{"id":"GHSA-gcv3-5v9q-fmhh","summary":"Froxlor has an authorization bypass in FTP shell assignment via missing server-side `available_shells` enforcement","details":"### Summary\nFroxlor 2.3.6 lets administrators configure `system.available_shells` as the approved shell list that customers may assign to FTP users. However, the server-side FTP account handlers do not enforce that whitelist when processing add or edit requests.\n\nAs a result, an authenticated customer with shell delegation enabled can submit an arbitrary shell such as `/bin/bash` even when the panel UI only offers more restricted choices. In deployments that use the default `nssextrausers` integration, the attacker-controlled shell is then propagated into the system account database, leading to real host shell access.\n\n### Details\nThe customer-facing FTP account page builds the shell selector from `system.available_shells`, which shows that the product intends the setting to act as the authorization boundary:\n\n```php\n// customer_ftp.php:138-149\n$shells = [\n    '/bin/false' =\u003e '/bin/false'\n];\n$availableshells = explode(',', Settings::Get('system.available_shells'));\nif (is_array($availableshells) && !empty($availableshells)) {\n    foreach ($availableshells as $shell) {\n        $shells[trim($shell)] = trim($shell);\n    }\n}\n```\n\nThe request handler forwards posted form data directly into the FTP API command implementation:\n\n```php\n// customer_ftp.php:170-172\nif ($action == 'edit' && Request::post('send') == 'send') {\n    $result = $log-\u003elogAction(USR_ACTION, LOG_INFO, \"edited ftp-account #\" . $id);\n    Commands::get()-\u003eapiCall('Ftps.update', Request::postAll());\n}\n```\n\nOn the server side, `Ftps::add()` and `Ftps::update()` only perform generic shell string validation. They do not verify that the submitted shell belongs to `system.available_shells`:\n\n```php\n// lib/Froxlor/Api/Commands/Ftps.php:119-123\nif (Settings::Get('system.allow_customer_shell') == '1' && $this-\u003egetUserDetail('shell_allowed') == '1') {\n    $shell = Validate::validate(trim($shell), 'shell', '', '', [], true);\n} else {\n    $shell = '/bin/false';\n}\n```\n\nThe validated shell is stored into `ftp_users.shell` and later consumed by the root-owned cron task that rebuilds NSS extrausers files:\n\n```php\n// lib/Froxlor/Cron/System/Extrausers.php:89-97\n$passwd_entries[] = $user['username'] . ':x:' . $uid . ':' . $gid . ':' . $gecos . ':' . $homedir . ':' . $shell;\n```\n\nBecause the default installer configuration sets `system.nssextrausers=1`, and the shipped Debian/Bookworm configuration enables `extrausers` in `nsswitch.conf`, the attacker-controlled shell becomes the effective login shell of the generated system user on standard supported deployments.\n\n### PoC\nAn attacker needs a normal customer account and a deployment where customer shell delegation is enabled for that customer.\n\nRelevant runtime prerequisites:\n\n- `system.allow_customer_shell=1`\n- the attacking customer has `shell_allowed=1`\n- the deployment uses `system.nssextrausers=1` with the shipped `libnss-extrausers` integration\n\nFroxlor requires a valid CSRF token for POST requests, so the attacker performs the exploit from an authenticated session.\n\nComplete PoC flow:\n\n1. Log in as a customer and obtain a valid `csrf_token`.\n2. Identify one FTP account owned by that customer.\n3. Submit an edit request that sets an arbitrary shell outside the administrator-approved `system.available_shells` list:\n\n```http\nPOST /customer_ftp.php?page=accounts&action=edit&id=17 HTTP/1.1\nHost: target.example\nContent-Type: application/x-www-form-urlencoded\nCookie: \u003cauthenticated customer session\u003e\n\ncsrf_token=VALID_CSRF_TOKEN&\nsend=send&\nid=17&\nusername=test1ftp1&\nftp_description=poc&\npath=/&\nshell=/bin/bash&\nlogin_enabled=1\n```\n\n4. Wait for Froxlor's master cron to process the queued `REBUILD_NSSUSERS` task.\n\nResult:\n\n- the request is accepted even if `/bin/bash` is not present in `system.available_shells`\n- `ftp_users.shell` is updated to `/bin/bash`\n- `/var/lib/extrausers/passwd` is regenerated with `/bin/bash` as the FTP user's login shell\n- the attacker can then authenticate to the host using that FTP user's credentials and obtain an interactive shell\n\n\n### Impact\nThis issue lets a low-privileged customer bypass an administrator-defined authorization boundary and promote an FTP-only account into a real shell account. On shared-hosting systems managed by Froxlor, that materially changes the trust model and can expose the host to lateral movement, local privilege-escalation follow-on attacks, data theft from colocated services, and persistence on the server.\n\nBecause the vulnerable flow is executed through the normal authenticated web interface and a root-owned provisioning task later materializes the chosen shell at the operating-system level, the vulnerability is stronger than a UI-only restriction bypass.","aliases":["CVE-2026-41235"],"modified":"2026-08-24T00:36:48.557886262Z","published":"2026-05-29T15:36:26Z","database_specific":{"severity":"HIGH","github_reviewed":true,"github_reviewed_at":"2026-05-29T15:36:26Z","nvd_published_at":"2026-06-04T19:16:29Z","cwe_ids":["CWE-863"]},"references":[{"type":"WEB","url":"https://github.com/froxlor/froxlor/security/advisories/GHSA-gcv3-5v9q-fmhh"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-41235"},{"type":"PACKAGE","url":"https://github.com/froxlor/froxlor"},{"type":"WEB","url":"https://github.com/froxlor/froxlor/releases/tag/2.3.7"}],"affected":[{"package":{"name":"froxlor/froxlor","ecosystem":"Packagist","purl":"pkg:composer/froxlor/froxlor"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.3.6"},{"fixed":"2.3.7"}]}],"versions":["2.3.6"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-gcv3-5v9q-fmhh/GHSA-gcv3-5v9q-fmhh.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"},{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:P"}]}