What Are Supercookies and Evercookies and How Do They Respawn?
Having supercookies evercookies explained is critical for digital privacy because these persistent tracking mechanisms bypass standard cookie deletion by storing unique identifiers in Flash LSOs, HTML5 Web Storage, Silverlight Isolated Storage, Canvas fingerprinting, HSTS supercookies, ETag tracking, and HPKP caches. Evercookies actively respawn deleted HTTP cookies by retrieving stored identifier hashes from hidden storage vectors, making Send.win’s zero-trace ephemeral profile environments essential to permanently eliminate persistence.
Most internet users assume that clicking “Clear Browsing Data” or enabling private browsing mode wipes away all tracking artifacts. Unfortunately, aggressive advertising technology companies and web analytics firms developed complex storage mechanisms that survive standard cookie deletions. Understanding how these persistent tracking vectors function—and having supercookies evercookies explained in technical detail—is essential for anyone seeking true session privacy, data confidentiality, and identity protection online.
Deep Dive: The Technical Anatomy of Supercookies
The term “supercookie” refers to any tracking mechanism that operates outside standard HTTP cookie storage and resists routine user deletion. Unlike standard cookies, which are domain-restricted text files easily removed through browser preferences, supercookies exploit low-level network headers, browser caching mechanisms, and auxiliary storage subsystems to maintain persistent identity vectors.
1. Flash Local Shared Objects (LSOs)
In the early days of persistent web tracking, Adobe Flash Local Shared Objects (LSOs) were the preferred vector for supercookies. Flash LSOs could store up to 100 kilobytes of data per domain in a dedicated system folder completely separate from the browser’s cookie directory. When users cleared their browser cookies, Flash LSOs remained intact. Tracking scripts routinely checked Flash LSO storage and copied the saved tracking ID back into standard cookie storage whenever a cookie was missing.
2. HTML5 Web Storage (localStorage, IndexedDB, WebSQL)
With the deprecation of Flash, web trackers migrated to modern HTML5 storage APIs. HTML5 introduced localStorage, sessionStorage, IndexedDB, and the legacy WebSQL database specification. While standard cookies are sent with every HTTP request and capped at 4KB per cookie, HTML5 Web Storage APIs allow websites to store megabytes of persistent data client-side. Because many browser cleanup utilities historically focused exclusively on deleting standard .txt cookie files, trackers utilized HTML5 Web Storage as a resilient backup repository for user IDs.
3. Silverlight Isolated Storage
Similar to Flash LSOs, Microsoft Silverlight provided its own Isolated Storage mechanism. Enterprise websites and multimedia applications running Silverlight plug-ins could read and write binary files to a sandboxed filesystem path on the host operating system. Tracking companies leveraged Silverlight Isolated Storage to maintain redundant copies of user tracking keys that survived standard browser cookie purges.
4. HSTS Supercookies (HTTP Strict Transport Security)
HSTS (HTTP Strict Transport Security) is a security header designed to force web browsers to connect to websites securely via HTTPS. However, tracking companies weaponized HSTS into a tracking supercookie vector. A tracking server creates a unique binary identifier for a user by setting HSTS policies across a specific combination of subdomains (e.g., sub01.tracker.com, sub02.tracker.com, up to 32 subdomains). When the user visits another website hosting the tracker’s script, the script fires invisible image requests to all subdomains. The browser automatically upgrades requests to HTTPS only for subdomains flagged in its HSTS cache. By reading which subdomains connect via HTTPS versus HTTP, the tracker decodes the user’s binary ID without using any cookies at all.
5. HTTP ETag Tracking (Entity Tags)
HTTP ETags (Entity Tags) are web server response headers used for browser cache validation. When your browser downloads an asset (such as an image or JavaScript file), the server sends an ETag header containing a unique hash representing that specific file version. On subsequent visits, your browser includes an If-None-Match header containing the previously received ETag value. Trackers generate a unique ETag string for every individual user upon their first visit. As long as the file remains cached in the browser, your browser transmits your unique ETag hash with every HTTP request, allowing the tracker to track you indefinitely.
