Why does exposing the Last-Modified timestamp of extension files create tracking risk?

Because the timestamp reflects when an extension was installed or updated, and that moment is highly specific to a device and browser profile. If a website can read it, the value can act as a stable browser identifier across visits. In practice, one exposed file can provide a stronger fingerprint than many weaker browser attributes combined.

Why a Last-Modified Timestamp Becomes a Tracking Signal

The risk is not the file metadata by itself, but what it reveals about the browser profile behind it. An extension file that was installed or updated at a precise moment can behave like a durable, high-entropy attribute, especially when it is stable across page visits and uncommon across the user population.

That makes the timestamp useful for fingerprinting because it can narrow a browser to a small set of users without relying on cookies, local storage, or login state. When a site can probe extension files, the value can persist long enough to support cross-visit correlation and reduce the effectiveness of ordinary privacy controls.

For the same reason, this is different from a generic browser attribute such as screen size or timezone. The update moment is often more specific, more irregular, and less shared across users, which gives it stronger identifying value when combined with other signals.

Why the Signal Is More Stable Than It First Appears

Extension installation and update timing tends to be driven by user behaviour, browser policy, extension release cadence, and device-specific usage history. That combination creates a pattern that is often surprisingly durable. Even if the extension itself is common, the exact timestamp can still separate one profile from another because people do not install or update extensions on the same schedule.

This matters most when the timestamp is exposed in a way that can be read repeatedly. A one-time observation is useful, but repeated observations let an adversary test whether the browser profile is the same one that returned earlier, even if IP address, user agent, or session state changes.

In practice, the tracking value increases when the signal is paired with other low-noise attributes. A single exposed timestamp may not uniquely identify every browser, but it can materially strengthen a fingerprint that is already built from several weaker attributes.

Risk and Threat Considerations

Exposing a Last-Modified value creates privacy risk because it can act as a stable marker for a specific browser profile, even when the user is not logged in. The exposure is most concerning when the same extension file can be probed across visits, because that turns a harmless-looking timestamp into a durable correlation point.

Failure mechanism: a website reads the file metadata, converts the update or install moment into a high-specificity attribute, and combines it with other browser signals to recognise the same profile later.

Impact: the user becomes easier to track across sessions and possibly across sites, which weakens anonymity, reduces the value of anti-fingerprinting measures, and can enable profiling without explicit consent.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

CIS Controls v8, NIST CSF 2.0 and NIST SP 800-63 set the governance and control requirements practitioners need to meet.

Framework	Control / Reference	Relevance
CIS Controls v8	CIS 5 — Account Management	Browser-readable metadata can aid persistent user profiling and tracking.
Recommendation — Minimise exposure of identifying metadata and review browser-facing artefacts for unnecessary persistence.
NIST CSF 2.0	PR.AA — Identity Management, Authentication, and Access Control	Tracking risk rises when a browser attribute can uniquely re-identify a user profile.
	PR.DS — Data Security	The timestamp is sensitive metadata because it can reveal user-specific history.
	GV.RM — Risk Management Strategy	Browser fingerprinting is a privacy and trust risk that needs explicit acceptance criteria.
Recommendation — Limit exposure of attributes that can function as persistent identifiers. Protect metadata that can be correlated into a tracking signal. Classify fingerprintable metadata as a privacy risk and set handling rules for it.
NIST SP 800-63	5.2.5 — Session Termination	Persistent browser signals can undermine session and user separation expectations.
Recommendation — Avoid relying on browser state that enables long-lived correlation across visits.

Practitioner Guidance

What to prioritise: treat any browser-exposed file metadata as a potential fingerprinting surface, not just a convenience detail. The practical question is whether the value is both accessible to web content and stable enough to survive routine browsing behaviour.

What to verify: check whether the timestamp is observable without elevated privileges, whether it changes predictably on update, and whether it is identical across multiple installs or profiles. If the answer is no, it is a weak identifier; if yes, it should be treated as tracking-relevant.

Common mistake: assuming that only cookies or explicit identifiers create tracking risk. Metadata that looks operational can still be more distinctive than expected, especially when it reflects user-specific history rather than device-wide state.

Practitioner takeaway: the main control objective is to reduce exposure of high-specificity, browser-readable metadata, because anything that behaves like a persistent profile marker can be repurposed for fingerprinting.