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OYSTERFRESH is a JavaScript malware loader used in a phishing campaign attributed by CERT-UA to the Belarus-linked Ghostwriter threat actor, also tracked as UAC-0057 and UNC1151, targeting Ukrainian government organizations since spring 2026. Delivery observed in the reporting chain involved phishing emails sent from compromised accounts, themed around the legitimate Prometheus Ukrainian online learning platform, with PDF attachments containing links that downloaded a ZIP archive carrying the OYSTERFRESH JavaScript file. OYSTERFRESH displays a decoy document to distract the victim, writes an obfuscated and encoded payload named OYSTERBLUES into the Windows Registry, and downloads and launches the OYSTERSHUCK component, which decodes OYSTERBLUES. Reported decoding methods used by OYSTERSHUCK include string reversal, ROT13 transformation, and URL decoding. The follow-on OYSTERBLUES payload profiles the infected host by collecting the computer name, username, operating system version, last boot time, and running processes, then sends that information to command-and-control infrastructure via HTTP POST. It can receive additional JavaScript from the C2 and execute it via eval(), and CERT-UA assessed that the infection chain ultimately delivers Cobalt Strike. High-confidence host and network indicators mentioned in the reporting include file names certificate.js (OYSTERFRESH), amplifier.js (OYSTERSHUCK), Oyster.js (OYSTERBLUES), EdgeSystemConfig.dll (CSBEACON/Cobalt Strike Beacon), and EdgeTaskMachine.js; the registry key HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\Blue'Oyster'; Run keys MicrosoftEdgeUpdate and WindowsEdgeStartup; the scheduled task MicrosoftEdgeUpdateTaskMachine; the URL hXXps://a3ufz.xsjdsb[.]icu/wp-json/prometheus-plus/certs-at-home/downloads; and domains mickeymousegamesdealer.alexavegas[.]icu, productionsamplesoftheyear.cgdirector[.]icu, and advancedaisolutionsforeveryone.a1si[.]icu. CERT-UA also noted Ghostwriter infrastructure is commonly hidden behind Cloudflare and frequently uses .icu domains.
2 distinct threat actors attributed by public researchers. Open in Mallory to see the full evidence chain and overlapping campaigns.
The mentioned JS file is classified as OYSTERFRESH, which provides display of a decoy document, entry into the operating system registry in an obfuscated and encoded form of the OYSTERBLUES software tool, as well as loading and launching the OYSTERSHUCK component.
The phishing email contained a PDF attachment with a malicious link that downloaded a ZIP archive carrying malware identified as OysterFresh.
10 distinct techniques documented for this family, organized by ATT&CK tactic.
This activity, which began in the spring of 2026, involves sending phishing emails to government entities using compromised accounts.
Ghostwriter targeted Ukrainian government agencies with phishing emails delivering malware and Cobalt Strike payloads... phishing emails sent from already-compromised accounts — making the sender look legitimate — carrying PDF attachments.
Inside the PDF is a link that, when clicked, downloads a ZIP archive containing a JavaScript file.
The mentioned JS file is classified as OYSTERFRESH... OYSTERBLUES... waits for instructions, which arrive as JavaScript code executed on the fly using the eval() function.
Typically, the email contains a PDF attachment with a link that, when clicked, leads to the download of a ZIP archive containing a JavaScript file.
Inside the PDF is a link that, when clicked, downloads a ZIP archive containing a JavaScript file.
This file, dubbed OYSTERFRESH, displays a decoy document while stealthily writing an obfuscated payload, OYSTERBLUES, to the Windows Registry.
OYSTERFRESH... provides display of a decoy document, entry into the operating system registry in an obfuscated and encoded form of the OYSTERBLUES software tool.
OYSTERFRESH... provides display of a decoy document, entry into the operating system registry in an obfuscated and encoded form of the OYSTERBLUES software tool.
The JavaScript file, dubbed OYSTERFRESH, is designed to display a decoy document as a distraction mechanism...
This file, dubbed OYSTERFRESH, displays a decoy document while stealthily writing an obfuscated payload, OYSTERBLUES, to the Windows Registry.
...as well as downloading and launching OYSTERSHUCK, which is responsible for decoding OYSTERBLUES.
67 indicators attributed across vendor reports, sandbox runs, and researcher write-ups. Values are masked here and available in full in Mallory.
5 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.