6. HTTP Public Key Pinning (HPKP)
HTTP Public Key Pinning (HPKP) allowed web servers to instruct browsers to remember specific cryptographic public keys associated with a domain to prevent man-in-the-middle attacks. Similar to HSTS tracking, malicious actors discovered they could abuse HPKP state tables to store persistent bit arrays inside the browser’s security policy cache. Due to these privacy concerns and implementation risks, HPKP was officially deprecated by major web standards bodies, though legacy browser installations remain susceptible to HPKP cache extraction.
7. Canvas and Hardware Fingerprinting
When storage-based supercookies are blocked or purged, tracking networks combine storage vectors with passive hardware fingerprinting. By rendering hidden HTML5 Canvas elements, sampling WebGL graphics performance, and analyzing AudioContext sound wave processing, trackers compute a unique cryptographic hash tied to your system graphics card and display drivers. To learn more about how hardware attributes build persistent identity profiles, read our comprehensive guide on browser fingerprint explained.
The Mechanics of Evercookies: The Unkillable Zombie Tracker
While an individual supercookie relies on a single persistent storage mechanism, an evercookie combines virtually all available storage mechanisms simultaneously into a self-healing tracking array. Created by security researcher Samy Kamkar in 2010, Evercookie demonstrated how impossible it is to remove tracking data using standard browser tools.
How Evercookie Synchronization Works
When a web page containing the Evercookie script loads, it writes a unique tracking ID into every accessible storage layer across your browser and system. This includes:
- Standard HTTP Cookies
- HTML5
localStorageandsessionStorage - IndexedDB and WebSQL databases
- HTTP ETag Caches
- HSTS Security Policy Caches
- Web History Caches (visited link color matching)
- Canvas rendering hash caches
- Window
nameDOM properties
When you visit the site again, Evercookie queries every single storage vector simultaneously. If you cleared your standard HTTP cookies, deleted your HTML5 localStorage, and wiped your IndexedDB databases, but forgot to clear your HTTP ETag cache, Evercookie finds your tracking ID in the ETag cache. The script then instantly writes that tracking ID back into your HTTP cookies, HTML5 storage, and IndexedDB databases. To permanently delete an evercookie, you must erase every single storage layer simultaneously—if even a single vector survives, the entire tracking profile respawns instantly.
The Role of Window Name Property in Persistence
The DOM window.name property persists across page navigations within a browser tab. Evercookie scripts populate window.name with the tracking ID. Even if the user navigates across different domain names within the same tab, the tracking ID remains intact in memory and can be accessed by scripts running on destination pages unless explicitly overwritten.
Web History Cache Exploitation (CSS :visited)
Evercookie also exploited CSS :visited selector styling rules to determine whether a browser had previously accessed specific tracking URLs. By probing link colors programmatically via JavaScript, Evercookie could reconstruct binary values encoded as URL visit histories, giving trackers yet another secret storage channel.
Comparing Cookie and Supercookie Persistence
Understanding the differences between standard web tracking methods and persistent supercookies highlights why advanced isolation technologies are necessary. Having supercookies evercookies explained in comparative context clarifies the technical baseline:
| Tracking Type | Storage Location | Survives “Clear Cookies”? | Survives Browser Restart? | Defeated by Ephemeral Isolation? |
|---|---|---|---|---|
| Standard Cookie | Browser Cookie Jar | No | Yes (if persistent) | Yes |
| HTML5 Web Storage | Browser Local Domain Storage | No (in modern browsers) | Yes | Yes |
| ETag Cache Header | Browser Resource Cache | No (only if Cache cleared) | Yes | Yes |
| HSTS Supercookie | Network Security State Cache | No | Yes | Yes |
| Evercookie Array | Multi-Vector Storage & Cache | No (Respawns automatically) | Yes | Yes |
| Send.win Ephemeral Profile | Sandboxed Ephemeral Storage Container | Self-Destructs on Close | No (Zero-Trace) | Complete Prevention |
Why Standard Browser Protections Fail Against Evercookies
Mainstream browsers struggle to defeat evercookies because these tracking techniques abuse legitimate web features required for performance and security. For instance, browsers cannot simply disable HTTP caching or HTTP Strict Transport Security without drastically reducing web page loading speeds and compromising user security against man-in-the-middle attacks.
Performance vs. Privacy Tradeoffs
Browser developers face a constant dilemma: HTTP caching reduces server load and speeds up web page delivery. If browsers disabled asset caching entirely to prevent ETag tracking, internet browsing would become noticeably slower and consume far more mobile data.
