CVE-2026-105240

Executive Summary

CVE-2026-105240 is an improper neutralization of a null byte (NUL character) vulnerability in the OutputDebugStringAppender component of Apache log4net. When logged content contains a NUL character, the debug output record is terminated at that point, causing all subsequent layout-rendered content — including exception text and trailing fields — to be silently discarded. Only Windows-based applications using the OutputDebugStringAppender are affected, and the vendor's stated remediation is to upgrade to log4net 3.5.0.

Technical Analysis

The vulnerability resides in Apache log4net's OutputDebugStringAppender, an appender type that routes log events to the Windows debugging subsystem via the OutputDebugString API. The underlying flaw class is CWE-158 (Improper Neutralization of Null Byte or NUL Character). The Windows debug-output channel is NUL-terminated at the operating-system boundary: a NUL character embedded within a rendered log record terminates the debug output record at that position.

Because log4net renders the full layout (message, exception text, and trailing fields) into a single string before delivering it to the appender, any NUL character present anywhere within the attacker-influenced portion of that rendered record causes everything after the NUL to be silently dropped. The data is not merely truncated at display time — the remainder of the record, including the exception text and trailing fields, is lost from the debug output entirely. This is an integrity-affecting issue (CVSS integrity impact: Low) rather than a confidentiality or availability issue; the CVSS vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N reflects a network-reachable, low-complexity, unauthenticated attack with low integrity impact and no direct confidentiality or availability impact.

The attack surface is bounded by two preconditions stated in the advisory:

1. The application must run on Windows.
2. The application must be configured to use the OutputDebugStringAppender.

Applications that do not use this appender — including those using file, console, or network appenders — are not affected by this behavior. The version range is log4net from 1.2.9 before 3.5.0.

Insufficient technical detail is available in the provided description to specify the exact code path, method name, or parameter at which the NUL is introduced or consumed; the advisory describes behavior at the appender/layout boundary rather than naming specific internal functions. The critical enabling condition derivable from the description is that "a party whose data reaches a log message" can influence logged content — i.e., the attacker does not need to reach the logging library directly, only to get data that it controls into a log message.

Exploitation Scenarios

No public proof-of-concept, Nuclei template, or exploit code is available for CVE-2026-105240 (POC_STATUS: None). The exploitation chain below is inferred from the vulnerability class and the mechanics stated in the NVD description. No specific function names, endpoints, or file paths beyond those in the advisory are asserted.

1. Reconnaissance / target identification. An attacker identifies a Windows-hosted application that uses Apache log4net (version 1.2.9 through below 3.5.0) and is configured with the OutputDebugStringAppender. Effective targeting requires knowledge of, or inference about, the victim's logging configuration, since only this appender is affected. This is a low-privilege, network-reachable precondition (PR:N, AV:N).

2. Trigger of the flaw class. The attacker supplies input that will be written into a log message — for example, by submitting crafted data to an application field, header, or parameter that the application subsequently logs. The payload includes a NUL character positioned so that it lands within the rendered log record.

3. Payload/execution at the vulnerability-class level. When log4net renders the log event (message plus layout-defined content such as exception text and trailing fields) and passes it to OutputDebugStringAppender, the embedded NUL terminates the debug output record at that point. Everything the layout rendered after the NUL — including exception text and trailing fields — is silently lost. The attacker does not need code execution or authentication; control over logged input is sufficient. The mechanism is record truncation/manipulation, not memory corruption or injection of executable content.

4. Post-exploitation impact. The practical effect is log integrity loss in the Windows debug output channel: exception details, stack traces, and trailing structured fields that would normally aid detection and forensics are dropped for the affected records. An attacker can therefore suppress or hide the remainder of a log record — concealing evidence that would otherwise appear alongside the (attacker-controlled) logged content. Impact is limited to integrity of the debug output records (I:L); there is no stated confidentiality or availability impact, and no privilege escalation or remote code execution is described.

Because the debug output channel is typically consumed by debuggers or diagnostic tooling rather than persisted as the primary audit log, the blast radius is contingent on whether an organization relies on OutputDebugStringAppender output for monitoring or troubleshooting.

Impact Assessment

CVE-2026-105240 carries a CVSS 3.1 base score of 5.3 (Medium), vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N. Subscore interpretation:

• Attack Vector (Network): the flaw is reachable over a network because the triggering data enters through logged input, which may originate remotely.
• Attack Complexity (Low): no special conditions beyond the appender configuration and Windows platform are required.
• Privileges Required (None): the attacker needs no authenticated access, only the ability to influence logged content.
• User Interaction (None): no victim action is required to trigger record truncation.
• Scope (Unchanged): impact is confined to the vulnerable component.
• Confidentiality (None) / Integrity (Low) / Availability (None): the sole impact is loss/integrity degradation of debug output records.

Affected systems: Windows applications built on Apache log4net 1.2.9 up to (but not including) 3.5.0 that use the OutputDebugStringAppender.

Business impact: the primary risk is degraded log fidelity and potential concealment of exception details and trailing log fields — an anti-forensics and monitoring-evasion concern rather than a data-breach or outage concern. Where debug output feeds incident detection or troubleshooting, attackers could suppress the portions of records most useful to defenders. The EPSS score is not yet available from FIRST.org, and the CVE is not listed in CISA KEV, so there is no evidence of active exploitation at the time of writing. Blast radius is limited by the appender- and platform-specific preconditions.

Detection Guidance

The following rules are heuristic detection content targeting the vulnerability class (NUL injection into logged records terminating the Windows debug output channel). They are provided as starting points and should be tuned to the environment; because exploitation is application-semantic rather than network-protocol-explicit, several rules rely on behavioral signals (truncated debug records, NUL bytes in logging telemetry) rather than a single reliable signature.

