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Fwd: [discuss] Batteries & tracking

From: Joseph Lorenzo Hall <joe@cdt.org>
Date: Mon, 3 Aug 2015 12:41:13 -0400
Message-ID: <CABtrr-U5OpUL-Ot9F-dDZhFDkmidQWa1kTmqwq+7t5aZZcZXPw@mail.gmail.com>
To: "public-privacy (W3C mailing list)" <public-privacy@w3.org>
http://www.theguardian.com/technology/2015/aug/03/privacy-smartphones-battery-life

*A little-known feature of the HTML5 specification means that websites can
find out how much battery power a visitor has left on their laptop or
smartphone – and now, security researchers have warned that that
information can be used to track browsers online.*

*The battery status API is currently supported in the Firefox, Opera and
Chrome browsers, and was introduced by the World Wide Web Consortium (W3C,
the organisation that oversees the development of the web’s standards) in
2012, with the aim of helping websites conserve users’ energy. Ideally, a
website or web-app can notice when the visitor has little battery power
left, and switch to a low-power mode by disabling extraneous features to
eke out the most usage.*

*W3C’s specification explicitly frees sites from needing to ask user
permission to discover they remaining battery life, arguing that
<http://www.w3.org/TR/2012/CR-battery-status-20120508/> “the information
disclosed has minimal impact on privacy or fingerprinting, and therefore is
exposed without permission grants”. But in a new paper from four French and
Belgian security researchers <http://eprint.iacr.org/2015/616.pdf>, that
assertion is questioned.*

*The researchers point out that the information a website receives is
surprisingly specific, containing the estimated time in seconds that the
battery will take to fully discharge, as well the remaining battery
capacity expressed as a percentage. Those two numbers, taken together, can
be in any one of around 14 million combinations, meaning that they operate
as a potential ID number. What’s more, those values only update around
every 30 seconds, however, meaning that for half a minute, the battery
status API can be used to identify users across websites.*

*For instance, if a user visits a website in Chrome’s private browsing mode
using a VPN, the website should not be able to link them to a subsequent
visit with private browsing and the VPN off. But the researchers warn that
that may no longer work: “Users who try to revisit a website with a new
identity may use browsers’ private mode or clear cookies and other client
side identifiers. When consecutive visits are made within a short interval,
the website can link users’ new and old identities by exploiting battery
level and charge/discharge times. The website can then reinstantiate users’
cookies and other client side identifiers, a method known as respawning.”*

*Worse still, on some platforms, the researchers found that it is possible
to determine the maximum battery capacity of the device with enough
queries, creating a semi-permanent metric to compare devices.*


-- 
Joseph Lorenzo Hall
Chief Technologist
Center for Democracy & Technology
1634 I ST NW STE 1100
Washington DC 20006-4011
(p) 202-407-8825
(f) 202-637-0968
joe@cdt.org
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Received on Monday, 3 August 2015 16:42:04 UTC

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