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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Yes. A tracking pixel can reveal that an email image was requested, and the 2025 USENIX Security researchers found that differences among some tested email services’ image-loading and phishing-report workflows could expose signals useful for evading inspection. Their experiments do not show that every provider is vulnerable today—or that every tracking pixel is malicious.
What an email tracking pixel can reveal
A tracking pixel is usually a tiny remote image embedded in an HTML email. When a mail client or an email service requests that image from its host, the request can tell the host that the image was loaded and when. The image URL may also contain an identifier associated with a recipient or message.
The European Data Protection Board’s October 7, 2024 Guidelines 2/2023 on Technical Scope of Art. 5(3) of the ePrivacy Directive defines a tracking pixel as a hyperlink to a resource, usually an image, embedded in content such as a website or email. CNIL’s April 14, 2026 recommendation describes an invisible one-pixel image whose identifier can indicate that a person read an email or visited a page.
A request can also expose technical information. The UK Information Commissioner’s Office says pixels can record the time, location, and operating system of the device used to read an email. Italy’s Garante discusses identifiers and request details such as IP address, user ID, message ID, delivery ID, and timestamp. What is visible depends on the implementation and the systems handling the request.
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A pixel does not, by itself, steal a password or infect a device. It is a way to register a remote request. Nor does every request prove that a person deliberately opened or read a message: email services may fetch images for their own processing.
How image loading can expose email-security behavior
Email services may handle a message through several systems. A service can prefetch remote content, route images through a proxy, or inspect a message after a user reports it as phishing. Those processes can make requests to remote content at different times or in distinguishable ways.
Chand, Nikiforakis, and Vadrevu’s 2025 study, “Doubly Dangerous: Evading Phishing Reporting Systems by Leveraging Email Tracking Techniques,” examined whether those differences could reveal activity in security workflows. The authors found distinguishable signals in the tested services’ prefetching, proxying, and phishing-report inspection. In their experiments, those signals could help distinguish ordinary message interaction from security-system activity.
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That distinction matters because a phishing inspection system may visit or examine content reported by a user. If the content can infer that inspection is happening, its behavior could differ for a security crawler and a human recipient. The researchers demonstrated this possibility in a controlled simulation; they did not establish how frequently criminals use the technique in real campaigns.
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The paper appeared in the proceedings of the 34th USENIX Security Symposium in August 2025. Its authors tested a defined set of popular email services and reporting workflows. The figures below describe those experiments, not all email platforms or current provider behavior.
| Measure | What the researchers reported |
|---|---|
| Email-open tracking | Seven of the eight popular email services tested were susceptible to email-open tracking through the researchers’ expanded tracking-vector set under default settings. This is a result for that sample and configuration, not a finding about every service. |
| Phishing-report workflows | The separate reporting-system experiment covered Gmail, Outlook, Proton Mail, and Tuta Mail, which offered dedicated phishing-report buttons in the tested configuration. The researchers identified distinguishable network behavior in reporting and inspection subsystems. |
| Population represented by selected services | The paper estimated that the selected services represented more than 2 billion users, based on user counts it cited. That estimate is not a count of people individually tested or shown to be vulnerable. |
| Systematic measurement | The researchers describe measuring thousands of emails over 44 days. This period and volume concern their systematic experiment. |
| Controlled evasion simulation | In a later end-to-end simulation, the researchers’ smart evasive sites received 275 crawler visits and were not blocked during the experiment; baseline sites received 114 crawler visits and were blocked. These are experimental results, not estimates of real-world campaign prevalence. |
The authors say they disclosed their results to affected providers and that the disclosures led to remedial changes and a vulnerability reward. The paper and its official publication record do not identify the exact fix for each provider or establish whether each mitigation remains in place today.
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What this means for email users and security teams
The study concerns a side channel: information about the behavior of systems handling an email, rather than a direct compromise of a recipient’s device. Its practical security concern is that a difference in how a message is processed may help evasive phishing infrastructure behave differently when examined.
Users should not treat an image request as proof that they personally opened an email, since prefetching and other service processes can load remote content. Security teams should likewise distinguish a human interaction from automated image retrieval when interpreting tracking signals.
Blocking remote images can reduce ordinary requests to tracking hosts, but it is not a guarantee against every tracking vector or against the service-side behaviors examined by the researchers. It can also prevent legitimate images from displaying until they are allowed. User privacy settings and provider anti-phishing safeguards address different layers of the problem.
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Mitigations and their trade-offs
The researchers’ proposals focus mainly on email-service operators, because the reported weakness arose from observable differences between service subsystems. The table distinguishes those proposals from controls an individual user may be able to apply.
| Where the control sits | Possible control | What it addresses and the trade-off | Evidence status |
|---|---|---|---|
| Email provider | Reduce or homogenize observable differences among prefetching, opening, proxying, and report-inspection requests. | Makes it harder to infer which subsystem generated a request. The study presents this as a service-side defensive direction. | Researchers’ recommendation; the sources do not verify current provider-by-provider deployment. |
| Email provider | Consider suppressing or caching remote objects while handling a phishing report, and process reports promptly. | Can limit signals from report handling. Suppressing objects may remove image content that other systems use; caching requires storage. | Researchers’ recommendations and trade-offs, not a confirmed description of current provider controls. |
| Mail client or user | Restrict automatic loading of remote images, where the client offers that setting. | Can reduce routine remote-image requests, but may hide useful email images and does not promise protection from all tracking vectors or provider-side signals. | A general privacy control; not shown by the study to defeat the tested technique in every configuration. |
| Email sender | Use tracking only for a defined purpose and handle identifiers and consent according to the applicable rules. | Addresses privacy obligations and expectations, rather than directly standardizing provider report-inspection behavior. | Requirements depend on jurisdiction, purpose, and implementation. |
Privacy rules vary by jurisdiction and purpose
Tracking pixels can serve legitimate functions as well as marketing measurement. Italy’s Garante lists examples that include deliverability, spam prevention, audience measurement, personalization, phishing detection, and formatting. Whether a particular use is permitted depends on its purpose, implementation, and applicable law.
In the UK, the ICO says PECR regulation 6 applies when a pixel stores information on, or accesses information stored on, a user’s device. Its guidance also describes information a pixel may record, including the time, location, and operating system associated with reading an email.
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CNIL’s final French recommendation of April 14, 2026 covers public and private organizations and relevant technical providers using email tracking pixels. It addresses who is responsible, when consent is required or an exemption applies, and how informed consent may be obtained and withdrawn. CNIL notes an exemption for individual deliverability measurement on emails related to a service requested by the recipient; that is not a general exemption for campaign tracking.
Italy’s Garante guidance of April 17, 2026 describes pixel identifiers and request information, and discusses exceptions for certain statistical measurement, authentication-security, and required service-message cases. It says prior consent is required in remaining cases outside the exceptions it describes. These are jurisdiction-specific positions, not a single global rule.
The U.S. Federal Trade Commission’s March 2023 discussion of hidden pixel tracking addresses broader privacy risks, including collection or sharing of personal and potentially sensitive information. It is relevant context for hidden tracking, but it is not a finding about the phishing-reporting weakness in the USENIX study.
What the evidence does—and does not—establish
The USENIX paper is a measured study of selected services and a simulated evasion experiment, not an audit of every mail service or a live count of attacks using this method. Its findings show why seemingly ordinary image requests can expose security-workflow signals under tested conditions. They do not establish that all providers remain vulnerable, that every image load is a human open, or that tracking pixels are inherently malicious.
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