{"id":55807,"date":"2020-01-29T00:00:00","date_gmt":"2020-01-28T21:00:00","guid":{"rendered":"https:\/\/prohoster.info\/blog\/blog_prohoster\/uyazvimost-v-opensmtpd-pozvolyayushhaya-udalyonno-vypolnit-kod-s-pravami-root"},"modified":"2020-02-18T14:03:57","modified_gmt":"2020-02-18T11:03:57","slug":"uyazvimost-v-opensmtpd-pozvolyayushhaya-udalyonno-vypolnit-kod-s-pravami-root","status":"publish","type":"post","link":"https:\/\/prohoster.info\/sk\/blog\/news\/uyazvimost-v-opensmtpd-pozvolyayushhaya-udalyonno-vypolnit-kod-s-pravami-root","title":{"rendered":"Zranite\u013enos\u0165 v OpenSMTPD, ktor\u00e1 umo\u017e\u0148uje vzdialen\u00e9 vykonanie k\u00f3du s pr\u00e1vami root","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

V rozv\u00edjanom projekte OpenBSD po\u0161tovom serveri OpenSMTPD<\/a><\/noindex> bola zisten\u00e1 kritick\u00e1 zranite\u013enos\u0165<\/a><\/noindex> (CVE-2020-7247), ktor\u00e1 umo\u017e\u0148uje vzdialene vykon\u00e1va\u0165 shell pr\u00edkazy na serveri s pr\u00e1vami pou\u017e\u00edvate\u013ea root. Zranite\u013enos\u0165 bola zisten\u00e1 po\u010das op\u00e4tovn\u00e9ho auditu, ktor\u00fd vykonala spolo\u010dnos\u0165 Qualys Security (predch\u00e1dzaj\u00faci audit OpenSMTPD sa uskuto\u010dnil<\/a><\/noindex> v roku 2015, a nov\u00e1 zranite\u013enos\u0165 je pr\u00edtomn\u00e1 od m\u00e1ja 2018). Probl\u00e9m bola odstr\u00e1nen\u00e1<\/a><\/noindex> v vydan\u00ed OpenSMTPD 6.6.2. V\u0161etk\u00fdm pou\u017e\u00edvate\u013eom sa odpor\u00fa\u010da naliehavo nain\u0161talova\u0165 aktualiz\u00e1ciu (pre OpenBSD je oprava k dispoz\u00edcii cez syspatch).<\/p>\n

Boli navrhnut\u00e9 dve varianty \u00fatoku. Prv\u00fd variant funguje v predvolenej konfigur\u00e1cii OpenSMTPD (prij\u00edmanie po\u017eiadaviek iba z localhostu) a umo\u017e\u0148uje lok\u00e1lne zneu\u017eitie probl\u00e9mu, ke\u010f m\u00e1 \u00fato\u010dn\u00edk mo\u017enos\u0165 pristupova\u0165 k lok\u00e1lnemu sie\u0165ov\u00e9mu rozhraniu (loopback) na serveri (napr\u00edklad na hostingov\u00fdch syst\u00e9moch). Druh\u00fd variant sa prejav\u00ed v pr\u00edpade, \u017ee je OpenSMTPD nastaven\u00fd na prij\u00edmanie vonkaj\u0161\u00edch sie\u0165ov\u00fdch po\u017eiadaviek (po\u0161tov\u00fd server prij\u00edmaj\u00faci extern\u00fa po\u0161tu). V\u00fdskumn\u00edci pripravili prototyp exploit, ktor\u00fd \u00faspe\u0161ne funguje ako s variantom OpenSMTPD zo s\u00fa\u010das\u0165ou OpenBSD 6.6, tak aj s prenosnou verziou pre in\u00e9 opera\u010dn\u00e9 syst\u00e9my (vykonan\u00e9 v Debian Testing).<\/p>\n

Probl\u00e9m je sp\u00f4soben\u00fd chybou v funkcii smtp_mailaddr(), ktor\u00e1 sa vol\u00e1 na overenie spr\u00e1vnosti hodn\u00f4t v poliach «MAIL FROM» a «RCPT TO», ur\u010duj\u00facich odosielate\u013ea\/prij\u00edmate\u013ea a pren\u00e1\u0161an\u00fdch po\u010das pripojenia k po\u0161tov\u00e9mu serveru. Na overenie \u010dasti po\u0161tovej adresy, ktor\u00e1 je pred znakom «@», sa v smtp_mailaddr() vol\u00e1 funkcia
\nvalid_localpart(), ktor\u00e1 pova\u017euje za platn\u00e9 (MAILADDR_ALLOWED) znaky «!#$%&’*\\\/ ?^`{|}~+-=_», v s\u00falade s po\u017eiadavkami RFC 5322. <\/p>\n

