Heap data disclosure via unterminated UTF-8 decoding
Published Apr 30, 2026 · Updated Apr 30, 2026
Out-of-bounds read in Exim before 4.99.2 allows remote attackers to disclose heap data via malformed UTF-8 message headers. The ${from_utf8:} expansion operator trusts the leading byte's continuation count and, without checking for the string terminator, advances past truncated input into adjacent heap memory. Exposure requires a configuration that applies the operator to externally supplied header data and returns its result in an SMTP rejection for a later message on the same connection.
Summary
What happened
Out-of-bounds read in Exim before 4.99.2 allows remote attackers to disclose heap data via malformed UTF-8 message headers. The ${from_utf8:} expansion operator trusts the leading byte's continuation count and, without checking for the string terminator, advances past truncated input into adjacent heap memory. Exposure requires a configuration that applies the operator to externally supplied header data and returns its result in an SMTP rejection for a later message on the same connection.
The record
- CVE
- CVE-2026-40686
- Published
- Apr 30, 2026
- Updated
- Apr 30, 2026
- Vendor
- LibSPF2 Project
- Product
- Exim
- Classifications
- CWE-125
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 30, 2026CVE publishedPublication date reported by the CVE source.
- Apr 30, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=0 <4.99.2
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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