{"id":82266,"date":"2020-05-20T19:42:07","date_gmt":"2020-05-20T17:42:07","guid":{"rendered":"https:\/\/prohoster.info\/blog\/novosti-interneta\/udalyonno-ekspluatiruemaya-uyazvimost-v-pochtovom-servere-qmail"},"modified":"2020-05-20T19:42:07","modified_gmt":"2020-05-20T17:42:07","slug":"udalyonno-ekspluatiruemaya-uyazvimost-v-pochtovom-servere-qmail","status":"publish","type":"post","link":"https:\/\/prohoster.info\/da\/blog\/news\/udalyonno-ekspluatiruemaya-uyazvimost-v-pochtovom-servere-qmail","title":{"rendered":"Fjernudnyttet s\u00e5rbarhed i mailserveren qmail","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

Sikkerhedsforskere fra firmaet Qualys viste<\/a><\/noindex> mulighed udnyttelse<\/a><\/noindex> s\u00e5rbarheder i mailserveren qmail, kendt<\/a><\/noindex> siden 2005 (CVE-2005-1513), men har forblevet udbedret, da forfatteren af qmail h\u00e6vdede, at det var urealistisk at skabe en fungerende udnyttelse, der kunne anvendes til at angribe systemer i standardkonfiguration. I Qualys lykkedes det at forberede en udnyttelse, der modbeviser denne antagelse og muligg\u00f8r initiativet til fjernudf\u00f8relse af kode p\u00e5 serveren ved at sende en specielt udformet besked.<\/p>\n

Problemet skyldes et integer overflow i funktionen stralloc_readyplus(), som kan opst\u00e5 ved behandling af en meget stor besked. For at udnytte dette kr\u00e6ves et 64-bit system med mere end 4 GB virtuel hukommelse. Ved den f\u00f8rste analyse af s\u00e5rbarheden i 2005 h\u00e6vdede Daniel Bernstein, at den antagelse i koden, at st\u00f8rrelsen af det allokerede array altid ligger inden for et 32-bit v\u00e6rdi, er baseret p\u00e5, at ingen giver gigabyte hukommelse til hver proces. I l\u00f8bet af de seneste 15 \u00e5r har 64-bit systemer p\u00e5 servere udkonkurreret 32-bit, mens m\u00e6ngden af leveret hukommelse og netv\u00e6rksb\u00e5ndbredde er steget markant. <\/p>\n

De medf\u00f8lgende pakker med qmail tog Bernsteins kommentar i betragtning, og ved opstart af processen qmail-smtpd begr\u00e6nsede de den tilg\u00e6ngelige hukommelse (for eksempel er der i Debian 10 et loft p\u00e5 7 MB). Men ingeni\u00f8rer fra Qualys fandt ud af, at dette ikke var tilstr\u00e6kkeligt, og at en fjernangreb ogs\u00e5 kunne udf\u00f8res p\u00e5 qmail-local processen, som i alle kontrollerede pakker forblev uden begr\u00e6nsninger. Som bevis blev der udviklet en prototypeudnyttelse, der er egnet til at angribe den Debian-pakke med qmail i standardkonfiguration.
\nFor at organisere fjernudf\u00f8relse af kode under angrebet kr\u00e6ves der 4 GB ledig diskplads og 8 GB RAM p\u00e5 serveren.
\nExploiten giver mulighed for at k\u00f8re vilk\u00e5rlige shell-kommandoer med rettighederne fra enhver bruger i systemet, undtagen root og systembrugere, der ikke har deres egen underkatalog i kataloget \"\/home\" (processen qmail-local k\u00f8res med rettighederne fra den lokale bruger, der modtager leveringen).<\/p>\n

Angrebet udf\u00f8res
\ngennem at sende en meget stor e-mail, der inkluderer flere linjer i hovedet, med en st\u00f8rrelse p\u00e5 omkring 4GB og 576MB. Behandlingen af en s\u00e5dan linje i qmail-local f\u00f8rer til heltals-overl\u00f8b ved fors\u00f8get p\u00e5 at levere beskeden til den lokale bruger. Heltals-overl\u00f8bet f\u00f8rer derefter til buffer-overl\u00f8b ved kopiering af data og muligheden for at overskrive hukommelsessider med libc-kode. Gennem manipulation af layoutet af de overf\u00f8rte data er det blandt andet muligt at omskrive adressen til funktionen \"open()\", s\u00e5 den erstattes med adressen til funktionen \"system()\". <\/p>\n

Dern\u00e6st, i forbindelse med kaldet til qmesearch() i qmail-local \u00e5bnes filen \".qmail-extension\" gennem funktionen open(), hvilket f\u00e5r funktionen
\nsystem(\".qmail-extension\"). Men da en del af filen \"extension\" dannes baseret p\u00e5 adressen til modtageren (for eksempel \"localuser-extension@localdomain\"), kan angribere organisere overf\u00f8rslen af en kommando til at k\u00f8re ved at angive brugeren \"localuser-;command;@localdomain\" som modtager af beskeden. <\/p>\n

Under analyse af koden blev der ogs\u00e5 identificeret to s\u00e5rbarheder i det ekstra patch qmail-verify, som er inkluderet i Debian-pakken. Den f\u00f8rste s\u00e5rbarhed (CVE-2020-3811<\/a><\/noindex>) tillader omg\u00e5else af e-mailadressekontrollen, og den anden (CVE-2020-3812<\/a><\/noindex>) f\u00f8rer til l\u00e6kage af lokal information. Is\u00e6r giver den f\u00f8rste s\u00e5rbarhed mulighed for at omg\u00e5 valideringen af adressen, der bruges i exploiten til at overf\u00f8re kommandoen (valideringen fejler for adresser uden dom\u00e6ne, s\u00e5som \"localuser-;command;\"). Den anden s\u00e5rbarhed kan bruges til at tjekke eksistensen af filer og kataloger i systemet, herunder dem der kun er tilg\u00e6ngelige for root (qmail-verify k\u00f8rer med root-rettigheder), gennem direkte adgang til den lokale h\u00e5ndterer.<\/p>\n

For circumventing the problem, Bernstein recommended running qmail processes with a set limit on available memory (\"softlimit -m12345678\"), which blocks the issue. An alternative protective method mentioned is limiting the maximum message size processed through the file \"control\/databytes\" (which by default is not created, leaving qmail vulnerable under default settings). Furthermore, \"control\/databytes\" does not protect against local attacks from system users, as the limit is only considered by qmail-smtpd.<\/p>\n

Problemet vedr\u00f8rer pakken netqmail<\/a><\/noindex>, som er en del af Debian-pakkerepositorierne. For denne pakke er der forberedt et s\u00e6t patches, der adresserer b\u00e5de gamle s\u00e5rbarheder fra 2005 (ved at tilf\u00f8je strenge hukommelsesgr\u00e6nser i koden med funktionen alloc()) og nye problemer i qmail-verify. Separat er forberedt<\/a><\/noindex> den opdaterede version af qmail-verify patch. Udviklerne afgreningen<\/a><\/noindex> notqmail<\/a><\/noindex> har forberedt deres patches til blokering af gamle problemer og har ogs\u00e5 p\u00e5begyndt arbejdet med at fjerne alle potentielle integer overflow fejl i koden.<\/p>\n

<\/a><\/noindex><\/p>\n

Kilde: opennet.ru<\/a><\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"

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