The exposure of a security trade-off in WhatsApp has disappointed activists, diplomats and others who use it regularly for their work, but in a world of increasing surveillance and ever more aggressive hacking, many say they were already wary of trusting its promise of total privacy.

Keeping plans and communications hidden from hostile authorities or rivals can be a constantly shifting game of digital cat and mouse. “[I believe] nothing is particularly safe in these messenger systems,” said a western diplomat who regularly uses WhatsApp but not for sensitive information, “just for keeping in touch and logistics”.

Facebook, which owns WhatsApp, claims no one can intercept messages on the system, not even the company and its staff, ensuring privacy for its billion-plus users. But in implementing its end-to-end encryption protocol, WhatsApp made a trade-off between security and usability which could make some messages vulnerable.

“It’s just another threat to the security of activists,” said Zaina Erhaim, a Syrian journalist and campaigner, who said she did not herself use the app much but knew of other Syrians who did.

Many of Syria’s boldest activists have learned to be wary of any communications system promising digital protections, because of the widespread use of torture by both the government of Bashar al-Assad and opponents including Isis. That can make it impossible for any activist who is seized to keep their secrets. “When captured, they are forced to decrypt their files,” Erhaim said.

“There could be some fears,” said a medical aid worker, who said the app was popular with his colleagues although they did not use it for the most sensitive conversations. “I think people use it for privacy and convenience.”

If news of the security trade-off pushes dissidents and others away from WhatsApp, it will not be the first time that a once-trusted programme has been sidelined. A decade ago Skype was the communication method of choice, but confidence in its systems was already failing badly before leaks by Edward Snowden revealed that the US government was regularly listening in on calls.

Other systems that have maintained a reputation for security have become victims of their own encryption success, banned by the authorities they were being used against. At the end of last year Egypt blocked access to Open Whisper Systems’ Signal app, a rival to WhatsApp that was recommended by Snowden.

It uses the same protocol as WhatsApp but does not suffer from the same vulnerability. It was favoured by some dissidents because its settings allow for the automatic deletion of new messages.

WhatsApp, by contrast, stores messages by default, making a user potentially more vulnerable if they are detained.

• This article was amended on 28 June 2017 following a review and amendments to the news article which prompted this article.