Pri tom sa priame escapovanie re\u0165azca vykon\u00e1va vo funkcii mda_expand_token(), ktor\u00e1 nahr\u00e1dza len znaky «!#$%&’*?`{|}~» (MAILADDR_ESCAPE). N\u00e1sledne sa re\u0165azec pripraven\u00fd v mda_expand_token() pou\u017e\u00edva pri vyvol\u00e1van\u00ed agenta doru\u010denia (MDA) pomocou pr\u00edkazu ‘execle(«\\\/bin\\\/sh», «\\\/bin\\\/sh», «-c», mda_command,…’. V pr\u00edpade zaslania e-mailu do mbox cez \\\/bin\\\/sh sa spust\u00ed re\u0165azec «\\\/usr\\\/libexec\\\/mail.local -f %%{mbox.from} %%{user.username}», pri\u010dom hodnota «%{mbox.from}» obsahuje escapovan\u00e9 \u00fadaje z parametra «MAIL FROM».<\/p>\n

Podstata zranite\u013enosti spo\u010d\u00edva v tom, \u017ee smtp_mailaddr() m\u00e1 logick\u00fa chybu, kv\u00f4li ktorej v pr\u00edpade zadan\u00ed pr\u00e1zdneho dom\u00e9ny v e-maile funkcia vracia \u00faspe\u0161n\u00fd k\u00f3d overenia, aj ke\u010f \u010das\u0165 adresy pred «@» obsahuje neplatn\u00e9 znaky. \u010ealej, pri pr\u00edprave re\u0165azca funkciou mda_expand_token() sa escapuj\u00fa nie v\u0161etky mo\u017en\u00e9 \u0161peci\u00e1lne znaky shellu, ale len \u0161peci\u00e1lne znaky platn\u00e9 v po\u0161tovej adrese. Tak je na spustenie svojej pr\u00edkazu sta\u010d\u00ed pou\u017ei\u0165 v lok\u00e1lnej \u010dasti e-mailu znak «;» a medzeru, ktor\u00e9 nepatria do s\u00fapravy MAILADDR_ESCAPE a nie s\u00fa escapovan\u00e9. Napr\u00edklad:<\/p>\n

$ nc 127.0.0.1 25<\/p>\n

HELO professor.falken
\n MAIL FROM:<;sleep 66;>
\n RCPT TO:<root>
\n DATA
\n .
\n QUIT<\/p>\n

Po tomto seden\u00ed OpenSMTPD pri dod\u00e1van\u00ed do mbox spust\u00ed cez shell pr\u00edkaz <\/p>\n

\/usr\/libexec\/mail.local -f ;sleep 66; root<\/p>\n

Pri tom s\u00fa mo\u017enosti \u00fatoku obmedzen\u00e9 t\u00fdm, \u017ee lok\u00e1lna \u010das\u0165 adresy nem\u00f4\u017ee presiahnu\u0165 64 znakov, a \u0161peci\u00e1lne znaky ‘$’ a ‘|’ s\u00fa pri escapovan\u00ed nahraden\u00e9 za «:». Na ob\u00eddenie tohto obmedzenia sa vyu\u017e\u00edva fakt, \u017ee telo e-mailu sa pren\u00e1\u0161a po spusten\u00ed \\\/usr\\\/libexec\\\/mail.local cez vstupn\u00fd tok, t.j. pomocou manipul\u00e1ci\u00ed s adresou je mo\u017en\u00e9 spusti\u0165 iba pr\u00edkazov\u00fd interpret\u00e1tor sh a vyu\u017ei\u0165 telo e-mailu ako s\u00fabor in\u0161trukci\u00ed. Ke\u010f\u017ee na za\u010diatku e-mailu s\u00fa uveden\u00e9 servisn\u00e9 SMTP hlavi\u010dky, na ich presko\u010denie sa navrhuje pou\u017ei\u0165 v\u00fdzvu pr\u00edkazu read v slu\u010dke. Pracuj\u00faci exploit nadob\u00fada pribli\u017ene tak\u00fdto vzh\u013ead:<\/p>\n

