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Rogue Authority flow diagram of QuartzLine: the signed installer opens a working PDF viewer while a hidden zenith module checks in to C2 behind Cloudflare and waits for commands or browser-profile rewrites.
Diagram: Rogue Authority, how the QuartzLine "zenith" backdoor starts and checks in, Oct. 9, 2026 (fair use).

Detection

Expel Says a Working PDF Viewer Called QuartzLine Hid a Backdoor That Stole Chrome Data and Dropped a RAT, With 38 Confirmed Incidents Across 32 Customers

The signed app, tracked by Mandiant as CLOUDHAZE, arrived through ads and redirectors, then pulled browser history, swapped in a new build and installed a second-stage RAT. A later payload called IronBark now spreads on its own. Why a blocked C2 lookup still means an infected host, and the tasks and paths to hunt.

Ava Okello, DetectionLondon6 min read

LONDON - A Windows PDF viewer that opens files, searches text and zooms like any other has been quietly working as a backdoor on corporate machines, and the security firm Expel says its analysts have now confirmed 38 incidents across 32 customers.

The app, called QuartzLine and tracked by Mandiant as CLOUDHAZE, has turned up as paperline.exe in its original build and as fielddesk.exe in a later one. It was signed with a legitimate extended validation code-signing certificate issued to Guangzhou Guoying Communication Technology Co., Ltd., which has since been revoked, Expel's James Parsons and Christine Billie wrote. They first published on Oct. 15 and updated the post on Wednesday with a second delivery chain, new indicators and more removal steps drawn from casework through Oct. 20.

"If your DNS filter or proxy blocked these domains, the device that tried to connect is still infected," the researchers wrote.

Grayware that takes orders

An independent teardown by the researcher site Rogue Authority on Oct. 9 found that QuartzLine's hidden module can run any Windows command and tamper with Chromium-based browser profiles. But every server reply Rogue Authority could decrypt said there was nothing to do, and public sandboxes scored the file as low risk or clean.

Expel says the operator does send commands in customer environments. Downloads often passed through a redirector domain, and in at least two cases began with a Google Ads click. Where search terms were visible, users were not looking for a PDF viewer at all. They searched for things like a consent form or a wallpaper, clicked an ad on a third-party site and got a working viewer with a backdoor inside.

Expel's timeline of one QuartzLine infection, from the Sept. 22 download and QuartzlineUpdateCheck task through Chrome history theft, a self-deleting carrier, a build swap and the Oct. 10 Caira RAT drop.
Screenshot: Expel, timeline of one QuartzLine infection in a customer environment, updated Oct. 21, 2026 (fair use).

In one environment, the user downloaded the installer from an Amazon Web Services S3 bucket on Sept. 22. It copied itself to the user's local application data folder, created a scheduled task named QuartzlineUpdateCheck and began checking in with westbeavers[.]com. On Sept. 29, the operator dropped chrome-history-summarizer-v0.0.1.exe, which uploaded an archive of the user's Chrome browsing history to an S3 bucket. A file named quartzline_takeover_carrier.exe landed on Oct. 5 and deleted itself within a second. On Oct. 9 a PowerShell script swapped in a new QuartzLine build, and on Oct. 10 a second script dropped caira.exe, a remote access trojan, with its own scheduled task, CairaServiceCheck. On Oct. 14 the host looked up three new command-and-control domains.

IronBark arrives on its own

In another environment, the RAT pulled down a third payload, IronBark, on the same day it landed. Expel says IronBark also has its own ad funnel: a lure site, purplepandagroup[.]com, redirects to an installer hosted in an S3 bucket and signed with a separate certificate issued to "Amir Dow," also revoked. Since Sept. 22, Expel has seen that installer at five customers, and it ran at four.

When IronBark runs, it force-closes Chrome and relaunches it with the crash-restore notice hidden. In one case, Chrome's protected stored data, where saved passwords and cookies live, was decrypted at that relaunch. Expel says it has not seen that data leave the device but treats it as exposed. Both QuartzLine and IronBark also change Chrome's default search engine and new-tab page to attacker-controlled domains. In one case, a hunt built only on QuartzLine hashes, certificates and domains missed IronBark entirely.

Expel indicator table listing QuartzLine malvertising redirectors, S3 delivery buckets and the westbeavers, healskills and motherotters command-and-control domains.
Screenshot: Expel, part of the QuartzLine and IronBark domain indicator table, updated Oct. 21, 2026 (fair use).

How the SOC caught it

The first wave reached Expel's queue on the evening of Friday, Oct. 9, when the operator pushed the second-stage RAT to infected hosts at multiple customers at once. The SOC flagged the cluster to the threat intelligence team on Oct. 13, and endpoint and network detections moved to production on Oct. 14. Many of the alerts since then have come from DNS filters and web proxies blocking the C2 domains. In at least one case, Expel could not trace the blocked traffic to any device with an endpoint detection and response agent, so the network block was the only signal anyone had.

Expel table of QuartzLine, Caira and IronBark scheduled task names and their command lines under the local application data folder.
Screenshot: Expel, QuartzLine, Caira and IronBark scheduled tasks to hunt, updated Oct. 21, 2026 (fair use).

What defenders can do

  • Map any blocked DNS or proxy request to the QuartzLine, Caira or IronBark domains back to a host, starting with unmanaged devices and anything without EDR coverage, and treat that host as infected.
  • Hunt for the scheduled tasks QuartzlineUpdateCheck, CairaServiceCheck and IronbarkUpdateCheck and any task whose name starts with HostHealthCache, matching on task name and path rather than the full command line, which can carry a per-host ID.
  • Remove the QuartzLine, Quartz, Caira, IronBark, HostHealthCache and DevicePerfCache folders under the user's local application data and delete the tasks, or reimage. Expel has seen the task left behind and still running after the file was deleted.
  • Search proxy logs for requests to paths under /quarry/, such as /quarry/geode and /quarry/talc, and for desktop traffic that sends no User-Agent header, which the Rogue Authority teardown flagged.
  • Reset credentials and sign out active sessions for anyone who used an affected device, and hunt IronBark indicators separately from QuartzLine.

Sources:


Ava Okello covers detection engineering, EDR telemetry and SOC hunting for SOCtember from London.

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