Attacker can use a specially crafted base64 exchange between Dovecot and Client to fake SCRAM TLS channel binding. This requires that the attacker is able to position itself between Dovecot and the client connection. If successful, the attacker can eavesdrop communications between Dovecot and client as MITM proxy. Install fixed version. No publicly available exploits are known.

The remote Debian 11 host has packages installed that are affected by multiple vulnerabilities as referenced in the dla-4617 advisory.

    -------------------------------------------------------------------------     Debian LTS Advisory DLA-4617-1                debian-lts@lists.debian.org     https://www.debian.org/lts/security/                       Guilhem Moulin     June 05, 2026                                 https://wiki.debian.org/LTS
    -------------------------------------------------------------------------

    Package        : dovecot     Version        : 1:2.3.13+dfsg1-2+deb11u4     CVE ID         : CVE-2026-33603 CVE-2026-40020 CVE-2026-42006     Debian Bug     : 1136444

    Multiple vulnerabilities were discovered in dovecot, a POP3/IMAP server,     which could lead to Denial of Service or information leak.

    CVE-2026-33603

        An attacker can use a specially crafted base64 exchange between         Dovecot and Client to fake SCRAM TLS channel binding and later         eavesdrop communications between Dovecot and client as MITM proxy.

    CVE-2026-40020

        An attacker can use the IMAP `SETACL` command to inject the anyone         permission to user's dovecot-acl file even if         `imap_acl_allow_anyone=no`, thereby allowing folders to be spammed         to all users. (The impact was limited to being able to spam folders         to other users. No unexpected access is gained.)

    CVE-2026-42006

        An attacker can cause uncontrolled memory usage with excessive         bracing over IMAP. This stems from an incomplete fix for         CVE-2026-27857.

    For Debian 11 bullseye, these problems have been fixed in version     1:2.3.13+dfsg1-2+deb11u4.

    We recommend that you upgrade your dovecot packages.

    For the detailed security status of dovecot please refer to     its security tracker page at:
    https://security-tracker.debian.org/tracker/dovecot

    Further information about Debian LTS security advisories, how to apply     these updates to your system and frequently asked questions can be     found at: https://wiki.debian.org/LTS     Attachment:
    signature.asc     Description: PGP signature

Tenable has extracted the preceding description block directly from the Debian security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote Ubuntu 22.04 LTS / 24.04 LTS / 25.10 / 26.04 LTS host has packages installed that are affected by multiple vulnerabilities as referenced in the USN-8365-1 advisory.

    It was discovered that Dovecot incorrectly treated some variable expansion pipelines as safe in     authentication filters. An attacker could possibly use this issue to perform SQL or LDAP injection     attacks. This issue only affected Ubuntu 25.10 and Ubuntu 26.04 LTS. (CVE-2026-27851)

    It was discovered that Dovecot incorrectly verified SCRAM TLS channel binding in certain base64 exchanges.
    A remote attacker could possibly use this issue to obtain sensitive information in a machine-in-the-middle     attack. (CVE-2026-33603)

    It was discovered that Dovecot incorrectly enforced Sieve script CPU limits. An attacker could possibly     use this issue to cause Dovecot to use excessive resources, leading to a denial of service.
    (CVE-2026-40016)

    It was discovered that Dovecot incorrectly handled certain IMAP SETACL commands. An attacker could     possibly use this issue to spam folders to other users. (CVE-2026-40020)

    It was discovered that Dovecot incorrectly handled excessive IMAP bracing. An attacker could possibly use     this issue to cause Dovecot to use excessive resources, leading to a denial of service. (CVE-2026-42006)

Tenable has extracted the preceding description block directly from the Ubuntu security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote Fedora 43 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-693373747f advisory.

    * CVE-2026-27851: lib-var-expand: Safe filter marks all following pipelines safe.
    * CVE-2026-33603: auth: CRAM-SHA-*-PLUS channel binding could be faked.
      MITM attacker with a certificate trusted by the client could have       bypassed the requirement for channel binding.
    * CVE-2026-40020: IMAP folders can be shared-spammed to everyone.
    * CVE-2026-42006: An attacker can cause uncontrolled memory usage with       excessive bracing over IMAP. The fix in CVE-2026-27857 was incomplete.
    * indexer-worker, quota-status, script-login, program-client-local: Root       privileges are now dropped permanently before serving requests.
    * indexer-worker: Default restart_request_count changed to 1 to work       correctly after permanent root privilege drop.
    * lmtp: Add back service_extra_groups=$SET:default_internal_group that was       incorrectly removed in v2.4.3.
    * master: inet_listener_reuse_port has been replaced by service_reuse_port.
      The new setting properly pre-creates all listener sockets at startup and       assigns one unique socket per process. Using this allows evenly distributing       incoming connections to login processes.


