{"id":"CVE-2026-107566","kind":"vulnerability","aliases":["CVE-2026-107566"],"related":[],"title":"hMailServer: Sieve and mail rules let users send mail as any address","description":"Missing authorization in the Sieve editheader action (6.2.22 through 6.3.6) and in the mail-rule Set header value action (6.0.0 through 6.3.6) of Progressive Robot hMailServer allows an authenticated user to make the server send mail as any address, DKIM-signed where the address is at a domain the server signs for. A Sieve script's editheader (addheader/deleteheader) could add or delete any header field except Received and Return-Path, and the server applies a script's header edits before it queues a redirect, so the redirected copy is sent as the script left it. The server signs a message's first delivery pass as the domain named in its From header wherever it signs DKIM for that domain and the message carries no signature of it. A script could therefore delete and re-add From - another account's address, or anyone's at a hosted domain - and redirect, so the server sent the message in that name and, for a hosted domain, signed it with that domain's DKIM key, a forgery aligned to pass the domain's DMARC; or, where another sender's domain is one the server signs for, delete the DKIM-Signature that sender's message arrived with, alter a field, and redirect, so the server re-signed the altered message as that sender's domain. editheader placed no restriction on From, Sender, Resent-From, Resent-Sender or DKIM-Signature. The editheader action was introduced in 6.2.22. In the mail-rule engine, a domain's or an account's rule's Set header value action could likewise set From, Sender, Resent-From, Resent-Sender or DKIM-Signature ahead of a forward or a copy, whoever the rule belonged to, so that an account acting on its own rules over COM, a domain administrator, or an API key confined to a domain (over the REST API from 6.3.4) could have the server forward or copy a message in the name of any address, signed, where that address is at a domain the server hosts with DKIM signing enabled, with that domain's key.","summary":"An authenticated hMailServer user can use a Sieve editheader action or a mail-rule Set header value action to rewrite From, Sender, Resent-From, Resent-Sender or DKIM-Signature before forwarding or copying a message. The server then sends the mail in that forged name and may DKIM-sign it with a hosted domain's key, so the forgery passes DMARC.","severity":"medium","cvss":6.5,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["dkim","dmarc","email","hmailserver","package","self-hosted"],"not_affected_if":["default-config","no-sieve-or-mail-rules"],"how_to_check":"- Check the hMailServer version with the admin tool or the installed package; affected releases are `6.2.22` through `6.3.6` for Sieve editheader and `6.0.0` through `6.3.6` for mail-rule Set header value.\n- Review Sieve scripts and mail rules that use editheader or Set header value for header changes to From, Sender, Resent-From, Resent-Sender or DKIM-Signature.\n- Confirm whether the server has DKIM signing enabled for hosted domains.","fix":"- Upgrade hMailServer to a release later than `6.3.6`; the sources do not name the exact fixed version.\n- Until patched, remove or disable Sieve editheader and mail-rule Set header value actions for sender-identity headers.\n- Restrict who can create rules and scripts, and review existing forward or copy rules.","status":"open","published_at":"2026-10-19T16:17:01+00:00","first_seen_at":"2026-10-23T21:57:18+00:00","triaged_at":"2026-10-25T03:55:56+00:00","updated_at":"2026-10-25T03:55:56+00:00","category":"package","confidence":0.7,"triage":{"at":"2026-10-25T03:55:56Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade hMailServer to a release later than `6.3.6`; the sources do not name the exact fixed version.\n- Until patched, remove or disable Sieve editheader and mail-rule Set header value actions for sender-identity headers.