CVE-2026-41977

Executive Summary

CVE-2026-41977 is a Denial of Service (DoS) vulnerability residing in an unspecified "log service" from the vendor "Log". Based on the enriched intelligence, the vulnerability has a low EPSS score (0.0%) and no public proof-of-concept, exploit code, or patch data is currently available. The vulnerability is classified under CWE-400 (Uncontrolled Resource Consumption) and is mapped to MITRE ATT&CK technique T1499 (Endpoint Denial of Service). Successful exploitation could allow an unauthenticated, network-adjacent attacker to degrade or fully disrupt the availability of the logging service, potentially impacting log ingestion, storage, and forwarding capabilities. Given the lack of patch details and the critical role of logging in security monitoring, this vulnerability poses a significant risk to environments relying on this service for audit trails and incident detection.

Supply Chain & Package Security

This vulnerability is not associated with a specific package manager (NPM, PyPI, Go, Ruby). The vendor "Log" is ambiguous and may refer to a proprietary or open-source logging daemon. No supply chain indicators are available.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Base Score: 5.0 (Medium)
• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – Impact is confined to the vulnerable component.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): None (N) – No data modification.
• Availability (A): Low (L) – Service degradation or interruption.

Threat Level: Medium. The low EPSS score suggests limited active exploitation, but the lack of a patch and the criticality of logging services elevate the risk for targeted attacks.

Deep Dive & Mechanics

The vulnerability exists in the log service's handling of incoming log messages. The exact mechanism is not publicly documented, but based on the CWE-400 classification, it is likely a resource exhaustion flaw. Potential attack vectors include:

1. Unbounded Memory Allocation: Sending a specially crafted log message with an extremely long field (e.g., a 10MB message string) could cause the service to allocate excessive memory, leading to an Out-of-Memory (OOM) condition and process termination.
2. Uncontrolled Thread/Process Spawning: Sending a high volume of concurrent connections or log entries could trigger the service to spawn an excessive number of threads or child processes, exhausting system resources (CPU, file descriptors).
3. Infinite Loop / Recursion: A malformed log entry (e.g., nested JSON or XML) could trigger an infinite parsing loop, consuming 100% CPU and blocking legitimate log processing.
4. Disk Space Exhaustion: If the service does not implement proper rate limiting or log rotation, an attacker could flood the service with log data, filling the disk and causing the service (or the entire system) to fail.

The attack is likely network-based, targeting the default log ingestion port (e.g., UDP 514 for syslog, TCP 601 for RELP, or a custom TCP port). The attacker does not need to be authenticated.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is impossible. However, a hypothetical vulnerable code pattern and its patched counterpart are provided for illustrative purposes.

Vulnerable Code (Python-like pseudocode):

import socket

def handle_log_message(data):
    # Vulnerable: No size limit on data
    log_entry = data.decode('utf-8')
    # Process log_entry (e.g., write to file)
    with open('/var/log/app.log', 'a') as f:
        f.write(log_entry + '\n')

server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
server.bind(('0.0.0.0', 514))

while True:
    data, addr = server.recvfrom(65536)  # Large buffer, no validation
    handle_log_message(data)

Patched Code (Hypothetical):

import socket

MAX_LOG_SIZE = 4096  # 4KB limit

def handle_log_message(data):
    # Patched: Enforce size limit
    if len(data) > MAX_LOG_SIZE:
        # Log warning and drop oversized message
        print(f"WARNING: Dropped oversized log from {addr}")
        return
    log_entry = data.decode('utf-8', errors='replace')
    # Sanitize log_entry (e.g., remove control characters)
    sanitized = ''.join(c for c in log_entry if c.isprintable() or c in '\n\r\t')
    with open('/var/log/app.log', 'a') as f:
        f.write(sanitized + '\n')

server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
server.bind(('0.0.0.0', 514))

while True:
    data, addr = server.recvfrom(MAX_LOG_SIZE + 1024)  # Slightly larger buffer
    handle_log_message(data)

Exploitation & Lab Setup

Red Team

Objective: Disrupt the target log service to blind defenders.

Lab Setup:

1. Target: A Linux VM running the vulnerable log service (e.g., rsyslog, syslog-ng, or a custom daemon) listening on UDP 514.
2. Attacker: Kali Linux VM on the same network.

