GHSA-937x-gpqr-72gg 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 asymmetryThe 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:On a denylist `UploadSet` the allow-check returns `True` for the mixed-case form:`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 componentsPermalinks 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):`UploadSet` uses a **denylist** — `AllExcept(...)` or populated `config.deny`. (Allowlists fail closed — §7c.)Application passes a **user-influenced `name`** to `save()`.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. Asymmetry7b. End-to-end (denylist `AllExcept(SCRIPTS)`)7c. Negative controls8. 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. ImpactBypass 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 remediationNormalize before the policy check so the name-override path matches the default path:`ext = extension(basename).lower()` (or `extension(lowercase_ext(basename))`) before `extension_allowed`.Or run the overridden `basename` through `lowercase_ext()` (as `get_basename` does) before both validation and save.Or make `extension_allowed` / the policy containers case-insensitive.Option (1) or (2) is the minimal, behavior-preserving fix.11. Evidence files`poc_case_fold.py` (§12) — self-contained PoC; runs against `pip install Flask-Reuploaded==1.5.0`.`/tmp/flask_reuploaded_case_fold_proof.txt` — captured verdict.Source permalinks pinned to commit `ae31c3f91da40b465ca5e8f57d93f063b4553e23`.12. Full PoC script (`poc_case_fold.py`)
CVSS v3.1 score: 7.5