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CVE-2026-72836 – Authentication Bypass / Cross-Account File Access – FileBrowser v2 before 2.63.19

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Summary

CVE-2026-72836 is an authentication bypass vulnerability in File Browser (filebrowser/filebrowser v2) caused by improper handling of case sensitivity (CWE-178) in home-directory scope ownership checks. On case-insensitive filesystems, two usernames differing only in letter case are stored as distinct accounts but resolve to the same physical directory, letting an attacker who self-registers a case-variant username gain unauthorized read, write, and delete access to another user’s files through normal authenticated endpoints. It affects all versions of File Browser prior to 2.63.19.

Technical details

  • Root cause: username/scope-ownership comparisons in File Browser’s storage layer are case-sensitive, while the underlying filesystem (e.g., Windows/NTFS or other case-insensitive filesystems) treats differently-cased names as identical paths.
  • Trigger conditions: the Signup and CreateUserDir features must be enabled, and the deployment must run on a case-insensitive filesystem; an attacker registers a new account whose username differs from a victim’s only in letter case (e.g., "CaseVictim" vs. "casevictim").
  • Attack vector: network — exploited entirely through the application’s normal authenticated HTTP API/endpoints, with no need for the victim’s credentials.
  • Impact: unauthorized access to another user’s home directory, enabling file disclosure, modification, and deletion, effectively bypassing per-user file isolation.

Affected software

  • File Browser (filebrowser/filebrowser v2) — all versions prior to 2.63.19
  • Package: pkg:golang/github.com/filebrowser/filebrowser/v2

Severity

CVSS v3.1 Base Score: 8.1 (High) — Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
(Also rated 9.2/Critical under CVSS v4.0: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N)

Mitigation and recommended actions

  • Immediate: upgrade File Browser to version 2.63.19 or later, which rejects case-folded home directory collisions and enforces case-insensitive scope matching.
  • If upgrading is not immediately possible: disable public self-registration (Signup) and/or CreateUserDir, or run the deployment on a case-sensitive filesystem to prevent the collision condition; audit existing user accounts for case-variant username pairs and remove or rename duplicates.
  • Review authenticated endpoint access logs for accounts created with usernames that are case-variants of existing accounts.

References

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