Self-destructing messages are a great idea, in theory. For people who want to keep personal messages private from people who may be in the same room—such as a snoopy friend, family member, or an abusive partner—the ability to set a timer on a message before it disappears forever can provide a comforting sense of security.

Perhaps the most known example is Signal, the popular messaging app with end-to-end encryption, which introduced self-destructing messages back in 2016. All users had to do was go into the settings for a conversation, select the self-destructing option, and set a timer.

But as reported by Motherboard’s Lorenzo Franceschi-Bicchierai this morning, Signal’s self-destructing messages don’t always self-destruct everywhere.

When people use the desktop Signal app on iOS, such as their laptop or iMac, they’ll automatically get notifications for the messages they receive. But even when these messages are labeled as self-destructing, and even after these messages are deleted from the Signal app itself, the messages survive in the iOS notification database.

#HEADSUP: #Security Issue in #Signal. If you are using the @signalapp desktop app for Mac, check your notifications bar; messages get copied there and they seem to persist — even if they are "disappearing" messages which have been deleted/expunged from the app. pic.twitter.com/CVVi7rfLoY — Alec Muffett (@AlecMuffett) May 8, 2018

Self-destructing messages are supposedly designed to wipe the information contained in messages across every machine or host server that may transit the message. So in order for Signal’s messages to truly be considered “self-destructing,” the messages have to be deleted everywhere. For people who rely on Signal self-destructing messages for a sense of safe, secure, and private communication, this setting represents a major vulnerability.

It’s noteworthy that domestic violence is one of the most common situations in which a person would need to conceal their messages from a person in the room. In a dangerous situation, a person may need to message a friend or someone they trust quickly and quietly. This makes self-destructive messaging a crucial option of defense for women and people of color, who are disproportionately more likely to be victims of domestic violence. And this situation isn’t the only instance on Signal where these populations are at a higher risk. Other Signal users have to use an individual’s real phone number in order to contact another user, meaning the best option for women or people of color who fear stalking or harassment involves getting dummy SIM card before using Signal.

This Signal vulnerability also has implications for parties that may not be in the same room as a user. iOS operating systems are incredibly difficult to hack, but it’s not impossible. That means that a hacker or piece of malware that’s able to penetrate the operating system would have access to Signal messages that were supposed to self-destruct.

Apple also cooperates with law enforcement in a way that could involve handing over user data stored in any of its databases—including desktop notification databases that could include these Signal messages. In the second half of 2016, Apple said that it received somewhere between 5,750 and 5,999 National Security Orders, although it did not clarify the nature of these orders, or how these orders were applied in a court of law.

Luckily, users can take action. According to a blog by security researcher Patrick Wardle, the settings for the Signal desktop app lets users edit their notification settings. People can specify that they only want to see the name of a messenger in their iOS notifications, or see nothing in their notifications at all.

But changing your settings can only save your future Signal messages. According to Wardle, anything that’s already been stored on the iOS notification database can be stored there, theoretically, forever.

This vulnerability in Signal, arguably one of the most trusted and respected secure messaging apps, proves just how difficult it is to engineer secure options for users that covers every possible place information could be stored.

The new version of Gmail introduced self-destructing emails, which would block the email recipient from forwarding the email to other parties. It’s an attractive prospect, but this iOS notification vulnerability could potentially also apply to the Gmail update. And similar vulnerabilities exist on other platforms. WhatsApp, which features end-to-end encryption, automatically saves pictures transmitted via the app to a user’s photo library and iCloud, and it backs up data to Google Drive.

While Signal is still one of the best options, if not the best option, for fast private messaging, this desktop vulnerability highlights that it’s far from a completely secure system, and it’s incredibly difficult to design one.