CVE-2026-33603

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.

Dovecot is an IMAP server for Linux/UNIX-like systems, written with security primarily in mind. It also contains a small POP3 server. It supports mail in either of maildir or mbox formats.

Security Fix(es):

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)

Attacker can upload a malicious Sieve script over ManageSieve service (or locally) to bypass configured CPU time limits for Sieve up to 130 times of the configured limit. Attacker can use this to degrade server performance and bypass configured CPU time limits for Sieve scripts. Install fixed version, or alternatively prevent direct access to Sieve scripts via ManageSieve or local access. No publicly available exploits are known.(CVE-2026-40016)

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. This causes folders to be spammed to all users. The impact is limited to being able to spam folders to other users, no unexpected access is gained. Install to fixed version. No publicly available exploits are known.(CVE-2026-40020)

An attacker can cause uncontrolled memory usage with excessive bracing over IMAP. The fix in CVE-2026-27857 was incomplete, only blocking one way of doing this, so there was still another way left open. In particular, the fix was for closing braces, but you could still use open braces to bypass the limit. Using excessive bracing, attacker can cause memory usage up to configured memory limit. Install fixed version, or configure vsz_limit for imap process to low value. No publicly available exploits are known.(CVE-2026-42006)

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.

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-2026-40020: imap-acl: reject CR/LF injection in SETACL identifier
• CVE-2026-42006: lib-imap: fix list_count_limit to actually limit '(' (incomplete fix for CVE-2026-27857)
• CVE-2026-33603: login-common: reject non-base64 SASL responses to prevent SCRAM channel binding bypass
• CVE-2026-40016: lib-sieve: enforce sieve_max_cpu_time inside :contains/:matches matcher loops

These are all security issues fixed in the dovecot24-2.4.4-1.1 package on the GA media of openSUSE Tumbleweed.

This update for dovecot24 fixes the following issues

• CVE-2026-27851: lib-var-expand: safe filter leaks to all following pipelines (bsc#1265146).
• CVE-2026-33603: login: base64 input can contain tabs that bypass IPC protection (bsc#1265147).
• CVE-2026-40016: Sieve: contains/: matches O(NxM) substring match bypasses sieve_max_cpu_time limit (bsc#1265148).
• CVE-2026-40020: IMAP folders can be shared-spammed to everyone (bsc#1265149).
• CVE-2026-42006: imap-login: uncontrolled memory usage with excessive bracing over IMAP (bsc#1265150).

Changes for dovecot24:

• Update to 2.4.4

This update for dovecot24 fixes the following issues

• CVE-2026-27851: lib-var-expand: safe filter leaks to all following pipelines (bsc#1265146).
• CVE-2026-33603: login: base64 input can contain tabs that bypass IPC protection (bsc#1265147).
• CVE-2026-40016: Sieve: contains/: matches O(NxM) substring match bypasses sieve_max_cpu_time limit (bsc#1265148).
• CVE-2026-40020: IMAP folders can be shared-spammed to everyone (bsc#1265149).
• CVE-2026-42006: imap-login: uncontrolled memory usage with excessive bracing over IMAP (bsc#1265150).

Changes for dovecot24:

• Update to 2.4.4

This update for dovecot22 fixes the following issues

• CVE-2026-33603: login: base64 input can contain tabs that bypass IPC protection (bsc#1265147).
• CVE-2026-40020: IMAP folders can be shared-spammed to everyone (bsc#1265149).
• CVE-2026-42006: imap-login: uncontrolled memory usage with excessive bracing over IMAP (bsc#1265150).

This update for dovecot23 fixes the following issues:

• CVE-2024-23184: parsing of messages containing many address headers (From, To, Cc, Bcc, etc.) could be excessively CPU intensive (bsc#1229184).
• CVE-2024-23185: large headers can cause resource exhaustion when parsing message (bsc#1229183).
• CVE-2025-59028: Invalid base64 authentication can cause DoS for other logins (bsc#1260894).
• CVE-2025-59031: decode2text.sh OOXML extraction may follow symlinks and read unintended files during indexing (bsc#1260895).
• CVE-2025-59032: pigeonhole: ManageSieve panic occurs with sieve-connect as a client (bsc#1260902).
• CVE-2026-27852: DoS by sending mail with bad header (bsc#1276799).
• CVE-2026-27855: OTP driver vulnerable to replay attack (bsc#1260900).
• CVE-2026-27856: Doveadm credentials were not checked using timing-safe checking function (bsc#1260899).
• CVE-2026-27857: sending excessive parenthesis causes imap-login to use excessive memory (bsc#1260898).
• CVE-2026-27858: pigeonhole: managesieve-login can allocate large amount of memory during authentication (bsc#1260901).
• CVE-2026-27859: excessive RFC 2231 MIME parameters in email would can excessive CPU usage (bsc#1260897).
• CVE-2026-33263: submission-login: Panic when mail_max_userip_connections is reached (bsc#1276794).
• CVE-2026-33603: login: base64 input can contain tabs that bypass IPC protection (bsc#1265147).
• CVE-2026-33604: SMTP Smuggling via Missing Dot-Stuffing (bsc#1276802).
• CVE-2026-33605: managesieve-login: Pre-auth crash (bsc#1276809).
• CVE-2026-33606: dsync: Mail content can cause dsync protocol injection (bsc#1276800).
• CVE-2026-33607: IMAP LIST match_sub() Exponential Backtracking -- CPU Denial of Service (bsc#1276795).
• CVE-2026-40014: CPU DoS via Crafted References Header (bsc#1276804).
• CVE-2026-40015: imap-hibernate can be crashed (bsc#1276812).
• CVE-2026-40016: Sieve :contains/:matches O(NxM) substring match bypasses sieve_max_cpu_time limit (bsc#1265148).
• CVE-2026-40019: managesieve-login pre-auth infinite loop (bsc#1276811).
• CVE-2026-40020: IMAP folders can be shared-spammed to everyone (bsc#1265149).
• CVE-2026-40203: IMAP Compression Can Reveal Whether a Small Synced Email Body Matches Sender-Chosen Text (bsc#1276815).
• CVE-2026-40205: OAuth2 passdb scope enforcement bypass via OR semantics in remote validation path (bsc#1276820).
• CVE-2026-42006: imap-login: uncontrolled memory usage with excessive bracing over IMAP (bsc#1265150).
• CVE-2026-42007: editheader RCE (bsc#1276817).
• CVE-2026-42008: XCLIENT FORWARD= bare token not namespaced (bsc#1276824).
• CVE-2026-42391: imap: Pre-login memory/CPU growth with ID command (bsc#1276835).
• CVE-2026-42393: doveadm_password or api key length can still be leaked with timing comparisons (bsc#1276827).
• CVE-2026-42395: Single NUL-Byte XCLIENT FORWARD Payload Crashes (bsc#1276826).
• CVE-2026-52681: Sieve resource usage tracking lost when active script changes (bsc#1276828).
• CVE-2026-52687: IMAP: COMPRESS ZSTD can cause excessive memory usage (bsc#1276837).
• CVE-2026-73208: auth: db-oauth2: aud claim used as fallback for missing scope claim (bsc#1276830).
• CVE-2026-73209: imap-login crash: Self-recursion on zero-output decompress chunks (bsc#1276833).

Changes for dovecot23:

• Update to version 2.3.21.1.