{"id":"CVE-2026-107569","kind":"vulnerability","aliases":["CVE-2026-107569"],"related":[],"title":"hMailServer: a crafted loop-count header lets mail flood the server","description":"Unchecked input for a loop condition in Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to stop the server carrying mail. The server bounds the mail it generates in response to a message - a rule forward, reply or copy, an account forward, a Sieve redirect, vacation or notify, a bounce, an out-of-office reply - by a counter it carries in the message's X-hMailServer-LoopCount header, and makes no more once the counter reaches the configured rule loop limit (5 by default); that is what ends a forwarding or auto-reply cycle in a bounded number of hops. X-hMailServer-LoopCount is an ordinary header that is not stripped from mail arriving from outside, and the server read it with a signed conversion and no validation, so a count below zero passed every test against the limit: a sender who sets X-hMailServer-LoopCount to a negative value, or on the Linux builds to a value above the 32-bit signed range that the conversion wraps to a negative one, lifts the bound, and a message that enters a generated-mail cycle then loops up to about two thousand million times, each hop a queued message and a delivery, filling the queue and disk and stopping mail service. The Sieve notify action (from 6.2.22) consulted no limit at all, and its increment of the count could overflow a signed integer; and an account could reset the counter on its own mail through a Sieve editheader of the header (from 6.2.22) or a rule Set header value action (all versions) ahead of a forward or redirect.","summary":"hMailServer 6.0.0 through 6.3.6 fails to bound its auto-forward and auto-reply loop counter when the X-hMailServer-LoopCount header is negative or wraps negative. A remote sender can make a message loop about two billion times, filling the mail queue and disk and stopping mail service.","severity":"high","cvss":7.5,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["hmailserver","linux","mail-server","package","self-hosted","windows"],"not_affected_if":["patched-version"],"how_to_check":"- Check the installed hMailServer version in the Administrator console or the installed program files; affected versions are 6.0.0 through 6.3.6.\n- Review Sieve scripts and rules that use editheader or Set header value on X-hMailServer-LoopCount, since an account can reset the counter.","fix":"- Upgrade hMailServer to a version after 6.3.6; the fixed release is unknown from the sources given.\n- Until patched, watch the queue and disk for runaway growth from forwarding or auto-reply rules.","status":"open","published_at":"2026-10-19T16:17:01+00:00","first_seen_at":"2026-10-23T21:57:18+00:00","triaged_at":"2026-10-25T03:23:00+00:00","updated_at":"2026-10-25T03:23:00+00:00","category":"package","confidence":0.7,"triage":{"at":"2026-10-25T03:23:00Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade hMailServer to a version after 6.3.6; the fixed release is unknown from the sources given.\n- Until patched, watch the queue and disk for runaway growth from forwarding or auto-reply rules.","reach":"internet","title":"hMailServer: a crafted loop-count header lets mail flood the server","impact":"medium","summary":"hMailServer 6.0.0 through 6.3.6 fails to bound its auto-forward and auto-reply loop counter when the X-hMailServer-LoopCount header is negative or wraps negative. A remote sender can make a message loop about two billion times, filling the mail queue and disk and stopping mail service.","category":"package","nicknames":[],"who_cares":["hmailserver","mail-server","self-hosted","windows","linux"],"confidence":0.7,"needs_auth":"no","worst_case":"Mail queue and disk fill, and mail service stops","analyst_note":"Find the fixed hMailServer release and confirm whether a default install with no forwarding rules is exposed; the sources give only the NVD record.","how_to_check":"- Check the installed hMailServer version in the Administrator console or the installed program files; affected versions are 6.0.0 through 6.3.6.\n- Review Sieve scripts and rules that use editheader or Set header value on X-hMailServer-LoopCount, since an account can reset the counter.","needs_analyst":true,"not_affected_if":["patched-version"],"who_is_affected":"Any hMailServer deployment on versions 6.0.0 through 6.3.6 that accepts mail from outside is exposed, since the header is not stripped from inbound mail. Linux builds are affected by values above the 32-bit signed range as well as negative values. Teams running a newer release or not using hMailServer can skip it.","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"high"},"cost_usd":0.000696,"input_tokens":3732,"output_tokens":646,"severity_before":"high"},"analyst_status":null,"analyst_note":null,"search":"'-107569':15A,18A '-2026':14A,17A '32':248C '5':166C '6.0.0':30B,90C '6.2.22':304C,340C '6.3.6':32B,92C 'account':123C,323C 'action':302C,347C 'ahead':350C 'allow':93C 'arriv':200C 'attack':97C 'auto':38B,42B,177C 'auto-forward':37B 'auto-repli':41B,176C 'billion':68B 'bit':249C 'bounc':132C 'bound':35B,106C,182C,262C 'build':242C 'carri':102C,143C 'condit':85C 'configur':162C 'consult':305C 'convers':211C,254C 'copi':121C 'could':316C,324C 'count':6A,217C,315C 'counter':45B,141C,159C,327C 'craft':3A 'cve':13A,16A 'cycl':179C,272C 'default':168C 'deliveri':289C 'disk':75B,294C 'edithead':335C 'end':172C 'enter':267C 'everi':221C 'fail':33B 'fill':70B,290C 'flood':10A 'forward':39B,118C,124C,174C,353C 'generat':110C,270C 'generated-mail':269C 'header':7A,52B,152C,193C,338C,345C 'hmailserv':1A,19B,29B,50B,89C,150C,188C,232C 'hop':185C,283C 'host':27B 'increment':312C 'input':81C 'integ':320C 'let':8A 'lift':260C 'limit':165C,225C,307C 'linux':20B,241C 'loop':5A,44B,65B,84C,164C,274C 'loop-count':4A 'loopcount':51B,151C,189C,233C 'mail':9A,22B,72B,78B,103C,108C,199C,271C,297C,331C 'mail-serv':21B 'make':62B,154C 'messag':64B,115C,146C,265C,286C 'million':280C 'negat':54B,57B,236C,258C 'notifi':130C,301C 'number':183C 'offic':137C 'one':259C 'ordinari':192C 'out-of-offic':134C 'outsid':202C 'overflow':317C 'packag':24B 'pass':220C 'progress':87C 'queu':285C 'queue':73B,292C 'rang':251C 'reach':160C 'read':206C 'redirect':127C,355C 'remot':59B,95C 'repli':43B,119C,138C,178C 'reset':325C 'respons':112C 'robot':88C 'rule':117C,163C,343C 'self':26B 'self-host':25B 'sender':60B,227C 'server':12A,23B,101C,105C,205C 'servic':79B,298C 'set':229C,344C 'siev':126C,300C,334C 'sign':210C,250C,319C 'stop':77B,99C,296C 'strip':197C 'test':222C 'thousand':279C 'time':69B,281C 'two':67B,278C 'unauthent':96C 'uncheck':80C 'vacat':128C 'valid':214C 'valu':237C,245C,346C 'version':349C 'window':28B 'wrap':56B,255C 'x':49B,149C,187C,231C 'x-hmailserver-loopcount':48B,148C,186C,230C 'zero':219C","nicknames":[],"mitigations":[],"who_affected":"Any hMailServer deployment on versions 6.0.0 through 6.3.6 that accepts mail from outside is exposed, since the header is not stripped from inbound mail. Linux builds are affected by values above the 32-bit signed range as well as negative values. Teams running a newer release or not using hMailServer can skip it.","tier":"B","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:N/UI:N/S:U/C:N/I:N/A:H","cvss_by":"cve@gitlab.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-107569","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-107569","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"}