Reverse ASUS RT-AC5300
In this laboratory, we are going to use the model: RT-AC5300 firmware.
At this moment, we need to unzip the zip file and use binwalk to analyze the .trx file. Using the option "-e" we can extract all the files from the .trx file.
unzip RT-AC5300_3.0.0.4_380_2356-g34e8f6b.trx.zipbinwalk RT-AC5300_3.0.0.4_380_2356-g34e8f6b.trxbinwalk -e RT-AC5300_3.0.0.4_380_2356-g34e8f6b.trx
We can see the firmware uses little-endian, and this is probably a sign that it is using a 3x ARM version or minor.
cd _RT-AC5300_3.0.0.4_380_2356-g34e8f6b.trx.extractedls -lals -la squashfs-rootls squashfs-root/www
We got the WWW content 😎
After emulating it, seems like this:
Why not emulation your router firmware instead of getting a physical device? 🤓
Backing to the extracted folder from the .trx file (firmware itself), we have a file called 1C we need to analyze. We got some precious information, such as:
//CPIO file with the /dev and rootFS139264 0x22000 ASCII cpio archive (SVR4 with no CRC), file name: "/dev", file name length: "0x00000005", file size: "0x00000000"139380 0x22074 ASCII cpio archive (SVR4 with no CRC), file name: "/dev/console", file name length: "0x0000000D", file size: "0x00000000"139504 0x220F0 ASCII cpio archive (SVR4 with no CRC), file name: "/root", file name length: "0x00000006", file size: "0x00000000"139620 0x22164 ASCII cpio archive (SVR4 with no CRC), file name: "TRAILER!!!", file name length: "0x0000000B", file size: "0x00000000"//Linux kernel version - it's important to full emulation3163712 0x304640 Linux kernel version 2.6.36//UNIX path - cool for searching related files/content on web3735664 0x390070 Unix path: /home/yau/asuswrt/release/src-rt-7.14.114.x/src/linux/linux-2.6.36/arch/arm/include/asm/dma-mapping.h
We can confirm some details by using the Linux strings utility:
cat 1C | strings | grep "version"Linux version 2.6.36.4brcmarm ([email protected]) (gcc version 4.5.3 (Buildroot 2012.02) ) #1 SMP PREEMPT Fri Mar 4 20:12:49 CST 20162.6.36.4brcmarm SMP preempt mod_unload modversions ARMv7slabinfo - version: 2.1squashfs: version 4.0 (2009/01/31) Phillip Loughercat 1C | strings | grep "/etc/"/etc/initcat 1C | strings | grep "/root/"/root/initrd
In sum, we can confirm the following details:
Kernel version: 2.6.36.4
gcc version 4.5.3
Buildroot 2012.02 (used to compile the rootFS)
ARMv7 processor - Dual Core 32-bit ARMv7 (Cortex-A9) @ 1.4GHz
FS ini present on /etc and /root folders! 😱
In fact, we should check everytime the device configuration available on the vendor page to learn about it, and get the device datasheet in order to confirm some details.
Per-process emulation is useful when only one binary needs to be emulated. To this that, next install full dependencies:
sudo apt-get install qemusudo apt-get install qemu-utilssudo apt install qemu-kvm libvirt-clients libvirt-daemon-system bridge-utilssudo adduser kali libvirtsudo adduser kali kvmsudo apt install qemu-system-armsudo apt install qemu-user
The user-mode emulation can be made by using the following command:
cd squashfs-root/usr/sbinqemu-arm -L <prefix> ./httpdorqemu-arm-static ./httpd
The -L option is important for when the binary links to external dependencies such as uCLibc or encryption libraries. It tells the dynamic linker to look for dependencies with the provided prefix.
We can get the following error:
qemu-arm: Could not open '/lib/ld-uClibc.so.0': No such file or directory
By inspecting the /lib/ folder, we can see the following:
The file "ld-uClibc.so.0" is present, so we need to just add the current dir to the qemu-arm execution. Otherwise, if the UClibc.so file has another name, we could resolve that by doing a symbolic link, e.g.: squashfs-root/lib/ld-uClibc-0.9.29.so to squashfs-root/lib/ld-uClic.so.
