Executive brief
Zen Browser is a web browser based on Firefox. In versions prior to 1.19.9b, the browser's automatic update system fails to verify the digital signatures of update files. If the developer's update servers or distribution accounts were compromised, an attacker could push malicious software updates to all users, leading to complete control over their computers.
Technical details
Zen Browser (prior to 1.19.9b) was found to have stripped Mozilla Application Resource (MAR) signature verification code from its updater component (org.mozilla.updater). Analysis of the updater binary revealed that cryptographic symbols for signature verification were missing and replaced with a 'Not Applicable' string, while the MAR update files themselves contained zero signatures. While the browser performs a SHA-512 integrity check against an XML manifest, both the manifest and the update file originate from the same trust domain. Consequently, a compromise of the update server or the GitHub release pipeline would allow an attacker to distribute arbitrary unsigned code to users. The issue was resolved in version 1.19.9b by re-enabling MAR signature verification and implementing a proper signing infrastructure.
Affected products
- Zen Browser Zen Browser Prior to 1.19.9b
Timeline
- 2026-04-09: patched: Fix committed to repository
- 2026-04-24: advisory: GitHub Security Advisory published
- 2026-05-11: disclosed: CVE published to NVD