Security vs. Privacy Dilemmas
Similarly, HSTS is vital for preventing SSL-stripping cyberattacks. Browsers must retain HSTS domain tables to guarantee encrypted connections. Disabling HSTS state retention would expose users to severe network eavesdropping risks on public Wi-Fi networks.
User Friction in Manual Cache Purges
Furthermore, clearing browser caches manually is inconvenient for average users because it logs them out of every active account and forces websites to re-download all graphics, stylesheets, and scripts. As a result, users rarely purge their caches frequently enough to prevent ETag or HSTS tracking. To maintain online safety without breaking daily web workflows, users must practice robust safe browsing practices paired with sandboxed profile management.
Cross-Process Storage Contamination
Standard web browsers run all extensions, plugins, and browsing tabs under a single operating system user account profile. When persistent tracking scripts write identifiers to low-level cache files, those identifiers become accessible across all tabs and browser windows, rendering internal tab isolation ineffective.
How Send.win Eliminates Supercookies and Evercookies Forever
Instead of attempting to clean infected browser profiles after tracking vectors take root, Send.win eliminates supercookie and evercookie persistence through architectural session isolation.
Send.win creates independent, fully sandboxed browser environments that operate in total isolation from one another and from the host operating system. Each profile functions as a fresh, standalone browser installation with its own isolated cookie jars, HTML5 storage containers, HTTP cache pools, and DNS security caches. Having supercookies evercookies explained helps demonstrate why this architectural approach is the only permanent solution.
Key Features of Send.win Ephemeral Profile Security
- Isolated Storage Containers: Storage APIs (localStorage, IndexedDB, WebSQL) are restricted strictly to individual profile containers. Trackers inside one profile cannot read or write data to any other profile.
- Zero-Trace Ephemeral Wipes: When you close an ephemeral Send.win profile, all temporary files, cache structures, HSTS states, and ETag records are automatically destroyed. No residual artifacts remain on your storage disk for evercookies to respawn from.
- Synthetic Hardware Fingerprinting: Send.win masks hardware signatures by generating custom, consistent, and realistic hardware attributes (Canvas, WebGL, AudioContext, System Fonts) for every profile, preventing trackers from correlating profiles via passive hardware profiling.
- Flexible Deployment Options: Users can deploy profiles locally using the Sendwin Browser desktop app for Windows, macOS, and Linux, or run Cloud Browser Sessions directly in the cloud without installing any local software.
- Programmable Automation API: Developers managing multi-account systems can leverage Send.win’s Automation API—available on Pro ($9.99/mo, $6.99/mo annual) and Team ($29.99/mo, $20.99/mo annual) plans—to control isolated profile instances programmatically using Selenium, Puppeteer, or Playwright.
How Send.win Helps With Supercookies Evercookies Explained
Send.win is an antidetect browser built for exactly this kind of work — every profile is a clean, isolated identity:
- Isolated profiles – unique fingerprint, separate cookies and storage per profile
- Stealth engine – canvas, WebGL, fonts, and audio spoofed at the engine level
- Desktop app + cloud sessions – native app for Windows, macOS, and Linux, or run profiles in the cloud with no install
- Built-in residential proxies – with automatic timezone, locale, and WebRTC matching
- Team features – share logged-in profiles with teammates without sharing passwords
Try the instant cloud browser demo — no install, no signup — or download the desktop app. The 30-day free trial needs no credit card, and paid plans start at $6.99/month billed annually (see pricing).
| Plan | Pricing (Annual / Monthly) | Profiles | Storage | Automation API | Seats |
|---|---|---|---|---|---|
| Pro Plan | $6.99 / mo ($9.99 billed monthly) | 150 Profiles | 5 GB Storage | Included (Selenium/Puppeteer/Playwright) | 1 Seat |
| Team Plan | $20.99 / mo ($29.99 billed monthly) | 500 Profiles | 20 GB Storage | Included (Selenium/Puppeteer/Playwright) | 16 Seats |
By compartmentalizing every web session into a sandboxed environment, Send.win ensures that evercookies have no cross-session storage vectors to exploit. For a deeper look into managing sandboxed browser instances, explore our guide on session isolation.
