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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchIn a 2020–2021 invoice-themed phishing campaign, attackers used Morse code as one layer of obfuscation to conceal parts of malicious HTML and JavaScript. The goal was credential theft: victims opened an attachment made to look like an Excel file and were shown a fake Office sign-in page. The reports document a historical campaign, not evidence that the same operation is active today.
How the phishing lure worked
The campaign used invoice- or payment-themed emails with HTML attachments named to resemble spreadsheets, including variants such as company_invoice_number._xlsx.hTML. Opening an attachment rendered a fake spreadsheet or blurred document. An Office-style sign-in prompt claimed the user’s session had timed out and asked for credentials.
The page could be tailored with the recipient’s email address and company logo. Credentials entered into its form were sent to an attacker-controlled phishing kit. The spreadsheet-like appearance and familiar sign-in prompt were social-engineering elements; Morse code was part of how the underlying page and scripts were concealed.
Where Morse code fit in the encoding chain
The HTML attachment was assembled from multiple segments. Some script components were embedded in the file, while others were loaded from external locations through encoded links. Microsoft described a sequence of changing obfuscation methods that included Escape encoding, Base64, ASCII, character-code encoding, and Morse code.
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February 2021 wave
In one February 2021 variant, JavaScript links were encoded first in ASCII and then in Morse code. The added layers made the links less obvious to simple inspection.
May 2021 wave
In a May 2021 variant, a phishing-kit domain was Escape-encoded and the HTML was then encoded in Morse. BleepingComputer described a sample decoder that converted the Morse text to hexadecimal, then reconstructed JavaScript tags that were injected into the page.
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These examples show why the tactic should not be reduced to “Morse code phishing.” Morse was one concealment method in an evolving chain; the intended result remained a convincing sign-in page that captured credentials.
What the campaign timeline establishes
Microsoft Security reported ten observed iterations between July 2020 and July 2021. Across its year-long investigation, Microsoft said the attackers changed obfuscation and encryption mechanisms every 37 days on average. That figure describes this campaign’s observed changes; it is not a general rate for phishing operations. Morse code appeared in the February and May 2021 waves.
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The reports are historical case studies. They do not establish that this specific campaign remains active, how prevalent similar techniques are now, or how effective any current security product is in independent testing.
Why one indicator is not enough
Searching only for Morse-like text or a single attachment filename would miss other variants. The campaign combined several parts that defenders need to consider together:
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- The email: an invoice or payment theme designed to prompt action.
- The attachment: an HTML file named to resemble an Excel spreadsheet.
- The concealed code: embedded and externally loaded components encoded in changing ways.
- The destination page: a tailored fake document and sign-in prompt intended to collect credentials.
A useful defensive approach therefore looks beyond a fixed string or file pattern and considers what an attachment does after its contents are decoded or rendered.
Practical ways to reduce exposure
Microsoft’s recommendations for reducing risk include the following. They are vendor guidance, not a guarantee that any single control will stop every attack.
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- Restrict unnecessary HTML attachments. Where business needs allow, remove or limit incoming
.htmland.htmattachments. - Review mail-flow exceptions. Check allow rules and overrides that might bypass normal attachment or link protections.
- Enable attachment and link protections. Microsoft describes using sandboxing and dynamic analysis to inspect URLs, decoded scripts, and page behavior as part of its layered detection approach.
- Use multifactor authentication. Apply MFA, especially to privileged and high-value accounts, so a stolen password alone is less likely to provide access.
- Avoid password reuse. Unique passwords limit the damage if credentials entered on a phishing page are exposed.
- Train people to recognize suspicious requests. Invoice themes, spreadsheet-like filenames, and unexpected sign-in prompts deserve scrutiny, even when a message includes familiar branding.
- Use browser and network protections. Protections against malicious sites and domains can help reduce the chance that users reach a phishing destination.
Microsoft’s account of its own detection capabilities is a product description, not a neutral comparison of security vendors or an independently measured effectiveness result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to do if someone opened the attachment
If a recipient opened a suspicious attachment or entered a password, preserve the original email and attachment for security analysis. Determine whether the file was opened and whether credentials were submitted, then follow your organization’s established credential-compromise response process. The documented attack path makes those two questions especially important: opening the file exposed the recipient to the fake page, while submitting credentials could expose the account.
Sources and attribution
Microsoft Security’s August 12, 2021 report describes the campaign timeline, encoding changes, and defensive recommendations: Microsoft detects new Morse code phishing campaign. Microsoft attributed the average 37-day change interval to its campaign investigation; the statement is not attributed to a named individual.
BleepingComputer’s contemporaneous coverage explains a sample decoder and the reconstructed JavaScript tags: Attackers use Morse code, other encryption methods in evasive phishing campaign.
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