A Hidden Threat: How Local Backdoors Can Compromise Secure Apps Like Signal and 1Password
We trust secure applications to protect our most sensitive data. Tools like Signal, 1Password, and Slack are cornerstones of modern communication and data management, built on a promise of encryption and integrity. However, a significant vulnerability in the update mechanism of many popular desktop applications reveals that even the most secure software can be undermined by a clever attack, provided one critical condition is met: initial access to your computer.
This flaw isn’t about breaking encryption. Instead, it’s a sophisticated “bait and switch” that allows an attacker with local access to your machine to hijack the update process, effectively turning a routine software update into a malicious backdoor.
Understanding the Update Hijack Vulnerability
Under normal circumstances, the update process for a desktop application is designed to be secure. It generally follows these steps:
- The application automatically downloads the latest update package to a temporary folder on your computer.
- It then performs an integrity check—typically by verifying a cryptographic signature—to ensure the downloaded file is authentic and hasn’t been tampered with.
- Once verified, the app prompts you to “Restart to Update” or “Install Now.”
The vulnerability lies in the small but critical gap between step 2 and step 3. After the application has verified the update file as legitimate, the file sits on your hard drive, waiting for you to approve the installation. During this window, an attacker or malware already present on your system can replace the legitimate, verified file with a malicious version.
Because the app already performed its security check, it doesn’t re-verify the file before executing it. When you click that “Restart to Update” button, you are unknowingly instructing the application to run the attacker’s malicious code with the same privileges as the application itself.
This type of flaw is known in cybersecurity circles as a Time-of-Check to Time-of-Use (TOCTOU) vulnerability. The integrity of the file is checked at one point in time, but the file that is actually used later on is different.
Which Applications Are Affected?
This vulnerability is not specific to one developer but rather a logical flaw in how some applications handle updates. Research has shown that several high-profile applications were susceptible, including:
- Signal
- 1Password
- Slack
- GitHub Desktop
- Visual Studio Code
Many of these applications are built using the Electron framework, but the issue is not exclusive to it. The core problem is the update procedure itself, which could potentially affect any software that downloads and stages updates in an unsecured local directory.
It is crucial to emphasize that this attack is not a remote exploit. An attacker must first have a foothold on your system, whether through malware, phishing, or physical access. This vulnerability serves as a powerful method for privilege escalation and persistence, allowing a low-level threat to embed itself deeply into your system by hijacking a trusted application.
The Real-World Impact: From Data Theft to Full System Control
If successfully exploited, this vulnerability could have devastating consequences. By replacing a legitimate update with a malicious payload, an attacker could:
- Compromise your password manager: A modified 1Password update could be programmed to silently exfiltrate your entire vault, including all your passwords, secure notes, and financial information.
- Read your encrypted messages: A backdoored Signal client could capture your messages before they are encrypted or after they are decrypted on your device, completely bypassing Signal’s celebrated end-to-end encryption.
- Install persistent malware: The attacker could install ransomware, keyloggers, or spyware that runs with high privileges, granting them long-term control over your machine.
Ultimately, by compromising a trusted application, the attacker gains a powerful and stealthy way to control your system, steal your data, and remain undetected.
Protecting Yourself: Actionable Security Measures
While developers are responsible for patching this flaw in their applications, proactive security hygiene is your best defense against the initial infection that makes this attack possible.
- Practice Foundational Security: The number one defense is to prevent malware from getting onto your system in the first place. Use a reputable antivirus program, enable your firewall, and avoid downloading software or opening attachments from untrustworthy sources.
- Keep Everything Updated: This includes your operating system and all installed software. While this vulnerability specifically targets the update process, a fully patched system is much harder to compromise initially. Ensure you have installed the latest versions of the affected applications, as many have already released fixes.
- Limit Physical Access: Never leave your computer unlocked and unattended in a public or untrusted environment. Physical access is one of the easiest ways for an attacker to plant malware.
- Use Standard User Accounts: For daily tasks, avoid using an administrator account. Running as a standard user can limit the scope of what malware can do if it does manage to infect your system.
Security is a continuous process, not a final state. This vulnerability is a stark reminder that even with robust encryption and trusted software, attackers can find clever ways to exploit logical gaps in security procedures. By maintaining a secure system environment, you can close the door on the initial access required for such attacks to succeed.
Source: https://blog.trailofbits.com/2025/09/03/subverting-code-integrity-checks-to-locally-backdoor-signal-1password-slack-and-more/
