Medium [CVE-2026-33604] SMTP smuggling vulnerability allows spoofed email injection
This medium-severity Red Hat Linux advisory covers CVE-2026-33604 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7.
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Summary
An attacker that can get Dovecot to relay a message, for example through Sieve redirect or submission relay, can use a crafted line ending in the message body to bypass the outbound protection that prevents message content from being interpreted as SMTP commands.
A downstream mail server that hasn't yet fixed the SMTP smuggling vulnerability can be tricked into treating part of the message body as new SMTP commands, allowing injection of spoofed email. This is the same vulnerability class as CVE-2023-51764 and CVE-2023-51766.
Where you control the receiving mail servers, ensure they reject bare carriage returns in message data. Update to non-vulnerable version.
No publicly available exploits are known. A flaw was found in Dovecot.
A remote attacker can exploit this by crafting a message with a specific line ending. If Dovecot relays this message, it can bypass security protections, allowing the attacker to inject spoofed email into a vulnerable downstream mail server.
This vulnerability is related to SMTP (Simple Mail Transfer Protocol) smuggling. Red Hat severity: Moderate — CVSS 5.9 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N).
Weakness: CWE-93. Affected Red Hat products: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
Mitigation checklist
- Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
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