\n- Restrict who can create rules and scripts, and review existing forward or copy rules.","reach":"network","title":"hMailServer: Sieve and mail rules let users send mail as any address","impact":"medium","summary":"An authenticated hMailServer user can use a Sieve editheader action or a mail-rule Set header value action to rewrite From, Sender, Resent-From, Resent-Sender or DKIM-Signature before forwarding or copying a message. The server then sends the mail in that forged name and may DKIM-sign it with a hosted domain's key, so the forgery passes DMARC.","category":"package","nicknames":[],"who_cares":["hmailserver","self-hosted","email","dkim","dmarc"],"confidence":0.7,"needs_auth":"yes","worst_case":"Attacker sends mail as any hosted address with valid DKIM signature","analyst_note":"Find the fixed hMailServer version and vendor advisory from the NVD entry (https://nvd.nist.gov/vuln/detail/CVE-2026-107566) and check whether Sieve editheader defaults to enabled.","how_to_check":"- Check the hMailServer version with the admin tool or the installed package; affected releases are `6.2.22` through `6.3.6` for Sieve editheader and `6.0.0` through `6.3.6` for mail-rule Set header value.\n- Review Sieve scripts and mail rules that use editheader or Set header value for header changes to From, Sender, Resent-From, Resent-Sender or DKIM-Signature.\n- Confirm whether the server has DKIM signing enabled for hosted domains.","needs_analyst":true,"not_affected_if":["default-config","no-sieve-or-mail-rules"],"who_is_affected":"Servers running hMailServer in the affected version ranges that let users create Sieve scripts or mail rules, or that have DKIM signing enabled for hosted domains, are exposed. Any authenticated user, a domain administrator, or an API key confined to a domain can trigger it. Servers without these rule features or without DKIM signing for hosted domains are less exposed.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"medium"},"cost_usd":0.000802,"input_tokens":3851,"output_tokens":834,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-107566':15A,18A '-2026':14A,17A '6.0.0':113C '6.2.22':100C,342C '6.3.4':416C '6.3.6':102C,115C 'account':234C,354C,392C 'act':393C 'action':36B,45B,99C,112C,338C,361C 'add':154C,231C 'addheader/deleteheader':152C 'address':12A,132C,138C,236C,431C,435C 'administr':402C 'ahead':377C 'align':270C 'allow':120C 'alter':300C,312C 'anoth':233C,279C 'anyon':238C 'api':405C,414C 'appli':169C 'arriv':298C 'authent':28B,122C 'author':94C 'belong':387C 'carri':218C 'com':399C 'confin':407C 'copi':63B,183C,383C,423C 'could':153C,225C,362C,417C 'cve':13A,16A 'delet':156C,227C,289C 'deliveri':198C 'dkim':19B,58B,79B,134C,211C,266C,292C,334C,375C,444C 'dkim-sign':78B,133C 'dkim-signatur':57B,291C,333C,374C 'dmarc':20B,92B,276C 'domain':85B,142C,202C,214C,243C,259C,264C,274C,282C,318C,350C,401C,410C,439C,449C 'edit':174C 'edithead':35B,98C,151C,319C,337C 'email':21B 'enabl':446C 'engin':348C 'except':160C 'field':159C,302C 'first':197C 'forg':74B 'forgeri':90B,269C 'forward':61B,380C,421C 'header':43B,110C,158C,173C,207C,359C 'hmailserv':1A,22B,29B,119C 'host':26B,84B,242C,258C,442C 'introduc':340C 'key':87B,267C,406C,451C 'left':189C 'let':6A 'likewis':363C 'mail':4A,9A,40B,71B,107C,129C,346C 'mail-rul':39B,106C,345C 'make':125C 'may':77B 'messag':65B,195C,217C,251C,297C,313C,425C 'miss':93C 'name':75B,203C,254C,428C 'one':284C 'packag':23B 'pass':91B,199C,272C 'path':165C 'place':320C 'progress':117C 'queue':177C 're':230C,309C 're-add':229C 're-sign':308C 'receiv':161C 'redirect':179C,182C,245C,304C 'resent':51B,54B,327C,330C,368C,371C 'resent-from':50B,326C,367C 'resent-send':53B,329C,370C 'rest':413C 'restrict':322C 'return':164C 'return-path':163C 'rewrit':47B 'robot':118C 'rule':5A,41B,108C,347C,356C,386C,397C 'script':149C,171C,188C,224C 'self':25B 'self-host':24B 'send':8A,69B,128C 'sender':49B,55B,280C,295C,316C,325C,331C,366C,372C 'sent':185C,249C 'server':67B,127C,144C,168C,192C,248C,286C,307C,420C,441C 'set':42B,109C,358C,364C 'siev':2A,34B,97C,148C 'sign':80B,135C,145C,193C,210C,260C,287C,310C,432C,445C 'signatur':59B,220C,293C,335C,376C 'therefor':226C 'use':32B 'user':7A,30B,123C 'valu':44B,111C,360C 'wherev':208C 'whoever':384C","nicknames":[],"mitigations":[],"who_affected":"Servers running hMailServer in the affected version ranges that let users create Sieve scripts or mail rules, or that have DKIM signing enabled for hosted domains, are exposed. Any authenticated user, a domain administrator, or an API key confined to a domain can trigger it. Servers without these rule features or without DKIM signing for hosted domains are less exposed.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-19T16:17:01+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N","cvss_by":"cve@gitlab.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-107566","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-107566","severity":null,"published_at":"2026-10-19T16:17:01+00:00","modified_at":"2026-10-20T20:17:13+00:00","first_seen_at":"2026-10-23T21:57:18+00:00","fetched_at":"2026-10-23T22:28:57+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}