Email spoofing via SMTP message-boundary confusion
Published Dec 24, 2023 · Updated Nov 4, 2025
SMTP smuggling in Exim 4.97 and earlier allows remote attackers to inject spoofed messages through PIPELINING and CHUNKING. During DATA reception, Exim treats nonstandard LF-dot line endings as an end-of-data marker, then parses the remaining body bytes as new SMTP commands. Exploitation requires a relay that forwards one of those sequences while Exim advertises both extensions, allowing a second message to bypass relay validation and SPF-based sender checks.
Summary
What happened
SMTP smuggling in Exim 4.97 and earlier allows remote attackers to inject spoofed messages through PIPELINING and CHUNKING. During DATA reception, Exim treats nonstandard LF-dot line endings as an end-of-data marker, then parses the remaining body bytes as new SMTP commands. Exploitation requires a relay that forwards one of those sequences while Exim advertises both extensions, allowing a second message to bypass relay validation and SPF-based sender checks.
The record
- CVE
- CVE-2023-51766
- Published
- Dec 24, 2023
- Updated
- Nov 4, 2025
- Vendor
- Unknown vendor
- Product
- Unknown product
- Classifications
- CWE-345, T1566
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Dec 24, 2023CVE publishedPublication date reported by the CVE source.
- Nov 4, 2025Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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What is affected?
Affected products and versions are unavailable in this record.
Published CVSS scores
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Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Public exploit references
- SEC Consult SMTP Smuggling proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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