Flask-Reuploaded: Extension-denylist bypass via case-folding asymmetry in name-override path (incomplete-fix variant of CVE-2026-27641)

## 1. Header | Field | Value | |---|---| | **Title** | Extension-denylist bypass via case-folding asymmetry in name-override path (incomplete-fix variant of CVE-2026-27641) | | **Project** | Flask-Reuploaded (`flask_uploads`) | | **Affected** | `<= 1.5.0` (latest release; commit `ae31c3f91da40b465ca5e8f57d93f063b4553e23`) | | **Relationship** | Incomplete-fix variant of **CVE-2026-27641** (fixed in v1.5.0; this variant survives that fix) | | **CWE** | CWE-434 (Unrestricted Upload of File with Dangerous Type) + CWE-178 (Improper Handling of Case Sensitivity) | | **CVSS v3.1 (proposed)** | **~7.0–7.3 (High, conditional)** — `CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H`. Final score deferred to maintainer. | | **Verified against** | `pip install Flask-Reuploaded==1.5.0`, Python 3.11 | --- ## 2. Decisive evidence — the asymmetry The v1.5.0 fix for CVE-2026-27641 added an extension **re-validation** that runs when a caller supplies a `name` override to `UploadSet.save()`. It is routed through the **case-preserving** `extension()` helper, whereas the default upload path normalizes to lowercase via `lowercase_ext()` (inside `get_basename()`). The two paths disagree on case: ``` default path : get_basename("shell.PHP") = lowercase_ext(secure_filename("shell.PHP")) = "shell.php" -> extension("shell.php") = "php" -> extension_allowed("php") -> DENY (correct) name-override : secure_filename("shell.PHP") = "shell.PHP" (case preserved) -> basename = "shell.PHP" -> ext = extension("shell.PHP") = "PHP" (NOT lowercased) -> extension_allowed("PHP") -> ALLOW (bypass) ``` On a denylist `UploadSet` the allow-check returns `True` for the mixed-case form: ```python # extensions.py:97 def __contains__(self, item): return item not in self.items # "PHP" not in ('js','php','pl','py','rb','sh') -> True -> allowed ``` `extension_allowed("PHP")` passes the denylist that `extension_allowed("php")` is blocked by. --- ## 3. Vulnerability summary `UploadSet.save(storage, name=...)` lets the caller override the stored filename. After the CVE-2026-27641 fix, `name` is sanitized with `secure_filename()` and its extension re-validated. Because the re-validation compares a **case-preserving** extension against a policy whose denied tokens are lowercase, an attacker controlling `name` stores a file with a denied extension by varying case (`shell.PHP`, `evil.pHp`). On servers that resolve/execute extensions case-insensitively (Windows/macOS filesystems; Apache `AddHandler`/`AddType`), this re-enables the dangerous-upload → code-execution outcome the parent CVE addressed. The bypassed config is the one the library's own docs recommend for blocking scripts (see §6). --- ## 4. Affected components Permalinks pinned to commit `ae31c3f91da40b465ca5e8f57d93f063b4553e23` (v1.5.0). | Role | Location | |---|---| | Normalizing helper (default path) | `src/flask_uploads/flask_uploads.py:283` — `return lowercase_ext(secure_filename(filename))` | | `save()` entry | `src/flask_uploads/flask_uploads.py:285` | | name-override sanitize | `src/flask_uploads/flask_uploads.py:333` — `name = secure_filename(name)` | | name-override assign (case kept) | `src/flask_uploads/flask_uploads.py:341` — `basename = name` | | **Re-validation (non-normalizing)** | `src/flask_uploads/flask_uploads.py:344` — `ext = extension(basename)` | | Allow-check | `src/flask_uploads/flask_uploads.py:345` | | Policy check | `src/flask_uploads/flask_uploads.py:268` — `extension_allowed` | | Containment backstop (path only) | `src/flask_uploads/flask_uploads.py:364` | | Sink | `src/flask_uploads/flask_uploads.py:374` — `storage.save(target)` | | Case-preserving extractor | `src/flask_uploads/extensions.py:101` — `def extension` | | Case-normalizing extractor | `src/flask_uploads/extensions.py:109` — `def lowercase_ext` | | Denylist membership | `src/flask_uploads/extensions.py:97` — `AllExcept.