Actionable Steps to Detect and Eliminate Supercookies
If you want to minimize supercookie tracking on your existing setup, follow these technical mitigation steps:
Step 1: Perform Deep Cache Purges
When clearing browser history, ensure you select “Cached images and files” and “Hosted app data” in addition to standard cookies to remove ETag hashes.
Step 2: Flush HSTS and Security Caches
In Chromium-based browsers, navigate to chrome://net-internals/#hsts and manually delete domain security policies for suspected tracking domains.
Step 3: Partition Web Storage
Enable strict first-party isolation settings in your browser to restrict HTML5 storage access to the top-level origin domain.
Step 4: Isolate High-Risk Web Personas
Use Send.win to run sensitive web accounts, financial operations, and competitive research in isolated, dedicated profiles with separate proxy IPs. For additional anonymity strategies, read our guide on anonymous browsing.
Step 5: Automate Profile Cleanup
Utilize Send.win’s ephemeral mode to automatically wipe temporary files and cache structures upon closing browser windows.
Step 6: Audit Storage APIs with Developer Tools
Inspect active browser sessions via Developer Tools (Application tab) to identify unauthorized localStorage entries, IndexedDB databases, and Service Workers registered by third-party analytics domains.
Step 7: Enforce Isolated Proxy IP Assignment
Bind distinct residential or datacenter proxies to every Send.win profile. Decoupling your IP address prevents server-side tracking graphs from associating different browser profiles with a single physical network location or internet service provider node.
🏆 Send.win Verdict
Traditional cookie clearers and browser extensions cannot purge hidden evercookie vectors across Web Storage, HSTS caches, and HTTP ETags. Understanding supercookies evercookies explained reveals why Send.win’s zero-trace ephemeral profile environments are essential: they isolate storage buckets and destroy persistent tracking traces upon session closure.
Try Send.win free today — launch isolated browser profiles with a 30-day free trial (no credit card required).
Frequently Asked Questions
What is the main difference between a supercookie and a normal cookie?
Standard cookies are simple text files stored in a designated browser folder that users can easily view and delete. Supercookies exploit hidden browser storage locations (like HTML5 Web Storage, HSTS caches, and HTTP ETags) or network headers, making them much harder to detect and remove through standard browser controls.
How does an evercookie respawn after being deleted?
An evercookie writes its tracking identifier to multiple storage vectors simultaneously (cookies, localStorage, IndexedDB, ETag cache, HSTS cache). When you delete standard cookies, the evercookie script checks the remaining hidden storage locations on your next visit, retrieves the saved ID, and rewrites it back into your cookie storage.
Does private browsing or Incognito mode block supercookies?
Incognito mode prevents standard cookies and history from saving permanently after you close the window. However, within an active Incognito session, supercookies and hardware fingerprinting can still track your activity across tabs. Furthermore, some HSTS and cache-based supercookies can leak state between normal and private windows in poorly configured browsers.
Can antivirus software detect and remove evercookies?
Most antivirus software cannot detect evercookies because evercookies use legitimate browser APIs and HTTP headers rather than malicious executable code. Specialized privacy tools or isolated browser environments like Send.win are required to stop evercookie persistence.
Why don’t web browsers just disable ETags and HSTS?
Browsers cannot disable ETags or HSTS without degrading web performance and security. ETags reduce server bandwidth by allowing browsers to cache images and scripts, while HSTS protects users from dangerous SSL-stripping cyberattacks. Disabling them would make the web slower and far less secure.
What is HSTS supercookie tracking?
HSTS supercookie tracking encodes a binary tracking number by setting HTTPS security policies across a combination of subdomains. When a browser visits the tracking site, the browser’s automatic HTTP-to-HTTPS upgrade behavior reveals which subdomains were previously stored, decoding the user’s tracking ID without using traditional cookies.
How does Send.win protect against evercookies?
Send.win isolates browser profiles into sandboxed containers with independent cookies, HTML5 storage, and cache systems. In ephemeral mode, closing a profile destroys all storage structures, HSTS states, and ETag caches completely, ensuring evercookies have no surviving vectors from which to respawn.
Can I automate Send.win profiles with automation frameworks like Puppeteer?
Yes. Send.win includes an Automation API on both Pro ($9.99/mo) and Team ($29.99/mo) plans. Developers can connect Selenium, Puppeteer, or Playwright to automate isolated browser profiles without triggering fingerprint detection or supercookie tracking.