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CVE-2026-62685 – Unauthorized File Access via Username Normalization Collision – File Browser pri…

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Summary

CVE-2026-62685 is a high-severity directory scope collision vulnerability in File Browser (filebrowser/filebrowser), a widely deployed self-hosted web file management interface. When both the Signup and CreateUserDir features are enabled, the cleanUsername() function maps distinct usernames to the same home directory path without checking for prior occupancy, allowing an unauthenticated attacker who self-registers a crafted username to gain full read and write access to an existing user’s files. The vulnerability carries a CVSS v3.1 base score of 8.1 (High) and affects all versions of File Browser prior to v2.63.17.

Technical details

  • Root cause: The cleanUsername() function in settings/dir.go performs a lossy, many-to-one transformation on usernames — stripping .. sequences, replacing non-alphanumeric characters (except @, _, -, .) with hyphens, and collapsing consecutive hyphens. As a result, usernames such as team/one, team one, and team-one all normalize to the same directory path (team-one). Because MakeUserDir calls MkdirAll (which is idempotent), a second registrant silently reuses the first user’s existing directory with no warning or rejection.
  • Trigger conditions: Both Signup=true and CreateUserDir=true must be enabled in the File Browser configuration. These are non-default settings that require deliberate administrator enablement.
  • Attack vector: Network-reachable; no prior authentication or credentials required. An attacker only needs to self-register an account with a username that normalizes to the same path as an existing user’s home directory.
  • Impact: The second registrant inherits the first user’s home directory scope, gaining full read and write access to all files within it. This constitutes unauthorized file disclosure (confidentiality breach), file tampering or deletion (integrity breach), and potential availability impact — all rated HIGH in the CVSS vector.
  • Attack nature: The collision can be triggered deliberately (by crafting a username to target a known victim) or may occur accidentally between legitimate users.

Affected software

  • File Browser (filebrowser/filebrowser): All versions up to and including v2.63.16

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
  • CWE-647: Use of Non-Canonical URL Paths in Authorization Decisions
  • CWE-706: Use of Incorrectly-Resolved Name or Reference

Mitigation and recommended actions

  • Immediate — Patch: Upgrade File Browser to v2.63.17 or later. This release rejects signup requests where the normalized home directory path collides with an existing user’s scope.
  • If immediate patching is not feasible: Disable at least one of the two required configuration features — set Signup=false to prevent open self-registration, or set CreateUserDir=false to prevent automatic home directory creation on signup. Either change eliminates the vulnerable code path.
  • Review existing user accounts for unintended directory scope overlaps that may have been introduced prior to patching.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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