So, the final qemu command is the following:
qemu-arm -L . usr/sbin/httpd/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission deniedGenerating SSL certificate...1068532656:error:02001002:system library:fopen:No such file or directory:bss_file.c:391:fopen('/etc/cert.pem','r')1068532656:error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:393:1068532656:error:140DC002:SSL routines:SSL_CTX_use_certificate_chain_file:system lib:ssl_rsa.c:682:/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission denied/dev/nvram: Permission deniedGenerating SSL certificate...1068532656:error:02001002:system library:fopen:No such file or directory:bss_file.c:391:fopen('/etc/cert.pem','r')1068532656:error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:393:1068532656:error:140DC002:SSL routines:SSL_CTX_use_certificate_chain_file:system lib:ssl_rsa.c:682:/dev/nvram: Permission denied/dev/nvram: Permission denied
Another way to emulate the process is to perform a cross-architectural chroot with QEMU.
[email protected]:~$ sudo apt-get install binfmt-support qemu qemu-user-static debootstrap
You can also download the target qemu-user static file pre-compiled from GitHub 😼
Next, we copy the qemu-arm-static binary to the rootFS directory of the firmware. We then chroot into the firmware root and obtain a working shell:
which qemu-arm-static 130 ⨯/usr/bin/qemu-arm-staticcp /usr/bin/qemu-arm-static squashfs-root/cd squashfs-root/sudo chroot . ./qemu-arm-static /bin/sh
This is possible due to QEMU registering with the kernel during installation to handle binaries with certain magic bytes via the binfmt_misc mechanism.
We can try to run the httpd service, getting some errors - of course, because the /etc/ folder is not mounted, NVRAM system is not present - but we will just workaround this later! 😎
This is the best method to test a single binary or feature in a short period of time without losing a lot of hours configuring and preparing the environment.
Next, we can start e.g., telnetd service inside our emulated environment ;)
---inside chroot FS----/usr/sbin # telnetdcalled setup()/usr/sbin #---Guest machine---└─$ netstat -antpActive Internet connections (servers and established)Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program nametcp6 0 0 :::23 :::* LISTEN -
In the previous user emulation scenarios, we found problems to emulate correctly the NVRAM. In this way, we can use hooks in a simple library to intercept calls to libnvram using LD_PRELOAD.
You can simply access this project on GitHub, compiling the C files with the target arch (ARM) or just download the arm version from releases.
wget https://github.com/firmadyne/libnvram/releases/download/v1.0/libnvram.so.armelmkdir -p squashfs-root/firmadyne/cp libnvram.so /firmadyne/libnvram.somkdir -p /firmadyne/libnvram/ mkdir -p /firmadyne/libnvram.override/sudo chroot . ./qemu-arm-static /bin/shexport LD_PRELOAD="/firmadyne/libnvram.so"httpd
You can also compile the nvram file using a cross-compiler for ARM architectures.
wget https://uclibc.org/downloads/binaries/0.9.30.1/cross-compiler-armv5l.tar.bz2tar xjf cross-compiler-armv5l.tar.bz2git clone https://github.com/firmadyne/libnvram.gitcd libnvram../cross-compiler-armv5l/bin/armv5l-gcc nvram.c -o nvram -shared -ldlcp nvram ../../_RT-AC5300_3.0.0.4_380_2356-g34e8f6b.trx.extracted/squashfs-root/firmadynemv nvram libnvram.sosudo chroot . ./qemu-arm-static /bin/shexport LD_PRELOAD="/firmadyne/libnvram.so"httpd
It works 😋
sudo chroot . ./qemu-arm-static /bin/shexport LD_PRELOAD="/firmadyne/libnvram.so"httpd
If we need to hook any function, LD_PRELOAD is our best friend.
For example, the next hook will emulate the native open file call from gclib.
#define _GNU_SOURCE#include <stdio.h>#include <dlfcn.h>typedef FILE *(*fopen_t)(const char *pathname, const char *mode);fopen_t real_fopen;FILE *fopen(const char *pathname, const char *mode) {fprintf(stderr, "called fopen(%s, %s)\n", pathname, mode);return real_fopen(pathname, mode);}__attribute__((constructor)) static void setup(void) {real_fopen = dlsym(RTLD_NEXT, "fopen");fprintf(stderr, "called setup()\n");}
We can compile it onto ARM using a cross compiler:
wget https://uclibc.org/downloads/binaries/0.9.30.1/cross-compiler-armv5l.tar.bz2tar xjf cross-compiler-armv5l.tar.bz2cross-compiler-armv5l/bin/armv5l-gcc hook.c -o hooks.so -shared -ldlsudo chroot . ./qemu-arm-static /bin/shexport LD_PRELOAD="/firmadyne/libnvram.so /hooks.so"httpd
Even though we don't have the /etc/rc.d or /etc/init.d to run the appropriate RC script to kick off the userland services, we were able to execute single binaries, which can give us some input about the device operation, or even finding vulnerabilities at this level.