__contains__` | | Documented script denylist | `src/flask_uploads/extensions.py:34-35` | --- ## 5. Taint analysis - **Source:** the `name` argument of `UploadSet.save(storage, name=...)` — commonly user-derived; the parent CVE-2026-27641 already treats `name` as attacker-controllable. - **Guard (asymmetric):** `secure_filename(name)` stops traversal, but the extension policy check at `:344-345` uses non-normalizing `extension()` against a lowercase-tokened policy. The realpath containment at `:364` constrains the *path*, not the *extension* — orthogonal to this bypass. - **Sink:** `storage.save(target)` at `:374` writes the attacker-extensioned file into the served upload directory. --- ## 6. CVSS justification (preconditions stated honestly) `CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H` ≈ **7.0–7.3 (High)**. - **AV:N** — web upload endpoint. - **AC:H** — three preconditions (stated plainly): 1. `UploadSet` uses a **denylist** — `AllExcept(...)` or populated `config.deny`. (Allowlists fail closed — §7c.) 2. Application passes a **user-influenced `name`** to `save()`. 3. Deployment resolves/executes extensions **case-insensitively** (Windows/macOS FS; Apache `AddHandler`/`AddType`). - **PR:L** — uploads typically authenticated. **UI:N**. **S:U**. **C:H/I:H/A:H** — RCE under the web server's privileges on execution-capable upload dirs. **Why High, not a contrived misconfiguration** — the bypassed control is the library's documented script-blocking mechanism: - `extensions.py:34-35` — `SCRIPTS`: *"…you might want to add ``php`` to the DENY setting."* - `extensions.py:24-26` — `EXECUTABLES`: *"…it's better suited for use with `AllExcept`."* An app that followed this guidance to block scripts is exactly what this variant defeats. **Not claimed:** not Critical. Pure allowlists are unaffected; execution requires a case-insensitive surface. Final score deferred to the maintainer. --- ## 7. Proof of Concept (results) Against shipped `Flask-Reuploaded==1.5.0`: ### 7a. Asymmetry ``` default path : lowercase_ext("shell.PHP") -> "php" -> extension_allowed("php") -> DENY name-override : secure_filename("shell.PHP") -> "shell.PHP" -> extension(...) -> "PHP" -> extension_allowed("PHP") -> ALLOW ``` ### 7b. End-to-end (denylist `AllExcept(SCRIPTS)`) ``` [1] save(storage("shell.PHP")) -> UploadNotAllowed (default path blocked) [2] save(storage("upload.bin"), name="shell.PHP") -> saved "shell.PHP", on_disk=True (BYPASS) [2b] save(storage("upload.bin"), name="evil.pHp") -> saved "evil.pHp", on_disk=True; containment intact ``` ### 7c. Negative controls ``` [3] allowlist IMAGES, name="shell.PHP" -> UploadNotAllowed: File extension 'PHP' is not allowed (fail-closed) [4] allowlist IMAGES, name="photo.jpg" -> saved "photo.jpg" (legit unaffected) ``` --- ## 8. Patch pattern (internal precedent) The project ships a **case-normalizing** extractor (`lowercase_ext`, used by `get_basename`) specifically so configured extensions are *"compare[d] … in the same case"* (its docstring). The v1.5.0 re-validation instead calls the **case-preserving** `extension()`, so the new guard does not match the normalization the rest of the system relies on — the classic incomplete-fix shape where a hand-rolled check diverges from the canonical normalizer. --- ## 9. Impact - Bypass of the documented extension denylist for scripts/executables. - Storage of `.PHP` / `.pHp` / mixed-case dangerous files inside the served upload directory (path containment holds; extension policy defeated). - On case-insensitive execution surfaces → **remote code execution** under the web server's privileges — the parent CVE's end impact, re-enabled on denylist deployments. --- ## 10. Suggested remediation Normalize before the policy check so the name-override path matches the default path: 1. `ext = extension(basename).low