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CVE-2026-105241— Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch

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 中 SmtpPickupDirAppender 存在不当处理 Unicode 编码的漏洞。 当邮件文件写入器无法编码某些内容（例如未配对的 UTF-16 代理对）时，会导致写入操作抛出异常。在此情况下，批处理中的每一个缓冲事件（包括非问题事件）都会被丢弃，仅问题事件所携带的内容导致异常，但最终结果是整个批次的所有日志事件记录全部丢失，且在邮件拾取目录中可能残留一封不完整（截断）的邮件。如果攻击者能够控制数据并将其插入到日志消息中，则可阻止其他事件记录的保存，从而抑制日志记录。 仅使用

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< 4d2e10f0908199604b4326f9df6d0b43b871e333 affected
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I. Basic Information for CVE-2026-105241

Vulnerability Information
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Vulnerability Title
Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch
Source: CVE Program / CVE List V5
Vulnerability Description
Improper Handling of Unicode Encoding vulnerability in the SmtpPickupDirAppender of Apache log4net. Content that the mail file writer cannot encode, such as an unpaired UTF-16 surrogate, made the write throw. Every buffered event in the batch was discarded, not only the one carrying the content, and a truncated mail could be left in the pickup directory. A party whose data reaches a log message could suppress the records of other events. Only applications that use SmtpPickupDirAppender 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
Unicode编码处理不恰当
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 ~ 4d2e10f0908199604b4326f9df6d0b43b871e333 -

II. Public POCs for CVE-2026-105241

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III. Intelligence Information for CVE-2026-105241

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Other References for CVE-2026-105241 (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-2026-105243 5.3 MEDIUM Apache log4net: Oversize EventLogAppender record silently discarded
CVE-2026-105242 5.3 MEDIUM Apache log4net: Request validation failure drops the event in the aspnet-request converter
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-176

V. Comments for CVE-2026-105241

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