the industry moves to privacy-first infrastructure.

Start with a comprehensive cookie audit, enforce best-practice attributes on every cookie, classify your cookies against consent frameworks, and begin testing Privacy Sandbox APIs in parallel with your existing cookie-based systems. The transition will be gradual, but the groundwork you lay now determines how smoothly your ad tech operations adapt to a cookieless world.