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CVE-2026-105243— Apache log4net: Oversize EventLogAppender record silently discarded

Quick assessment

Affected
Apache Software Foundation Apache log4net
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Apache log4net 的 EventLogAppender 存在日志记录不足的漏洞。 当日志消息过长时，其内容会被截断至固定长度。在计入日志名和事件源名的长度后，该固定长度可能超过 Windows 事件日志所接受的上限。此时，事件日志无法存储任何内容，也不会生成任何报告。攻击者可通过构造足够长的日志消息，使其自身数据到达日志记录点，从而阻止整条日志记录的生成。 此漏洞仅影响在 Windows 平台上使用 EventLogAppender 的应用程序。 受影响的 Apache log4net 版本范围为：1.

CVSS 5.3 · Medium

Possible ATT&CK Techniques 2 AI

T1078 · Valid Accounts T1562

Affected Version Matrix 2

VendorProduct Version RangeStatus
Apache Software Foundation Apache log4net 1.2.9< 3.5.0 affected
02e1e115435888485f2e28b414d267e39e799e07< 28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed affected
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I. Basic Information for CVE-2026-105243

Vulnerability Information
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Vulnerability Title
Apache log4net: Oversize EventLogAppender record silently discarded
Source: CVE Program / CVE List V5
Vulnerability Description
Insufficient Logging vulnerability in the EventLogAppender of Apache log4net. Long messages were truncated to a fixed size that exceeds what the Windows Event Log accepts once the log and source names are counted, and the event log then stored nothing and reported nothing. A party whose data reaches a log message could suppress the whole record by making it long enough. Only applications on Windows that use EventLogAppender are affected. This issue affects Apache log4net: from 1.2.9 before 3.5.0. Users are recommended to upgrade to version 3.5.0, which fixes the issue.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
不充分的日志记录
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Apache Software Foundation Apache log4net 1.2.9 ~ 3.5.0 -
Apache Software Foundation Apache log4net 02e1e115435888485f2e28b414d267e39e799e07 ~ 28fbfb25678c48a8cc5bc9b94ead0dddfc39ffed -

II. Public POCs for CVE-2026-105243

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III. Intelligence Information for CVE-2026-105243

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Other References for CVE-2026-105243 (3)

Same Patch Batch · Apache Software Foundation · 2026-10-06 · 8 CVEs total

CVE-2026-94114 5.9 MEDIUM Apache Commons BCEL: Nested Code/Record attributes drive unbounded parse-time recursion in
CVE-2026-105244 5.3 MEDIUM Apache log4net: RemoteSyslogAppender silently deletes non-ASCII content
CVE-2026-105242 5.3 MEDIUM Apache log4net: Request validation failure drops the event in the aspnet-request converter
CVE-2026-105241 5.3 MEDIUM Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch
CVE-2026-105240 5.3 MEDIUM Apache log4net: NUL character truncates OutputDebugStringAppender records
CVE-2026-105239 5.3 MEDIUM Apache log4net: NUL character truncates EventLogAppender records
CVE-2026-105111 4.7 MEDIUM Apache Commons BCEL: Class2HTML emits unescaped class strings, enabling stored XSS

IV. Related Vulnerabilities

Same product: Apache log4net
Same vendor: Apache Software Foundation
Same weakness: CWE-778

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