Safe Usage Of Find -Exec And Command Injection Vulnerabilities

The Dangers of Unfiltered User Input in -exec

The -exec option in the find command allows users to execute other commands and scripts by passing arguments. However, this opens up find to command injection attacks if untrusted input is passed without sanitization. Attackers can inject malicious code into the arguments that gets executed with root privileges, allowing them to fully compromise systems.

How command injection attacks work by injecting malicious code

Command injection attacks take advantage of the ability to chain multiple commands together in a single argument by using separators like semicolons. The injected commands run with the same privileges as the original command. An example attack would be passing “; rm -rf /” as an argument, which would delete the root directory after running the intended command. There are many ways to inject hostile commands depending on the target system and shell.

Real-world examples of attacks exploiting -exec

There have been instances of command injection vulnerabilities in the find -exec option being exploited in the wild:

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In 2021, malware was found exploiting a vulnerability in a cloud provider’s log aggregation script that passed user input unsafely to find’s -exec.
Researchers discovered an attack campaign targeting Linux web servers in 2020 that exploited web apps calling find to gain persistence and install cryptocurrency miners.
A proof of concept at a major hacking conference demonstrated completely compromising a server by injecting commands into a find call that processed filenames from a public file upload.

Best practices for sanitizing user input

To prevent command injection into find’s -exec option, proper sanitization of any arguments is crucial:

Use contextual escaping or quotations around arguments to neutralize special characters like semicolons.
Validate, filter, and limit argument length and allowed values to reduce attack surface.
Employ escaping mechanisms designed for your target shell like Bash’s quote function.
Consider using -fprint over -exec echo and parsing output rather than executing commands.

Following best secure coding practices for your language and environment is key.

Securely Passing Arguments to -exec

Using find’s -fprint option to log results instead of -exec

The native -print option or -fprint for sending output to a file can serve as safer alternatives to -exec in situations where you only need to log path names that match the find criteria. This eliminates invoking arbitrary commands. The tradeoff is losing the ability to dynamically act on each found file. But combining find with parsing the logged output provides protection against injection while retaining similar functionality. Remember to sanitize inputs for any calls that generate filenames as well.

Validating arguments with regular expressions

Applying regular expression matching on arguments before passing them to the -exec handler is an effective technique for restricting inputs to only safe values. Whitelisting is preferrable over blacklisting prohibited characters. Some examples:

Strip out all non-alphanumeric characters
Explicitly define only allowed special chars like underscore
Match arguments against name length limits

Combined with escaping, regular expression validation prevents malicious strings like those seen in command injection from reaching -exec.

Running find as non-privileged user

Having the find process with permissions to read sensitive directories but not execute arbitrary commands greatly limits damage from command injection. The principle of least privilege should apply, typically meaning running find as non-root. While this is not foolproof, since compromising lower privilege accounts still offers pivots to root, it strongly mitigates exposure. Make sure file permissions disallow the find user from editing scripts or writingable directories that could enable further privilege escalation as well.

Safely Constructing -exec Commands

Quoting special characters in arguments

Shells expand unquoted arguments, allowing embedded semicolons, backticks, parentheses and other special characters to break out of the intended command’s context. Putting quotes around dynamic arguments prevents splitting on those characters and the execution of anything extra:

find . -type f -exec cmd '{}' \;
becomes dangerous with something like:
find . -type f -exec cmd '; rm -rf /' \;
But the quotes contain it:  
find . -type f -exec cmd '{}' \;

Quoting arguments also needs to pair properly with escaping for full protection.

Using alternative actions like -print and -ok

In cases where the goal is to simply output matching paths, -print does that without invoking commands specified in -exec. Even better, -ok requires confirmation before executing each found file with the given command. This puts a gate on all execution, although injection could still target the confirmation output. Bypassing -exec means these alternatives avoid its risks. Although they limit what actions can be performed, the safety often outweighs that flexibility.

Dropping privileges before executing commands

Having find give up root permissions provides inherent containment around the damage malicious injected payloads could inflict. The tool sudo allows safely escalating only for targeted actions needing higher privileges without find itself running all-powerful. Configure sudo to only permit certain pre-defined command formats and arguments to enforce what find can execute. While find still requires some permissions for traversing sensitive directories, privilege dropping codifies least privilege principles. This model segments and compartmentalizes capabilities to minimize exposure.

Detecting and Preventing Command Injection

Scanning find calls with static analysis

Static analysis looks at application source code without executing it, allowing inspection for dangerous patterns like command injection risks. Tools can scan code containing find -exec uses and match against known unsafe practices. This helps catch issues proactively before software gets deployed. Customized rules can search for failure to sanitize arguments passed to -exec properly. Embedding static analysis into developer environments through IDE plugins provides real-time, continuous feedback to coders on potential weaknesses like command injection during regular builds. Automating analysis processes to run scans in CI/CD pipelines helps keep software safe over time as code evolves.

Monitoring calls to find in production

Runtime monitoring and instrumentation tracks invocation of commands like find -exec to detect exploitation attempts against running software. Approaches include:

Wrapping find usage with tools checking for suspicious arguments
Alerting on anomalies by analyzing command logs for odd execution patterns
Sandboxing find calls and watching for errors indicating injection efforts

As protection in depth, production monitoring paired with protective measures catches attacks vectors that slip past other defenses. Striking a balance minimizes performance overhead while still effectively recognizing signs of command injection during active use.

Sandboxing -exec actions

Isolating find’s -exec handling into confined sandboxes limits damage attackers can inflict if exploiting command injection. Jails, containers, virtual machines, or other compartmentalization techniques reduce viable attack surface and prevent jumping to wider system control. While compromising the sandbox still provides footholds for hostile access, the scope remains controlled versus fully breaking out. Tradeoffs exist in smoothly passing information into and out of the sandbox. But robust solutions make sandboxes reasonably transparent while greatly enhancing safety. Employ sandboxing along with filtering and monitoring to incorporate defense in depth.