CLI tool

A CLI tool exists to compute mCaptcha challenges. It can be installed from multiple sources:


The CLI tool is available on,, the Rust language’s package registry. Rust language toolchain is required to install from, please see for Instructions to install it.

1cargo install mcaptcha-cli

Pre-compiled binaries

Nightly builds and stable releases are regularly published to for a variety of CPU architectures and operating systems.

  1. Download binary, checksum and GPG signature files
  1. Verify checksum
1sha256sum -c {FILENAME}.tar.gz.sha256
  1. Download mCaptcha’s GPG release keys and verify GPG signature
1gpg --keyserver --recv 73DAC973A9ADBB9ADCB5CDC4595A08135BA9FF73
2gpg --verify {FILENAME}.tar.gz.asc
  1. Install Binary
1tar -xvzf {FILENAME}.tar.gz && sudo cp {FILENAME}/mcaptcha-cli /usr/local/bin

Build from source

  1. Install Rust tool chain Please see here for instructions.
  2. Download source code
1git clone
  1. Compile and install
1cargo build --release && sudp cp ./target/release/mcaptcha-cli

Pass mCaptcha challenge

The CLI tool requires details about the challenge to work on it. The tool can be used in three different modes compute challenge:

  1. Protected Page: Compute mCaptcha challenge for the CAPTCHA at a protected page
  2. Widget URL: Compute PoW for captcha at widget URL
  3. (Developer mode) Offline: Computes PoW over given CAPTCHA parameters

From protected page URL

The most convenient mode: copy the URL of the webpage which has the mCaptcha widget (example: and run the CLI tool with it to get an authorization code: Compute challenge using the URL

103:39 atm@lab cli ±|feat-parse-webpage ✗|→ mcaptcha-cli protected-page
2Authorization token: eRAZJiMrW58uDYA1s64Tmwq1u30HutuF

Widget URL

If you have the widget URL (will be in format, it can be used to solve challenge as well:

1Compute PoW by fetching parameters from  CAPTCHA URL
3Usage: mcaptcha-cli online --url <URL>
6  -u, --url <URL>  URL of the CAPTCHA. Example:
7  -h, --help       Print help

Example usage:

113:32 atm@lab cli ±|online ✗|→ mcaptcha-cli online -u
2Authorization token: 3xleN26OctBuVu3X4t6CYyUjErhaxQvz

[Developer mode] Offline

Useful while debugging mCaptcha configurations, works on raw challenge parameters.

Help menu:

1Compute PoW with offline parameters
3Usage: mcaptcha-cli offline --salt <SALT> --phrase <PHRASE> --difficulty-factor <DIFFICULTY_FACTOR>
6  -s, --salt <SALT>                            Salt with which PoW should be computed
7  -p, --phrase <PHRASE>                        Phrase over which PoW should be computed
8  -d, --difficulty-factor <DIFFICULTY_FACTOR>  Difficulty Factor
9  -h, --help                                   Print help

Example usage:

113:28 atm@lab cli ±|online|→ mcaptcha-cli offline -s $(rand 32) -p $(rand 32) -d 50000
2difficulty: 50000
3nonce: 90507
4original phrase: f351f333d44b2c6b5bf7f033b065bbb8fb5e9dd153bd402e43ed04425f5a3859
5result: 340276562956196291522979356090220150471

Where rand is this script.

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