CVE-2026-105241: Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch
- Is it exploited?
Not on CISA's list. CISA has not added it to the Known Exploited Vulnerabilities catalog as of . If CISA adds it, this page shows it as known exploited.
- How likely is exploitation in the next 30 days?
- 0.8%
EPSS score, percentile 57.3 of all scored records. A probability estimated by FIRST, not a report of exploitation.
- How severe?
- 5.3 Medium
CVSS v3.1 base score, given by the CNA
- What should I do?
CISA has set no required action, because the vulnerability is not in KEV.
- Which versions are affected?
Affected
- from 1.2.9 to before 3.5.0
Source: CISA KEV 2026.10.29, as of ; FIRST EPSS, as of ; CVE List, as of . About the data.
What to do
Advisories and patches
This summary is based on the CVE record as of . Check the vendor's advisory before you act: affected versions and fixes can change.
Affected products and versions
As listed in the CVE record.
| Status | Versions |
|---|---|
| Affected | from 1.2.9 to before 3.5.0 |
Versions not listed are: unaffected.
| Status | Versions |
|---|---|
| Affected | from 02e1e1154358 to before 4d2e10f09081 |
Versions not listed are: unaffected.
Source: CVE List, as of . About the data.
About this vulnerability
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…
Summary: CVE record, as of .
- CISA has not added CVE-2026-105241 to its Known Exploited Vulnerabilities (KEV) catalog, as of 29 Oct 2026.
- The CVE record was published on 6 Oct 2026.
- The CVE record carries a CVSS v3.1 base score of 5.3 (Medium), given by its CNA.
- FIRST's EPSS model puts the probability of exploitation in the next 30 days at 0.8% (percentile 57.3), as of 29 Oct 2026. EPSS is a statistical estimate, not a report of exploitation.
- CISA's SSVC assessment in the CVE record: exploitation none, automatable yes, technical impact partial.
- The credits in the CVE record name Anthropic (Claude) as finder.
Source: CISA KEV 2026.10.29, as of ; CVE List, as of ; FIRST EPSS, as of . About the data.
Description
Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch
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 List, as of . About the data.
Weakness
CWE-176
- CWE-176: Improper Handling of Unicode Encoding
CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record.
Timeline
| Date | Event |
|---|---|
| CVE record published | |
| CVE record last updated |
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