Signal Flaw Allowed Code Execution With No User Interaction

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An update released over the weekend for the desktop version of the privacy-focused communications app Signal patches a critical vulnerability that could have been exploited for remote code execution with no user interaction required.

Several researchers were looking at an unrelated cross-site scripting (XSS) vulnerability when they noticed that the XSS payload was triggered in the Signal desktop application.

The white hat hackers discovered that they could execute arbitrary code in the app simply by sending a specially crafted message containing specific HTML elements to the targeted user.

“The Signal-desktop software fails to sanitize specific html-encoded HTML tags that can be used to inject HTML code into remote chat windows. Specifically the <img> and <iframe> tags can be used to include remote or local resources,” the researchers explained in an advisory.

They created proof-of-concept (PoC) payloads that could be used to crash Signal, obtain data from the targeted device’s /etc/passwd file, execute a remote JavaScript file, display a message in an iframe, play audio and video files, display a phishing page, and exfiltrate conversations.

“The critical thing here was that it didn’t required any interaction form the victim, other than simply being in the conversation. Anyone can initiate a conversation in Signal, so the attacker just needs to send a specially crafted URL to pwn the victim without further action,” Iván Ariel Barrera Oro, one of the researchers involved in finding the vulnerability, wrote in a blog post.

The vulnerability affects versions 1.7.1, 1.8.0, 1.9.0 and 1.10.0 on Windows, Linux and likely macOS. Signal developers patched the issue within a couple of hours with the release of version 1.10.1 on Saturday.

Based on an analysis of the source code, researchers determined that the flaw had been previously patched but the fix was removed – likely by accident – with a change made on April 10.

Related: WhatsApp Co-founder Invests $50 Million in Signal

Related: Signal Announces Private Contact Discovery

Related: Flaw Allows Hackers to Alter “Signal” Attachments

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.

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