Business Insider contacted Telegram after unverified documents alleging links between President-elect Donald Trump and Russia claimed the app had been hacked by the FSB.
But spokesman Markus Ra denied the claims. "We think the report is likely to be fake," he said.
The claim appears in a dossier containing a number of colourful, unverified allegations about Trump. The document was published in full by BuzzFeed, and the President-elect was reportedly given a summary of the document last week.
Reportedly written by a former British M16 employee, the document also contains supposed claims from an FSB cyber operative that the agency had successfully cracked Telegram's encryption to spy on political activists. According to the document, the FSB was concerned by "Russian internal political activists" using Telegram, which promises secure communications. The operative claimed the FSB had "cracked" Telegram and that it was "no longer secure to use".
Ra suggested the allegations might refer to an incident in May last year, when two Russian activists using Telegram were compromised. This didn't involve a hack of Telegram - rather the hacker obtained the activists' log-in codes sent via text message.
Telegram promises secure instant messaging, and rivals the likes of WhatsApp and Signal. It was built by Russian entrepreneur Pavel Durov and has gained notoriety for being terror group ISIS' platform of choice. It is also popular with political activists in Iran, Hong Kong and Russia, and is used by Russian opposition leader Alexei Navalny.
This isn't the first time Telegram's security has come under question, with multiple security specialists flagging its custom MTproto protocol in the last year.
But the app still ranks above rival services such as WhatsApp and Viber on the Electronic Frontier Foundation's Secure Messaging Scorecard.
Here's the full paragraph on the FSB compromising Telegram from the document published by BuzzFeed:
"In terms of other technical IT platforms, an FSB cyber operative flagged up the 'Telegram' enciphered commercial system as having been of especial concern and therefore heavily targeted by the FSB, not least because it was used frequently by Russian internal political activists and oppositionists. His/her understanding was that the FSB now successfully had cracked this communications software and therefore it was no longer secure to use."