datamodel-code-generator: Protobuf weak-import path traversal allows files to be written outside the temporary directory

Severity

Proposed: High. CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N = 7.5.

• Scored to match the project's own rating of the sibling path-traversal advisories CVE-2026-55389 / CVE-2026-55390, both AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N = 7.5. This finding is the same attack surface (unvalidated path from a processed schema) with the impact moved from confidentiality (read) to integrity (write), hence C:N/I:H.
• AV:N: the malicious schema can be fetched over the network via --url, consistent with how the siblings were scored.
• I:H: attacker fully controls the write path and can create arbitrary directory trees and overwrite existing files; only the file content is constrained (a fixed syntax = "proto{2,3}"; line).
• A:N chosen conservatively; overwriting build-critical files (lock files, __init__.py, entry points) can cause availability loss, which would argue A:L.
• Honest alternative: if the reviewer treats "the victim must run the tool on the file" as user interaction (UI:R), the score is 6.5 (Moderate). The project used UI:N for the identical read-path siblings, so UI:N is used here for consistency.

Details

Source pattern — the imported path is captured with no character restriction:

# protobuf.py:37
WEAK_IMPORT_PATTERN = re.compile(r'^\s*import\s+weak\s+"([^"]+)"\s*;', re.MULTILINE)

Sink — the captured path is joined and written with no boundary check:

# protobuf.py:383-393  (_write_missing_weak_imports)
for import_path in WEAK_IMPORT_PATTERN.findall(text):
    if any((include_path / import_path).exists() for include_path in include_paths):
        continue
    stub = self.weak_import_dir / import_path        # no resolve(), no is_relative_to()
    stub.parent.mkdir(parents=True, exist_ok=True)   # arbitrary directory tree
    stub.write_text(syntax, encoding=self.parser.encoding)

Two escape mechanisms, both from pathlib's / semantics:

• Absolute path: Path(".../__weak_imports__") / "/etc/x" == /etc/x (an absolute right operand replaces the base). Creates new files anywhere the process can write.
• ../ traversal: Path(".../__weak_imports__") / "../../x" climbs out of the temp directory.

Why the existence guard (line 389) does not protect the write: it checks include_path / import_path, using a different base (include_paths) than the actual write, which uses self.weak_import_dir / import_path. For a relative path the two bases resolve to different directories, so the guard checks a location that is not the real write target and misses it — enabling overwrite of existing files. (For an absolute path the guard and the write coincide, so an already- existing absolute target is skipped; absolute paths thus create new files but do not overwrite. Overwrite is reached via the relative-traversal path.)

Timing: _write_missing_weak_imports runs inside __enter__ (line 333), and protoc.main(...) runs later in the with body. protoc rejects the malformed virtual path (Backslashes, consecutive slashes, "." or ".." are not allowed in the virtual path) and the overall command exits non-zero — but the file has already been written; the filesystem side effect is not rolled back.

Attack path

1. Attacker crafts evil.proto containing e.g. import weak "/home/victim/.config/x"; and/or import weak "../../overwrite_me.txt";, and gets the victim to process it (third-party schema, or --url to an attacker-hosted file).
2. _write_missing_weak_imports writes the stub to the attacker-chosen path, creating any intermediate directories and overwriting an existing file (relative case).
3. protoc then fails, but the file(s) are already on disk.

Proof of Concept

Two self-contained scripts (poc/reproduce.sh for --input, poc/reproduce-url.sh for --url) install the released datamodel-code-generator==0.80.0, serve/point a malicious .proto, and assert the writes. The PoC drives the released package unmodified; the sink file's sha256 is recorded in REPRODUCE.md.

Local --input run against 0.80.0 — real output:

[*] installed version: 0.80.0
[test A] absolute-path arbitrary file + mkdir(parents):
  [PASS] new file created at attacker-controlled absolute path
  [PASS] content is the fixed syntax stub
[test B] relative-traversal overwrite of pre-existing file:
  [PASS] pre-existing victim.txt content was overwritten
  [PASS] victim.txt now holds the injected stub
==================== RESULT: 4 passed, 0 failed ====================

Remote --url run against 0.80.0 — real output (proto served over HTTP, then the same writes fire):

[*] serving malicious proto at http://127.0.0.1:<port>/evil_url.proto
  [PASS] server actually served the proto (HTTP 200)
  [PASS] absolute-path arbitrary file created
  [PASS] pre-existing victim overwritten via ../
==================== RESULT: 3 passed, 0 failed ====================

Impact

Any user or CI pipeline that runs an affected version of datamodel-code-generator on an untrusted Protobuf schema (a downloaded third-party .proto, or --url to a remote source) is exposed to arbitrary file/directory creation and overwrite by the schema author, with the process's own permissions. Concrete harms:

• Overwrite source, config, .env, lock files, or __init__.py / entry-point files → project corruption, broken builds, denial of service.
• Create arbitrary directory trees / drop files into watched or auto-loaded locations.

Direct arbitrary code execution has not been demonstrated. The written content is limited to a generated syntax = "proto2"; or syntax = "proto3"; line and is not fully attacker-controlled.

Resolution

The fix was merged into the parent repository's main branch in 5e94b8f.

The Protobuf input preparer now resolves the dedicated temporary weak-import directory and each candidate stub path. If a resolved candidate is outside that directory, it raises SchemaParseError before checking include paths, creating parent directories, or writing the stub. The containment check rejects absolute-path escapes and relative .. traversal that would leave the sandbox.

Input-preparation failures also immediately clean up the temporary directory.

Regression tests cover:

• rejection of absolute weak-import paths outside the sandbox;
• rejection of relative .. traversal outside the sandbox;
• preservation of a pre-existing file outside the sandbox.

Local tox validation of the patch passed across the Python 3.10–3.14 and formatter matrix, the CI compatibility environments, and the repository checks. Combined line and branch coverage, including the changed files, reached 100%.

A fixed package has not yet been published to PyPI; the latest published version at the time of this update is 0.80.0. Patched versions remains unset until a release containing this fix is available. The advisory remains private pending that release.

References

• GHSA-8359-h9fx-j6v9 / CVE-2026-55389 — datamodel-code-generator arbitrary local file read via JSON-Schema $ref path traversal (CWE-22/200/610, 7.5, patched 0.62.0). Same class, read primitive; sibling entry point.
• GHSA-442q-2j6p-642g / CVE-2026-55390 — datamodel-code-generator arbitrary local file read via XSD schemaLocation path traversal (CWE-22/200/610, 7.5, affected >= 0.59.0, <= 0.61.0, patched 0.62.0). Same class, read primitive; the fix sweep that missed the Protobuf entry point.
• Python pathlib — PurePath.__truediv__: an absolute right-hand operand discards the left operand; .. segments are not normalized. (https://docs.python.org/3/library/pathlib.html)

Discovery

Found via source review and a reproducing PoC on datamodel-code-generator/datamodel-code-generator, released 0.80.0 and main @ 834731d56c0a90c182a0919f316069cf5ff0659a, 2026-09.