bflask

Fast Go CLI for bruteforcing Flask signed session cookies.

Started as a Codex test and later published after working well in CTFs.

Warning

Use this tool only in authorized security testing, educational labs, and CTF environments.

Benchmark

Tool	Time
flask-unsign	677.76s
bflask	48.54s

Install

go install github.com/MyCode83/bflask@latest

From source:

go build .

Important

Requires Go 1.25 or newer.

Usage

bflask crack \
  -c "eyJ1c2VyIjoiYWRtaW4ifQ..." \
  -w rockyou.txt \
  -t 100

JSON output:

bflask crack -c "$COOKIE" -w keys.txt -j

With SHA-256 and a timeout:

bflask crack -c "$COOKIE" -w keys.txt -d sha256 --timeout 2m

Write the hit to a file:

bflask crack -c "$COOKIE" -w keys.txt -o result.json

Print only the recovered SECRET_KEY:

bflask -q crack -c "$COOKIE" -w keys.txt

Sign a cookie and print only the cookie value:

bflask -q sign -k supersecret -p '{"user":"admin"}' -s cookie-session -d sha256

Decode a cookie payload:

bflask decode -c "$COOKIE"

Decode only the raw payload:

bflask -q decode -c "$COOKIE" --raw

Configuration

Priority is CLI > ENV > CONFIG > DEFAULT.

Example config.yaml:

cookie: ""
wordlist: "./wordlist.txt"
threads: 50
salt: "cookie-session"
digest: "sha1"
verbose: false
timeout: "0s"
output: ""
json: false
quiet: false

Use a config file:

bflask --config config.yaml crack

Environment variables use the BFLASK_ prefix:

BFLASK_THREADS=200 BFLASK_DIGEST=sha256 bflask crack -c "$COOKIE" -w keys.txt

Running bflask --config config.yaml without a subcommand starts crack with the values from the config file. Running bflask without a config file or subcommand prints help.

How Flask Cookies Work

Flask's default session stores data client-side and signs it with SECRET_KEY through itsdangerous. The data is not encrypted. If the signing key is guessed, the payload can be verified and decoded. bflask checks candidate keys with URLSafeTimedSerializer-compatible HMAC signing, cookie-session salt by default, and SHA-1 by default.

Compressed Flask cookies start with a leading .. bflask supports decoding these zlib-compressed payloads when a valid key is found.

Sample Flask App

A tiny app for generating a test cookie is in examples/flask/app.py:

cd examples/flask
python -m flask --app app run

Visit http://127.0.0.1:5000/, copy the session cookie, and test it with a wordlist containing supersecret, salt 11, digest sha256.

Troubleshooting

invalid Flask cookie format: pass only the cookie value, not session=<value> or a full Cookie: header.

unsupported digest: use one of sha1, sha224, sha256, sha384, sha512, or md5.

No hit found: check the salt, digest, wordlist encoding, and whether the target uses Flask's default signed session serializer.