CVE-2021-35000

Executive Summary

CVE-2021-35000 is a local information disclosure vulnerability in the OpenBSD kernel's multicast routing implementation. The flaw stems from uninitialized memory access, allowing a local attacker with low privileges to read sensitive kernel memory. While the CVSS score is low (3.3), the vulnerability can be chained with other exploits to achieve privilege escalation and arbitrary code execution in the kernel context. No public proof-of-concept or patch details are currently available.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
• Base Score: 3.3 (Low)
• Attack Vector: Local (AV:L)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: Low (C:L)
• Integrity Impact: None (I:N)
• Availability Impact: None (A:N)

Threat Level: Low. Exploitation requires local access and low privileges. The primary risk is information disclosure, which can aid in further attacks.

Deep Dive & Mechanics

The vulnerability resides in the OpenBSD kernel's multicast routing code. When processing multicast routing requests, the kernel fails to properly initialize memory buffers before using them. This uninitialized memory may contain residual data from previous kernel operations, including sensitive information such as pointers, cryptographic keys, or other process data.

Attack Flow:

1. An attacker gains low-privileged code execution on the target OpenBSD system.
2. The attacker triggers the vulnerable multicast routing code path, causing the kernel to expose uninitialized memory.
3. The attacker reads the disclosed data, potentially extracting sensitive kernel information.
4. This information can be used to bypass kernel protections (e.g., KASLR) or craft further exploits for privilege escalation.

Vulnerable vs. Patched Code

No specific code diff is publicly available. The vulnerability is caused by missing memory initialization (e.g., memset or bzero) before using a buffer in the multicast routing handler. A patched version would ensure all allocated memory is zeroed before use.

Vulnerable Pattern (pseudocode):

void handle_multicast_route(struct mbuf *m) {
    struct some_struct data;
    // data is not initialized
    process_data(&data); // uses uninitialized data
}

Patched Pattern (pseudocode):

void handle_multicast_route(struct mbuf *m) {
    struct some_struct data;
    memset(&data, 0, sizeof(data)); // initialize memory
    process_data(&data);
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to an OpenBSD system (any version with multicast routing enabled).
• Ability to execute low-privileged code (e.g., via a compromised user account).

Exploitation Steps:

1. Reconnaissance: Identify the vulnerable kernel module (route or multicast).
2. Trigger: Write a small C program that sends crafted multicast routing requests to the kernel (e.g., using ioctl or setsockopt with IP_ADD_MEMBERSHIP).
3. Data Extraction: Read the returned data from the kernel, which may contain uninitialized memory.
4. Analysis: Parse the leaked data for sensitive information (e.g., kernel pointers, stack canaries).

Example Trigger Code (Conceptual):

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <net/if.h>
#include <stdio.h>
#include <string.h>

int main() {
    int sock = socket(AF_INET, SOCK_DGRAM, 0);
    struct ip_mreq mreq;
    mreq.imr_multiaddr.s_addr = inet_addr("224.0.0.1");
    mreq.imr_interface.s_addr = htonl(INADDR_ANY);
    setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
    // The kernel may leak data in the process
    return 0;
}

Blue Team

Detection:

• Monitor for unusual multicast routing requests from low-privileged processes.
• Use kernel auditing to log setsockopt calls with IP_ADD_MEMBERSHIP or similar.
• Look for repeated attempts to read kernel memory via /dev/kmem or similar interfaces.

Mitigation:

• Apply kernel patches as soon as available.
• Restrict local user access to the minimum necessary.
• Use kernel hardening features (e.g., kern.securelevel).

Indicators of Compromise (IOCs)

• File Hashes: None available.
• Network Indicators: None (local vulnerability).
• Process Behavior: Unusual multicast routing requests from non-network daemons.
• Kernel Logs: Entries related to multicast routing errors or unexpected memory access.

Detection Engineering

YARA Rule

rule CVE_2021_35000_OpenBSD_Multicast_Leak {
    meta:
        description = "Detects potential exploitation of CVE-2021-35000 via uninitialized memory in multicast routing"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2021-35000"
    strings:
        $s1 = "IP_ADD_MEMBERSHIP" ascii wide nocase
        $s2 = "setsockopt" ascii wide nocase
        $s3 = "multicast" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: CVE-2021-35000 OpenBSD Multicast Routing Exploitation Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit CVE-2021-35000 by triggering multicast routing with low privileges
author: Valters IT
date: 2025-04-10
logsource:
    product: openbsd
    service: kernel
detection:
    selection:
        EventID: 0x1234  # Hypothetical kernel audit event
        ProcessName: '*'
        Syscall: 'setsockopt'
        Protocol: 'IPPROTO_IP'
        Option: 'IP_ADD_MEMBERSHIP'
    condition: selection
falsepositives:
    - Legitimate multicast applications
level: medium

Suricata Rule

alert ip any any -> any any (msg:"CVE-2021-35000 OpenBSD Multicast Routing Exploit Attempt"; content:"|08 00 00 00|"; offset:0; depth:4; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Patch: Apply the official OpenBSD patch when released. Monitor the OpenBSD errata page.
2. Workaround: Disable multicast routing if not needed (sysctl -w net.inet.ip.mforwarding=0).
3. Access Control: Restrict local user accounts and use principle of least privilege.
4. Monitoring: Enable kernel auditing for multicast-related syscalls.
5. Hardening: Increase kern.securelevel to prevent kernel memory access.

Valters IT
DevSecOps & Cyber Threat Intelligence