To do this, we can use Buildroot 2012.02 to create your rootFS and compile kernel version 2.6.36.
To compile it, you can use a normal Linux distribution (Kali Linux), or try to use an old version of ubuntu 16.4.
To start using buildroot, you need to compile the manual and learn about it.
For this ASUS router, the best configs are the following:
make menuconfig
board config name:
To choose the right kernel defconfig:
or accessing: output/build/linux-2.6.36.4/arch/arm/configs
acs5k_defconfig mv78xx0_defconfigacs5k_tiny_defconfig mx1_defconfigafeb9260_defconfig mx21_defconfigam200epdkit_defconfig mx27_defconfigams_delta_defconfig mx31pdk_defconfigap4evb_defconfig mx3_defconfigassabet_defconfig mx51_defconfigat572d940hfek_defconfig n770_defconfigat91cap9adk_defconfig n8x0_defconfigat91rm9200dk_defconfig neocore926_defconfigat91rm9200ek_defconfig neponset_defconfigat91sam9260ek_defconfig netwinder_defconfigat91sam9261ek_defconfig netx_defconfigat91sam9263ek_defconfig nhk8815_defconfigat91sam9g20ek_defconfig ns9xxx_defconfigat91sam9rlek_defconfig nuc910_defconfigateb9200_defconfig nuc950_defconfigbadge4_defconfig nuc960_defconfigbcmring_defconfig omap3_defconfigcam60_defconfig omap_4430sdp_defconfigcarmeva_defconfig omap_generic_1510_defconfigcerfcube_defconfig omap_generic_1610_defconfigcm_x2xx_defconfig omap_generic_1710_defconfigcm_x300_defconfig omap_generic_2420_defconfigcns3420vb_defconfig omap_h2_1610_defconfigcolibri_pxa270_defconfig omap_innovator_1510_defconfigcolibri_pxa300_defconfig omap_innovator_1610_defconfigcollie_defconfig omap_osk_5912_defconfigcorgi_defconfig omap_perseus2_730_defconfigcpu9260_defconfig onearm_defconfigcpu9g20_defconfig orion5x_defconfigcpuat91_defconfig palmte_defconfigcsb337_defconfig palmtt_defconfigcsb637_defconfig palmz71_defconfigda8xx_omapl_defconfig palmz72_defconfigdavinci_all_defconfig pcm027_defconfigdove_defconfig picotux200_defconfigebsa110_defconfig pleb_defconfigecbat91_defconfig pnx4008_defconfigedb7211_defconfig pxa168_defconfigem_x270_defconfig pxa255-idp_defconfigep93xx_defconfig pxa3xx_defconfigeseries_pxa_defconfig pxa910_defconfigezx_defconfig qil-a9260_defconfigfootbridge_defconfig raumfeld_defconfigfortunet_defconfig realview_defconfigg3evm_defconfig realview-smp_defconfigg4evm_defconfig rpc_defconfigh3600_defconfig s3c2410_defconfigh5000_defconfig s3c6400_defconfigh7201_defconfig s5p6440_defconfigh7202_defconfig s5p6442_defconfighackkit_defconfig s5pc100_defconfightcherald_defconfig s5pv210_defconfigimote2_defconfig sam9_l9260_defconfigintegrator_defconfig shannon_defconfigiop13xx_defconfig shark_defconfigiop32x_defconfig simpad_defconfigiop33x_defconfig spear300_defconfigixp2000_defconfig spear310_defconfigixp23xx_defconfig spear320_defconfigixp4xx_defconfig spear600_defconfigjornada720_defconfig spitz_defconfigkafa_defconfig stamp9g20_defconfigkb9202_defconfig stmp378x_defconfigkirkwood_defconfig stmp37xx_defconfigks8695_defconfig sx1_defconfiglart_defconfig tct_hammer_defconfigloki_defconfig trizeps4_defconfiglpd270_defconfig u300_defconfiglpd7a400_defconfig u8500_defconfiglpd7a404_defconfig usb-a9260_defconfiglubbock_defconfig usb-a9263_defconfigmagician_defconfig versatile_defconfigmainstone_defconfig viper_defconfigmini2440_defconfig xcep_defconfigmmp2_defconfig yl9200_defconfigmsm_defconfig zeus_defconfig
To start the compilation process: make all or using a specific GCC version: make HOSTCC=gcc-4
In file included from clean-temp.h:22:0,from clean-temp.c:23:./stdio.h:477:1: error: 'gets' undeclared here (not in a function)_GL_WARN_ON_USE (gets, "gets is a security hole - use fgets instead");^
Fix
find . -name stdio.in.h./output/build/host-m4-1.4.16/lib/stdio.in.hsed -i -e '/gets is a/d' lib/stdio.in.h
This is frustrating me too, I cannot install texinfo, because texinfo is compatible with the tex documentation for this version of the compiler I have to use, which is what's breaking the documentation in the first place.
