Predictable Math.random() session signing key lets unauthenticated attackers forge admin cookies in Rejetto HFS (CVE-2026-61500)
A weapon-grade exploit for this unauthenticated no-interaction flaw is published in VulnCheck's XDB (catalogued 2026-10-01) against a file server normally published straight to the internet; no in-the-wild attacks are confirmed, but anything still on 3.2.0 or earlier is one public script away from handing over admin.
HFS 3.0.0 through 3.2.0 derives its session-cookie signing key from the non-cryptographic Math.random() generator and leaks outputs of that same generator to unauthenticated clients in login responses. An attacker who collects a handful of login responses can reconstruct the generator state, recover the signing key and forge an administrator session cookie, which yields full admin access and code execution through the server_code configuration feature.
Affected: Rejetto HFS (http_file_server) 3.0.0 through 3.2.0 (fixed in 3.2.1)
How to Test
Check the running version in the HFS admin interface or the application title/about screen; any build from 3.0.0 up to and including 3.2.0 is affected. On affected hosts, review HFS access logs for repeated unauthenticated login requests from a single source followed by authenticated administrator actions, and inspect the HFS configuration for unexpected `server_code` entries or added accounts, which is the path from forged session to code execution.
How to Patch
Upgrade to Rejetto HFS v3.2.1 (released 2026-07-13, https://github.com/rejetto/hfs/releases/tag/v3.2.1), which the vendor states fixes multiple vulnerabilities allowing administrative access. After upgrading, invalidate existing sessions and change the administrator password so any previously forged or recovered key is useless; where an upgrade must wait, remove the HFS admin interface from internet reachability.