Tenable has extracted the preceding description block directly from the Fedora security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote Debian 12 / 13 host has packages installed that are affected by multiple vulnerabilities as referenced in the dsa-6313 advisory.

    - -------------------------------------------------------------------------     Debian Security Advisory DSA-6313-1                   security@debian.org     https://www.debian.org/security/                       Moritz Muehlenhoff     May 31, 2026                          https://www.debian.org/security/faq
    - -------------------------------------------------------------------------

    Package        : dovecot     CVE ID         : CVE-2026-33603 CVE-2026-40016 CVE-2026-40020                      CVE-2026-42006 CVE-2026-27851

    Multiple vulnerabilities have been discovered in the Dovecot IMAP server     which way result in denial of service, SQL injection or     man-in-the-midddle attacks

    For the oldstable distribution (bookworm), these problems have been fixed     in version 1:2.3.19.1+dfsg1-2.1+deb12u6.

    For the stable distribution (trixie), these problems have been fixed in     version 1:2.4.1+dfsg1-6+deb13u6.

    We recommend that you upgrade your dovecot packages.

    For the detailed security status of dovecot please refer to     its security tracker page at:
    https://security-tracker.debian.org/tracker/dovecot

    Further information about Debian Security Advisories, how to apply     these updates to your system and frequently asked questions can be     found at: https://www.debian.org/security/

    Mailing list: debian-security-announce@lists.debian.org

Tenable has extracted the preceding description block directly from the Debian security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote Fedora 44 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-96eeb03b88 advisory.

    * CVE-2026-27851: lib-var-expand: Safe filter marks all following pipelines safe.
    * CVE-2026-33603: auth: CRAM-SHA-*-PLUS channel binding could be faked.
      MITM attacker with a certificate trusted by the client could have       bypassed the requirement for channel binding.
    * CVE-2026-40020: IMAP folders can be shared-spammed to everyone.
    * CVE-2026-42006: An attacker can cause uncontrolled memory usage with       excessive bracing over IMAP. The fix in CVE-2026-27857 was incomplete.
    * indexer-worker, quota-status, script-login, program-client-local: Root       privileges are now dropped permanently before serving requests.
    * indexer-worker: Default restart_request_count changed to 1 to work       correctly after permanent root privilege drop.
    * lmtp: Add back service_extra_groups=$SET:default_internal_group that was       incorrectly removed in v2.4.3.
    * master: inet_listener_reuse_port has been replaced by service_reuse_port.
      The new setting properly pre-creates all listener sockets at startup and       assigns one unique socket per process. Using this allows evenly distributing       incoming connections to login processes.


Tenable has extracted the preceding description block directly from the Fedora security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - Attacker can use a specially crafted base64 exchange between Dovecot and Client to fake SCRAM TLS channel     binding. This requires that the attacker is able to position itself between Dovecot and the client     connection. If successful, the attacker can eavesdrop communications between Dovecot and client as MITM     proxy. Install fixed version. No publicly available exploits are known. (CVE-2026-33603)

Note that Nessus relies on the presence of the package as reported by the vendor.

ID	Name	Product	Family	Severity
318856	Debian dla-4617 : dovecot-auth-lua - security update	Nessus	Debian Local Security Checks	medium
318619	Ubuntu 22.04 LTS / 24.04 LTS / 25.10 / 26.04 LTS : Dovecot vulnerabilities (USN-8365-1)	Nessus	Ubuntu Local Security Checks	critical
318284	Fedora 43 : dovecot (2026-693373747f)	Nessus	Fedora Local Security Checks	critical
318247	Debian dsa-6313 : dovecot-auth-lua - security update	Nessus	Debian Local Security Checks	critical
318245	Fedora 44 : dovecot (2026-96eeb03b88)	Nessus	Fedora Local Security Checks	critical
315673	Linux Distros Unpatched Vulnerability : CVE-2026-33603	Nessus	Misc.	medium

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CVE-2026-33603

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