$ nc 192.168.56.143 25<\/p>\n

HELO professor.falken
\n MAIL FROM:<;for i in 0 1 2 3 4 5 6 7 8 9 a b c d;do read r;done;sh;exit 0;>
\n RCPT TO:<root@example.org>
\n DATA
\n #0
\n #1
\n \u2026
\n #d
\n for i in W O P R; do
\n echo -n «($i) » && id || break
\n done > \\\/root\\\/x.»`id -u`».»$$»
\n .
\n QUIT<\/p>\n

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

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

\u0412 \u0440\u0430\u0437\u0432\u0438\u0432\u0430\u0435\u043c\u043e\u043c \u043f\u0440\u043e\u0435\u043a\u0442\u043e\u043c OpenBSD \u043f\u043e\u0447\u0442\u043e\u0432\u043e\u043c \u0441\u0435\u0440\u0432\u0435\u0440\u0435 OpenSMTPD \u0432\u044b\u044f\u0432\u043b\u0435\u043d\u0430 \u043a\u0440\u0438\u0442\u0438\u0447\u0435\u0441\u043a\u0430\u044f \u0443\u044f\u0437\u0432\u0438\u043c\u043e\u0441\u0442\u044c (CVE-2020-7247), \u043f\u043e\u0437\u0432\u043e\u043b\u044f\u044e\u0449\u0430\u044f \u0443\u0434\u0430\u043b\u0451\u043d\u043d\u043e \u0432\u044b\u043f\u043e\u043b\u043d\u0438\u0442\u044c shell-\u043a\u043e\u043c\u0430\u043d\u0434\u044b \u043d\u0430 \u0441\u0435\u0440\u0432\u0435\u0440\u0435 \u0441 \u043f\u0440\u0430\u0432\u0430\u043c\u0438 \u043f\u043e\u043b\u044c\u0437\u043e\u0432\u0430\u0442\u0435\u043b\u044f root. \u0423\u044f\u0437\u0432\u0438\u043c\u043e\u0441\u0442\u044c \u0432\u044b\u044f\u0432\u043b\u0435\u043d\u0430 \u0432 \u0445\u043e\u0434\u0435 \u043f\u043e\u0432\u0442\u043e\u0440\u043d\u043e\u0433\u043e \u0430\u0443\u0434\u0438\u0442\u0430, \u043f\u0440\u043e\u0432\u0435\u0434\u0451\u043d\u043d\u043e\u0433\u043e \u043a\u043e\u043c\u043f\u0430\u043d\u0438\u0435\u0439 Qualys Security (\u043f\u0440\u043e\u0448\u043b\u044b\u0439 \u0430\u0443\u0434\u0438\u0442 OpenSMTPD \u043f\u0440\u043e\u0432\u043e\u0434\u0438\u043b\u0441\u044f \u0432 2015 \u0433\u043e\u0434\u0443, \u0430 \u043d\u043e\u0432\u0430\u044f \u0443\u044f\u0437\u0432\u0438\u043c\u043e\u0441\u0442\u044c \u043f\u0440\u0438\u0441\u0443\u0442\u0441\u0442\u0432\u0443\u0435\u0442 \u0441 \u043c\u0430\u044f 2018 \u0433\u043e\u0434\u0430). \u041f\u0440\u043e\u0431\u043b\u0435\u043c\u0430 \u0443\u0441\u0442\u0440\u0430\u043d\u0435\u043d\u0430 \u0432 \u0432\u044b\u043f\u0443\u0441\u043a\u0435 OpenSMTPD 6.6.2. \u0412\u0441\u0435\u043c \u043f\u043e\u043b\u044c\u0437\u043e\u0432\u0430\u0442\u0435\u043b\u044f\u043c \u0440\u0435\u043a\u043e\u043c\u0435\u043d\u0434\u0443\u0435\u0442\u0441\u044f […]<\/p>\n","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[702],"tags":[],"class_list":["post-55807","post","type-post","status-publish","format-standard","hentry","category-news"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\n","aioseo_head_json":{"title":"\ud83e\udd47Zranite\u013enos\u0165 v OpenSMTPD, ktor\u00e1 umo\u017e\u0148uje vzdialen\u00e9 spustenie k\u00f3du s pr\u00e1vami root | ProHoster","description":"Vo vyv\u00edjanom projekte OpenBSD sa na e-mailovom serveri OpenSMTPD 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