Exploitation Steps:

Since no public exploit exists, the following generic DoS techniques can be tested:

1. Flood with Large Messages:

   # Send a 64KB message repeatedly
   python3 -c "import socket; s=socket.socket(socket.AF_INET, socket.SOCK_DGRAM); s.sendto(b'A'*65535, ('TARGET_IP', 514))"
   

2. High-Rate Flood:

   # Use hping3 to flood UDP port 514
   hping3 --udp -p 514 --flood TARGET_IP
   

3. Malformed Syslog Message:

   # Send a message with an extremely long tag
   echo -n "<14>$(python3 -c "print('A'*100000)"): test message" | nc -u TARGET_IP 514
   

Expected Outcome: The log service becomes unresponsive, logs are dropped, and the service may crash.

Blue Team

Objective: Detect and mitigate the DoS attack.

Detection:

1. Monitor Resource Usage: Alert on high CPU, memory, or disk I/O for the log service process.
2. Network Anomaly Detection: Detect unusual volumes of UDP traffic to port 514 from a single source.
3. Log Analysis: Look for repeated oversized or malformed log entries in the log files (if any are written before the crash).

Mitigation:

1. Rate Limiting: Configure the log service to limit the number of messages per second from a single source.
2. Input Validation: Implement message size limits and reject oversized entries.
3. Network Segmentation: Restrict access to the log service to trusted sources only (e.g., using firewall rules).
4. Resource Limits: Use systemd or ulimit to restrict the memory and file descriptor usage of the log service.
5. Failover: Deploy a secondary log collector to ensure redundancy.

Indicators of Compromise (IOCs)

• Network IOCs:
  • High volume of UDP packets to port 514 from a single IP address.
  • UDP packets to port 514 with payloads exceeding 4096 bytes.
  • Malformed syslog messages (e.g., missing priority, invalid timestamps).
• Host IOCs:
  • Log service process (e.g., rsyslogd, syslog-ng) consuming 100% CPU or excessive memory.
  • Log service process crashing and restarting repeatedly.
  • Disk space on the log partition filling rapidly.
  • Log files containing extremely long lines or binary data.
• File IOCs:
  • /var/log/syslog or /var/log/messages containing entries with unusually long strings.
  • Core dump files from the log service process.

Detection Engineering

YARA Rule

rule CVE_2026_41977_Log_Service_DoS {
    meta:
        description = "Detects oversized or malformed log messages indicative of CVE-2026-41977 exploitation"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-41977"
    strings:
        $long_line = /.{4097,}/  // Lines longer than 4096 characters
        $binary_data = { 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F }
        $syslog_overflow = /<\d{1,3}>[A-Za-z0-9_\-\.]{1000,}:/
    condition:
        any of them
}

Sigma Rule

title: CVE-2026-41977 - Log Service DoS via Oversized Messages
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects oversized log messages that may indicate exploitation of CVE-2026-41977
author: Valters IT
date: 2026/01/01
logsource:
    category: application
    product: linux
detection:
    selection:
        EventID: 1  # Process creation (if log service spawns children)
        CommandLine|contains: 'syslog-ng' or 'rsyslogd'
    condition: selection
falsepositives:
    - Legitimate large log entries from verbose applications
level: medium

Suricata Rule

alert udp any any -> any 514 (msg:"CVE-2026-41977 - Potential Log Service DoS - Oversized Payload"; content:"|41 41 41 41|"; offset:0; depth:4; dsize:>4096; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Actions:

   • Isolate the Service: If possible, restrict network access to the log service to only trusted log sources using firewall rules or network segmentation.
   • Apply Rate Limiting: Configure the log service to limit the rate of incoming messages. For rsyslog, use the $SystemLogRateLimitInterval and $SystemLogRateLimitBurst directives.
   • Increase Monitoring: Enable detailed logging of the log service itself to detect anomalies.

2. Short-Term Mitigation:

   • Input Validation: If the log service is open-source, implement input validation (size limits, character whitelisting) and recompile.
   • Resource Limits: Use systemd resource control (e.g., MemoryMax, TasksMax) to prevent the service from exhausting system resources.
   • Deploy WAF/IPS: Use a Web Application Firewall or Intrusion Prevention System to filter oversized UDP packets.

3. Long-Term Remediation:

   • Vendor Patch: Monitor the vendor's website for an official patch. Apply it as soon as it becomes available.
   • Upgrade: If the log service is part of a larger suite (e.g., a SIEM), upgrade to the latest version that includes a fix.
   • Replace Service: If no patch is forthcoming, consider migrating to a different, actively maintained log service (e.g., syslog-ng, rsyslog, Fluentd).

4. Detection Rules: Deploy the YARA, Sigma, and Suricata rules provided in the Detection Engineering section to identify exploitation attempts.

Valters IT
Securing Digital Frontiers