Ad Injector Dulls Chrome’s Luster

Netskope Advanced Threat Protection recently detected ads being injected into web traffic of multiple users. The source of these ad injections is a Javascript ad injector commonly known as Lnkr. In this blog post, we will provide an overview of Lnkr, a list of all the URLs we have uncovered that are hosting the Lnkr Javascript, and identify the source of the injections.

Lnkr Ad Injector

Lnkr is family of adware that injects ads into websites that a user visits. Lnkr has previously been found in browser plugins, standalone Windows adware applications, rootkits, Android packages, and even directly included on some websites. The most common active distribution vector is browser extensions that inject ads into all of the user’s web traffic.

Discovery

The Lnkr campaign began in early September and continues through the time of this writing. We were first alerted to the campaign when our Outbreak Detection System found ads being injected into websites that do not commonly serve ads, including online banking portals and internal websites.

Disclosures

On 5 December 2019, we notified Amazon of the Lnkr scripts hosted in AWS S3.

Customer Alert

Customers using Netskope’s Next-Gen SWG are protected from the injected ads. Affected customers will see alerts in their Netskope Skope IT console that contain URLs like the following:

http://nextextlink[.]com/metric/?mid=&wid=51824&sid=&tid=7501&rid=LOADED&custom1=[redacted]&custom2=[redacted]&t=1569547496304

The URL contains one of the domains listed at the end of this post and tracking information from the website where the ads are injected.

Netskope users will be able to recognize Lnkr infections, because an infected user will typically have a very high volume of alerts for one of the URLs listed at the end of this post.

Mitigation

Because the most common active distribution vector is Chrome extensions, we recommend removing all Chrome extensions on an affected system and doing a fresh install of Chrome. If the infection persists, the system might be infected with other malware that bundles the adware. 

Lnkr Analysis

The Lnkr Javascript is either directly included by a website or injected by the adware, as seen in Figure 1.

Figure 1: Website with Lnkr script

Though analytics.js, lknr5.js and lnkr30_nt.js are related to Lnkr, the main activity is carried out by the ‘analytics.js’ file in the snippet shown in Figure 1.

Upon visiting the webpage, the following actions take place:

• A jsonp request is launched in the format /optout/get?jsonp=__twb_cb_808309138&key=1940453547ec8d17dd&t=1573556950225 as shown in Figure 2.

Figure 2: jsonp request by Lnkr

• Several blank tracking GIFs are loaded with the callback arguments LAUNCHED, LOADED, BEFORE_OPTOUT, FINISHED in the format /metric/?mid=&wid=51807&sid=&tid=6464&rid=<status>&custom1=[redacted]&t=1573556950222 as shown in Figure 3.

Figure 3: Tracking GIFs

• Another jsonp request is launched in the format /optout/set/lat?jsonp=__twb_cb_274636224&key=1940453547ec8d17dd&cv=1573556950&t=1573556950743
• Based on the country in the jsonp response, as shown in Figure 2, the ScriptsToLoad function launches the associated Lnkr urls in the configuration as shown in Figure 4.

Figure 4: Scripts To load by Lnkr

• The ad injection occurs. Popular services like Google or Reddit are allow listed from injection, presumably to stay under the radar and avoid detection.
• Several blank tracking GIFs are again loaded with the callback arguments OPTOUT_RESPONSE_OK, MNTZ_INJECT, MNTZ_LOADED in the format /metric/?mid=cd1d2&wid=51807&sid=&tid=6464&rid=<status>&t=1573556950746 as shown in Figure 5.

Figure 5: Tracking GIFs

The script also contains functionality to redirect the searches to Adware-related websites. An excerpt of the redirect on the typo of the word, ‘booking’ is shown in Figure 7.

Figure 6:  Search redirect

The javascript also contains a webpage that its development is supported by optional advertisements as shown in Figure 7.

Figure 7: Optional Advertisement message

Though these settings are present in the script, they were not enabled or displayed in the webpage.

