Common Desktop Environment 2.3.1 / 1.6 libDtSvc Buffer Overflow

2020.04.18
Credit: Marco Ivaldi
Risk: High
Local: Yes
Remote: No
CWE: CWE-119


CVSS Base Score: 4.4/10
Impact Subscore: 6.4/10
Exploitability Subscore: 3.4/10
Exploit range: Local
Attack complexity: Medium
Authentication: No required
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: Partial

@Mediaservice.net Security Advisory #2020-06 (last updated on 2020-04-15) Title: Stack-based buffer overflow in CDE libDtSvc Application: Common Desktop Environment 2.3.1 and earlier Common Desktop Environment 1.6 and earlier2020-06-cde-libDtSvc.txt Platforms: Oracle Solaris 10 1/13 (Update 11) and earlier Other platforms are potentially affected (see below) Description: A difficult to exploit stack-based buffer overflow in the libDtSvc library distributed with CDE may allow local users to corrupt memory and potentially execute aritrary code in order to escalate privileges Author: Marco Ivaldi <marco.ivaldi@mediaservice.net> Vendor Status: Oracle <secalert_us@oracle.com> notified on 2019-12-15 CERT/CC notified on 2019-12-15 (tracking VU#308289) CVE Name: CVE-2020-2851 CVSS Vector: CVSS:3.0/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H (Base Score: 7.8) References: https://github.com/0xdea/advisories/blob/master/2020-06-cde-libDtSvc.txt https://www.oracle.com/security-alerts/cpuapr2020.html https://sourceforge.net/p/cdesktopenv/wiki/Home/ https://www.oracle.com/technetwork/server-storage/solaris10/ https://www.mediaservice.net/ https://0xdeadbeef.info/ 1. Abstract. A difficult to exploit stack-based buffer overflow in the _DtCreateDtDirs() function in the Common Desktop Environment version distributed with Oracle Solaris 10 1/13 (Update 11) and earlier may allow local users to corrupt memory and potentially execute arbitrary code in order to escalate privileges via a long X11 display name. The vulnerable function is located in the libDtSvc library and can be reached by executing the setuid program dtsession. Note that Oracle Solaris CDE is based on the original CDE 1.x train, which is different from the CDE 2.x codebase that was later open sourced. In detail, the open source CDE is not affected by this specific vulnerability, but following our report some additional work has been done by its maintainers to properly check bounds in the libDtSvc library. Most notably, insecure calls to strncat() that caused buffer overflows have been fixed. 2. Example Attack Session. In order to reproduce this bug, the following commands can be used: bash-3.2$ cat /etc/release Oracle Solaris 10 1/13 s10x_u11wos_24a X86 Copyright (c) 1983, 2013, Oracle and/or its affiliates. All rights reserved. Assembled 17 January 2013 bash-3.2$ uname -a SunOS nostalgia 5.10 Generic_147148-26 i86pc i386 i86pc bash-3.2$ id uid=54322(raptor) gid=1(other) bash-3.2$ grep 10.0.0.24 /etc/hosts 10.0.0.24 aaaa:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa [activate a valid display on 10.0.0.24:0] /usr/dt/bin/dtsession -display aaaa:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:0 Segmentation Fault 3. Discussion. The overflow occurs in the following code snippet of Oracle Solaris CDE (the Ghidra decompiler is probably doing something wrong as some variables seem to overlap, however its output is good enough for the purpose of this discussion): char * _DtCreateDtDirs(int param_1) { ... char local_f0 [104]; char local_88 [112]; char *heap_path2; char *tmp_ptr1; char *home; undefined *local_c; undefined local_8 [4]; ... if (param_1 != 0) { strcpy(local_f0,*(char **)(param_1 + 0x80)); strcpy(local_88,*(char **)(param_1 + 0x80)); ... } An X11 display data structure is passed to the _DtCreateDtDirs() function as its only parameter (param_1 in the pseudocode above). It contains the X11 display name at offset 0x80. This display name is copied into the stack buffers local_f0 and local_88 using the insecure function strcpy() twice, therefore two overflows occur. Based on the inferred stack layout, the following local variables are overflowed into before the saved return address can be reached: heap_path2 tmp_ptr1 home local_c local_8 This complicates exploitation, in particular because the heap_path2 and tmp_ptr1 pointers get in the way. A skilled attacker might be able to overwrite all variables with safe data and leverage memory corruption to obtain arbitrary code execution. However, there is an additional challenge: the ability to control a hostname to be passed in the X11 display name string. In our PoC above we have edited /etc/hosts, but this is obviously not possible for an unprivileged local attacker. A DNS server under the control of the attacker may be used for this purpose, but such an approach would introduce a number of additional complications. That said, as a rule of thumb all memory corruption issues have the potential to become serious security vulnerabilities until otherwise proven. Therefore, we recommend to treat this bug as a potential security vulnerability and to fix it as such. 4. Affected Platforms. All platforms shipping the Common Desktop Environment are potentially affected. This includes: * Oracle Solaris 10 1/13 (Update 11) and earlier [default installation] According to the CDE Wiki, the following platforms are officially supported: * All Official Ubuntu variants 12.04 - 18.04 * Debian 6, 7, 8, 9 * Fedora 17 at least * Archlinux * Red Hat * Slackware 14.0 * OpenBSD * NetBSD * FreeBSD 9.2, 10.x, 11.x * openSUSE Tumbleweed (gcc7) * openSUSE Leap 4.2 (gcc4) * SUSE 12 SP3 (gcc4) * Solaris, OpenIndiana 5. Fix. The maintainers of the open source CDE 2.x version have issued the following patches: https://sourceforge.net/p/cdesktopenv/mailman/message/36900154/ https://sourceforge.net/p/cdesktopenv/code/ci/6b32246d06ab16fd7897dc344db69d0957f3ae08/ Oracle, which maintains a different CDE codebase based on the 1.x train, has assigned the tracking# S1240932 and has released a fix for all affected and supported versions of Solaris in the Critical Patch Update (CPU) of April 2020. As a workaround, it is also possible to remove the setuid bit from the vulnerable executable as follows (note that this might prevent it from working properly): bash-3.2# chmod -s /usr/dt/bin/dtsession Please note that during the audit many other potentially exploitable bugs have surfaced in libDtSvc and in the Common Desktop Environment in general. Therefore, removing the setuid bit from all CDE binaries is recommended, regardless of patches released by vendors. Copyright (c) 2020 Marco Ivaldi and @Mediaservice.net. All rights reserved.


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