/home/ubuntu/buildroot-2012.02/output/toolchain/gcc-4.5.3/gcc/doc/service.texi:6: warning: node prev `Service' in menu `Trouble' and in sectioning `Bugs' differ/home/ubuntu/buildroot-2012.02/output/toolchain/gcc-4.5.3/gcc/doc/service.texi:6: warning: node up `Service' in menu `Bugs' and in sectioning `Top' differMakefile:4052: recipe for target 'doc/gcc.info' failedmake[2]: *** [doc/gcc.info] Error 1make[2]: Leaving directory '/home/ubuntu/buildroot-2012.02/output/toolchain/gcc-4.5.3-initial/gcc'Makefile:5027: recipe for target 'all-gcc' failedmake[1]: *** [all-gcc] Error 2make[1]: Leaving directory '/home/ubuntu/buildroot-2012.02/output/toolchain/gcc-4.5.3-initial'
Instead of editing files, when I run make I pass the argument MAKEINFO.
One minor point about this. I think it should be pointed out what this does. It basically overrides the MAKEINFO variable predefined by GNU make, which normally points to the program to call in order to get makeinfo invoked. So by setting it to true you are effectively replacing calls to makeinfo by calls to /bin/true (can be seen with make -npf /dev/null |grep ^MAKEINFO).
Fix
make MAKEINFO=true
Can't use 'defined(@array)'
Can't use 'defined(@array)' (Maybe you should just omit the defined()?) at kernel/timeconst.pl line 373./home/ubuntu/buildroot-2012.02/output/build/linux-3.2.6/kernel/Makefile:141: recipe for target 'kernel/timeconst.h' failedmake[2]: *** [kernel/timeconst.h] Error 255Makefile:945: recipe for target 'kernel' failedmake[1]: *** [kernel] Error 2make[1]: *** Waiting for unfinished jobs....make[1]: Leaving directory '/home/ubuntu/buildroot-2012.02/output/build/linux-3.2.6'package/Makefile.package.in:393: recipe for target '/home/ubuntu/buildroot-2012.02/output/build/linux-3.2.6/.stamp_built' failedmake: *** [/home/ubuntu/buildroot-2012.02/output/build/linux-3.2.6/.stamp_built] Error 2
Fix
Can’t use ‘defined(@array)’ (Maybe you should just omit the defined()?) at kernel/timeconst.pl line 373 has prompted where is the delicate error message, open timeconst.pl to see (under the kerner directory).
cd output/build/linux-3.2.6/kernel/vim timeconst.pl/!defined(@val))change this (line 373):372 @val = @{$canned_values{$hz}};373 if (!defined(@val)) {374 @val = compute_values($hz);375 }376 output($hz, @val);by:372 @val = @{$canned_values{$hz}};373 if ([email protected]) {374 @val = compute_values($hz);375 }376 output($hz, @val);
make
Compiling everything is a long journey, you can also test more recent kernel versions and so on to avoid some errors when you are building your environment for emulation.
At the end, you will get something like this on your output/images folder:
-rw-r--r-- 1 kali kali 62914560 Jul 27 18:30 rootfs.ext2-rw-r--r-- 1 kali kali 1437696 Jul 27 18:30 rootfs.squashfs-rw-r--r-- 1 kali kali 4085760 Jul 27 18:30 rootfs.tar-rwxr-xr-x 1 kali kali 446 Jul 27 18:30 start-qemu.sh-rwxr-xr-x 1 kali kali 8880 Jul 27 18:27 versatile-pb.dtb-rw-r--r-- 1 kali kali 2871872 Jul 27 18:27 zImage
Now, it's time to start your qemu emulator:
qemu-system-arm -M versatilepb -kernel zImage -dtb versatile-pb.dtb -drive file=rootfs.ext2,if=scsi,format=raw -append "rootwait root=/dev/sda console=ttyAMA0,115200"
Finally, merge the rootfs (host + router), and exploit it like a baws 😉
Other options instead of using qemu are docker containers.
docker pull kelvinlawson/arm-cortexsudo docker run --privileged=true --name=firmware -p 2221:22 -p 8888:80 -p 4443:443 -p 2223:23 -it 8d319a850335shell$: cd /sbinshell$: ./preninit