Sigma Rule

title: Potential NUL-Byte Injection into log4net OutputDebugStringAppender Records
id: 5c2b7f10-9a4e-4d1c-8f66-2f9a1c0d7e33
status: experimental
description: Detects logging activity where debug output records are terminated by embedded NUL characters, consistent with CVE-2026-105240 in Apache log4net OutputDebugStringAppender on Windows.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-105240
  - https://logging.apache.org/log4net/
author: Threat Intelligence Team
date: 2026/01/01
tags:
  - attack.t1565.001
  - cve.2026-105240
logsource:
  product: windows
  category: process_creation
detection:
  selection_image:
    Image|endswith:
      - '\w3wp.exe'
      - '\dotnet.exe'
  selection_cmdline:
    CommandLine|contains:
      - 'OutputDebugStringAppender'
  selection_nul:
    CommandLine|contains: '\x00'
  condition: selection_image and (selection_cmdline or selection_nul)
falsepositives:
  - Legitimate applications logging binary or encoded content containing NUL bytes
  - Diagnostic tooling that references appender configuration names
level: low

YARA Rule

rule CVE_2026_105240_log4net_NUL_OutputDebugString
{
    meta:
        description = "Detects NUL-byte patterns associated with CVE-2026-105240 in Apache log4net OutputDebugStringAppender configuration or logged content"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-105240"
        author = "Threat Intelligence Team"
        date = "2026-01-01"
        severity = "medium"

    strings:
        $appender = "OutputDebugStringAppender" ascii wide nocase
        $log4net  = "log4net" ascii wide nocase
        $nul      = { 00 }
        $ver1     = "1.2.9" ascii
        $ver2     = "3.5.0" ascii

    condition:
        uint16(0) == 0x5A4D and
        ($appender or ($log4net and ($ver1 or $ver2))) and
        $nul
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-105240 Potential NUL Byte Injection into log4net Logged Content"; flow:established,to_server; content:"|00|"; http_client_body; pcre:"/\x00/"; threshold:type limit, track by_src, count 1, seconds 60; classtype:attempted-admin; reference:cve,2026-105240; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-105240; sid:2026105240; rev:1;)

Elastic Detection

process where host.os.type == "windows" and
  event.category == "process" and
  (
    (process.name : ("w3wp.exe", "dotnet.exe") and process.command_line : "*OutputDebugStringAppender*") or
    process.command_line : "*\u0000*"
  )

Splunk SPL Query

index=windows (sourcetype=WinEventLog:* OR sourcetype=XmlWinEventLog:*)
| eval nul_present=if(like(_raw, "%" . "\x00" . "%"), 1, 0)
| where nul_present=1 OR match(_raw, "(?i)OutputDebugStringAppender")
| stats count earliest(_time) as first_seen latest(_time) as last_seen by host, _raw
| where count > 0
| sort - count

Wazuh OSSEC Rule

<group name="windows,log4net,cve-2026-105240,">
  <rule id="202610" level="6">
    <if_sid>60000</if_sid>
    <field name="win.eventdata.commandLine">OutputDebugStringAppender</field>
    <description>CVE-2026-105240: log4net OutputDebugStringAppender referenced in process execution on $(win.system.computer)</description>
    <mitre>
      <id>T1565.001</id>
    </mitre>
  </rule>
  <rule id="202611" level="8">
    <if_sid>60000</if_sid>
    <field name="win.eventdata.commandLine">\x00</field>
    <description>CVE-2026-105240: Possible NUL byte injected into logged content on $(win.system.computer)</description>
    <mitre>
      <id>T1565.001</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations.

   • Where feasible, stop using OutputDebugStringAppender in production configurations on Windows, or replace it with an appender that does not rely on the NUL-terminated Windows debug output channel (the advisory explicitly scopes the flaw to this appender and this platform).
   • Sanitize or reject NUL characters in any externally influenced input before it is written into a log message, at the application boundary.
   • Restrict who can influence logged content and monitor for NUL characters appearing in logging telemetry.
   • Ensure a secondary, reliable audit log channel is in place so that truncated debug output does not create a monitoring blind spot.

2. Patch/upgrade steps.

   • The Apache log4net advisory recommends upgrading to version 3.5.0, which fixes the issue. Affected range: 1.2.9 before 3.5.0.
   • Because Patch Confidence is 0/4 and no patch source has been confirmed by tracked feeds, verify the 3.5.0 release and its integrity against the official Apache log4net distribution and release notes before deployment, and validate the fix in a staging environment.
   • Inventory all Windows applications and build dependencies that reference log4net, and determine which are configured with OutputDebugStringAppender to prioritize remediation.

3. Long-term hardening.

   • Adopt a dependency inventory process that tracks logging-library versions and their appender configurations across all .NET/Windows applications.
   • Add input-validation and log-sanitization controls (including NUL-byte stripping) as a standard logging-layer policy.
   • Maintain immutable/off-host log storage so integrity manipulation of any single channel does not erase the audit trail.
   • Monitor the Apache log4net project and CVE feeds for the official 3.5.0 patch confirmation and any updates to the affected range.

References

• NVD — CVE-2026-105240: https://nvd.nist.gov/vuln/detail/CVE-2026-105240
• Apache log4net project and release notes (upgrade target 3.5.0): https://logging.apache.org/log4net/
• CWE-158 — Improper Neutralization of Null Byte or NUL Character: https://cwe.mitre.org/data/definitions/158.html
• FIRST.org EPSS (no score yet published for this CVE): https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (CVE not listed as of writing): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence are associated with CVE-2026-105240 in the provided intelligence. No public PoC, Nuclei template, or exploit code is available, and the CVE is not in CISA KEV.