The earliest evidence of Lnkr dates back to 2016 in a Softpedia news article that describes an Imgur browser extension injecting ads using the URLs shown in Figure 8.

Figure 8: Lnkr Urls related to Imgur uploader

In 2018, Lnkr appeared again, this time in Firefox add-ons that were masquerading as official Firefox updates. A Bugzilla report lists 70 affected add-ons that have been taken down and a MalwareBytes article lists both Chrome and Firefox extensions. BitDefender also reported a rootkit distributing the adware around the same time. Figure 9 shows a screenshot of the website used to trick users into installing the extensions, ublockerext[.]com/ff/.

Figure 9: ublockerext[.]com/ff/ website

At the time of this writing, the website was live but the links to the add-ons hosted there could not be installed, displaying a message that they were corrupt, as shown in Figure 10.

Figure 10: ublockerext[.]com addon installation message

Static analysis of the add-ons identified Lnkr code present in the background.js and content.js files. One of the addons we inspected contained a currently active Lnkr script hosted on Amazon S3 as shown in Figure 11.

Figure 11: Lnkr script hosted on Amazon S3

Though several Lnkr associated browser extensions have been removed from the respective app stores, the associated URLs hosting the scripts remain active. A majority of Lnkr domains were using Let’s Encrypt certificates. Based on our observations we believe this to be an actively ongoing campaign. 

Conclusion

The Lnkr campaign we detected is still ongoing. To shield yourself from any possible ad injection, we recommend you block the domains listed at the end of this post. We also recommend you audit the extensions installed in your Chrome browser at chrome://extensions and remove any affected extensions. As this is still an ongoing campaign, we will continue to monitor and report on any new developments. Netskope customers using our Next-Gen SWG are already protected against the injection.

Indicators of compromise

URLS currently serving Lnkr 

1018433480[.]rsc[.]cdn77[.]org

1480876790[.]rsc[.]cdn77[.]org

appmakedev[.]xyz

appslinker[.]net

blickkeily[.]com

blinkjork[.]com

browlinkdev[.]xyz

captiontxt[.]com

cilkonlay[.]com

clonyjohn[.]com

closemike[.]com

colextidapp[.]com

countsource[.]cool

cozytech[.]biz

dataprovider[.]website

datapro[.]website

devappstor[.]com

dimagesrc[.]com

dismagic[.]com

domclickext[.]xyz

dowlextff[.]com

evenffext[.]com

extnotecat[.]com

flexylincks[.]com

goldapps[.]org

groproext[.]com

higedev[.]cool

jonyclose[.]com

jonysource[.]com

jsfuel[.]com

killssource[.]com

larickway[.]com

leaderdigital[.]org

lifebounce[.]net

linkangood[.]com

lonelyfix[.]com

longsrc[.]com

loudsjack[.]com

lowffdompro[.]com

magictraps[.]com

masyclick[.]com

mikkymax[.]com

miniklixk[.]org

minisrclink[.]cool

mirextpro[.]com

netstats[.]space

nextextlink[.]com

oilcloze[.]com

onlinekey[.]biz

pagevalidation[.]space

pingclock[.]net

plankjock[.]com

polinaryapp[.]com

prilapptime[.]com

programdiag[.]com

promclickapp[.]biz

protesidenext[.]com

proudflex[.]org

proxdevcool[.]com

rasenalong[.]com

renetteapp[.]com

serenityart[.]biz

shortyclubs[.]com

skillapp[.]net

slickfluide[.]com

sourcebig[.]cool

srctestlink[.]com

statcounter[.]biz

sysfileff[.]com

vibeclimate[.]com

www[.]die-rheinische-affaire[.]de

thrillingos[.]herokuapp[.]com/mozilla/best-ytb-down/content/analytics

s3[.]amazonaws[.]com/cashe-js

s3[.]amazonaws[.]com/js-cache

s3[.]amazonaws[.]com/js-static

s3[.]amazonaws[.]com/jscache

s3[.]amazonaws[.]com/jsfile

s3.amazonaws.com/jscriptcdn