Chromium Blog: Writing Extensions More Securely

Chromium Blog

News and developments from the open source browser project

Writing Extensions More Securely

Friday, July 22, 2011

built-in protectionsmanyarticlesMinimize your permissions (*/*) permissions for hosts if you only need to access a couple, and don’t copy and paste your manifest from example code blindly. Review your manifest to make sure you’re only declaring what you need. This applies to permissions like tabs, history, cookies, etc. in addition to host permissions. For example, if all you’re using is chrome.tabs.create, you don’t actually need the tabs permission.

Use content_security_policy in your manifest

Starting in Chrome 14, we will begin supporting Content Security Policy in extensions via the content_security_policy manifest field. This allows you to control where scripts can be executed, and it can be used to help reduce your exposure to cross-site scripting vulnerabilities. For example, to specify that your extension loads resources only from its own package, use the following policy:

"content_security_policy": "default-src 'self'"

If you need to include scripts from specific hosts, you can add those hosts to this property.

Don’t use <script src> with an HTTP URL

When you include javascript into your pages using an HTTP URL, you’re opening your extension up to man-in-the-middle (MITM) attacks. When you do so in a content script, you have a similar effect on the pages you’re injected into. An attacker on the same network as one of your users could replace the contents of the script with content that they control. If that happens, they could do anything that your page can do.

If you need to fetch a remote script, always use HTTPS from a trusted source.

Don’t use eval()

The eval() function is very powerful and you should refrain from using it unless absolutely necessary. Where did the code come from that you passed into eval()? If it came from an HTTP URL, you’re vulnerable to the same issue as the previously mentioned <script> tag. If any of the content that you passed into eval() is based on content from a random web page the user visits, you’re vulnerable to escaping bugs. For example, let’s say that you have some code that looks like this:

function displayAddress(address) { // address was detected and grabbed from the current page
eval("alert('" + address + "')");
}

If it turned out that you had a bug in your parsing code, the address might wind up looking something like this:

'); dosomethingevil();

There’s almost always a better alternative to using eval(). For example, you can use JSON.parse if you want to parse JSON (with the added benefit that it runs faster). It’s worth the extra effort to use alternatives like this.

Don’t use innerHTML or document.write()

It’s really tempting to use innerHTML because it’s much simpler to generate markup dynamically than to create DOM nodes one at a time. However, this again sets you up for bugs in escaping your content. For example:

function displayAddress(address) { // address was detected and grabbed from the current page
myDiv.innerHTML = "<b>" + address + "</b>");
}

This would allow an attacker to make an address like the following and once again run some script in your page:

<script>dosomethingevil();</script>

Instead of innerHTML, you can manually create DOM nodes and use innerText to insert dynamic content.

Beware external content

In general, if you’re generating dynamic content based on data from outside of your extension (such as something you fetched from the network, something you parsed from a page, or a message you received from another extension, etc.), you should be extremely careful about how you use it. If you use this data to generate content within your extension, you might be opening your users up to increased risk.

You can also read a few more examples of the issues discussed here in our extension docs. We hope these recommendations help you create better and safer extensions for everyone.

Posted by Erik Kay, Software Engineer

Google

Labels

$200K 1
10th birthday 4
abusive ads 1
abusive notifications 2
accessibility 3
ad blockers 1
ad blocking 2
advanced capabilities 1
android 2
anti abuse 1
anti-deception 1
background periodic sync 1
badging 1
benchmarks 1
beta 79
better ads standards 1
billing 1
birthday 4
blink 2
browser 2
browser interoperability 1
bundles 1
capabilities 6
capable web 1
cds 1
cds18 2
cds2018 1
chrome 34
chrome 81 1
chrome 83 2
chrome 84 2
chrome ads 1
chrome apps 5
Chrome dev 1
chrome dev summit 1
chrome dev summit 2018 1
chrome dev summit 2019 1
chrome developer 1
Chrome Developer Center 1
chrome developer summit 1
chrome devtools 1
Chrome extension 1
chrome extensions 3
Chrome Frame 1
Chrome lite 1
Chrome on Android 2
chrome on ios 1
Chrome on Mac 1
Chrome OS 1
chrome privacy 4
chrome releases 1
chrome security 7
chrome web store 32
chromedevtools 1
chromeframe 3
chromeos 4
chromeos.dev 1
chromium 6
cloud print 1
coalition 1
coalition for better ads 1
contact picker 1
content indexing 1
cookies 1
core web vitals 2
csrf 1
css 1
cumulative layout shift 1
custom tabs 1
dart 8
dashboard 1
Data Saver 3
Data saver desktop extension 1
day 2 1
deceptive installation 1
declarative net request api 1
design 2
developer dashboard 1
Developer Program Policy 2
developer website 1
devtools 13
digital event 1
discoverability 1
DNS-over-HTTPS 4
DoH 4
emoji 1
emscriptem 1
enterprise 1
extensions 27
Fast badging 1
faster web 1
features 1
feedback 2
field data 1
first input delay 1
Follow 1
fonts 1
form controls 1
frameworks 1
fugu 2
fund 1
funding 1
gdd 1
google earth 1
google event 1
google io 2019 1
google web developer 1
googlechrome 12
harmful ads 1
html5 11
HTTP/3 1
HTTPS 2
iframes 1
images 1
incognito 1
insecure forms 1
intent to explain 1
ios 1
ios Chrome 1
jank 1
javascript 5
lab data 1
labelling 1
largest contentful paint 1
launch 1
lazy-loading 1
lighthouse 2
linux 2
Lite Mode 2
Lite pages 1
loading interventions 1
loading optimizations 1
long-tail 1
mac 1
manifest v3 2
metrics 2
microsoft edge 1
mixed forms 1
mobile 2
na 1
native client 8
native file system 1
New Features 5
notifications 1
octane 1
open web 4
origin trials 2
pagespeed insights 1
pagespeedinsights 1
passwords 1
payment handler 1
payment request 1
payments 2
performance 16
performance tools 1
permission UI 1
permissions 1
play store 1
portals 3
prefetching 1
privacy 2
privacy sandbox 4
private prefetch proxy 1
profile guided optimization 1
progressive web apps 2
Project Strobe 1
protection 1
pwa 1
QUIC 1
quieter permissions 1
releases 3
removals 1
rlz 1
safe browsing 2
Secure DNS 2
security 35
site isolation 1
slow loading 1
sms receiver 1
spam policy 1
spdy 2
spectre 1
speed 3
ssl 2
store listing 1
strobe 2
subscription pages 1
suspicious site reporter extension 1
TCP 1
the fast and the curious 14
TLS 1
tools 1
tracing 1
transparency 1
trusted web activities 1
twa 2
user agent string 1
user data policy 1
v8 6
video 2
wasm 1
web 1
web apps 1
web assembly 1
web developers 1
web intents 1
web packaging 1
web payments 1
web platform 1
web request api 1
web vitals 1
web.dev 1
web.dev live 1
webapi 1
webassembly 1
webaudio 3
webgl 7
webkit 5
WebM 1
webmaster 1
webp 5
webrtc 6
websockets 5
webtiming 1
writable-files 1
yerba beuna center for the arts 1

Feed

Give us feedback in our Product Forums.

Google
Privacy
Terms

Chromium Blog

Writing Extensions More Securely

Labels

Archive

Feed