Option Dynamic SSL Certificates

OWASP ZAP allows you to transparently decrypt SSL connections. For doing so, ZAP has to encrypt each request before sending to the server and decrypt each response, which comes back. But, this is already done by the browser. That's why, the only way to decrypt or intercept the transmission, is to do a ‘manipulator in the middle’ approach.


manipulator in the middle

In short words, every data send to and received from the server is encrypted/decrypted by using the original server's certificate inside ZAP. This way, ZAP knows the plain text. To establish a SSL protected session from you (your browser), ZAP is using it's own certificate. This is the one you can create. Every certificate created by ZAP will be signed for the same server name. In the example above, ZAP will create a certificate for the server's name www.example.com. This way, your browser will do regular SSL encryption.

ZAP Root CA certificate

Imagine you're visiting multiple SSL protected sites. Every time your browser connects such a site, a new SSL certificate is created. But, these certificates are not trusted by anyone (because self created by ZAP). In other words, your browser will not accept such certificates in the first place. You may familiar with such situations, when your browser complains certificate error but you manually can create an exception rule for that server.

Every certificate created by ZAP is in the direct chain of trust from the “ZAP Root CA” certificate. (For more details about chain of trust, use your favorite search engine ;-) ) This means, you (your browser) only have to trust the ZAP Root CA once, and any further certificates are automatically trusted. In other words, once you've added the ZAP Root CA certificate to your list of trusted Root CAs, your browser doesn't recognize the man in the middle.


On iOS 10.3 and onwards, you also need to enable full trust for the root certificate: Go to Settings > General > About > Certificate Trust Settings. Under “Enable full trust for root certificates”, turn on trust for the certificate.


When you are running ZAP for the first time then it will generate a Root CA certificate just for you. If you do not use the ‘browser launch’ feature then you have to install it within your browser or HTTP client application. See section installation for more details.

Every generated Root CA certificate is valid for one year. After that period you have to create a new one.
Every generated Root CA certificate is 2048 bit strong (RSA with SHA1).
Every generated Root CA certificate starts with serial number “1”. Every generated Root CA certificate consists of the following identifiers:

CN = OWASP Zed Attack Proxy Root CA
L = 87b77fe834b0a301
C = XX

As you can see, there's a Location identifier (L) which is only a hexadecimal number. This number is constructed out of two 32bit hash codes: user's name and user's home directory. This way you can identify your own certificate when using multiple installations. But there's no way, that anyone can figure out your name from this hash code.


When you're using multiple ZAP installation and you want to use the same Root CA certificate, so you can import it. Simply use one installation of OWASP ZAP to generate one Root CA certificate.
Copy the file ‘OWASP ZAP/config.xml’ from your users home directory to the PC, where you want to use the same certificate and press ‘import’ to import it.

Alternatively you can use the command line options:

  • -certfulldump <path> to dump the certificate from one ZAP instance
  • -certload <path> to load the certificate into another ZAP instance

You can also import certificates stored in pem files as long as they include both the certificate and the unencrypted private key in the following format:


And yes, that example will work - its the Superfish certificate!


In the options dialog of ZAP you're seeing the raw bytes (hexa-decimal encoded) of the certificate. The option “view” tries to use your system's default viewing tool for “.CER” files. On Windows, this is typically the same, when exporting the certificate and double clicking on it.


In the options dialog of ZAP you're seeing the raw bytes (hexa-decimal encoded) of the certificate. Many programs are using this simple format for import/export functions. When clicking ‘export’, these bytes are saved to disk. This is equal to selecting all and doing CTRL+C (copy to clipboard) and save it into a new .CER file (which is simple text as you see in the dialog).

Dynamic certificates

Each ZAP instance is using it's own root certificate. Of course, you can import root certificates, to use them on multiple machines. When running, there will be sub-certificated created, each time a HTTPS resource is requested. That means, the Root CA certificate is used as an issuer.

Every dynamically generated certificate is valid for 1000 days.
Every dynamically generated certificate is 2048 bit strong (RSA with SHA1).
Every dynamically generated certificate has a random serial number. Every dynamically generated certificate consists of the following identifiers:

CN = www.example.com
E = [email protected]
C = XX
OU = Zed Attack Proxy Project

Side note: Each time you start ZAP, internally a random serial number offset is generated. Every dynamically generated certificate will use this offset plus an increasing counter. For example, first dynamically certificate has serial number 2314, the second one 2315, the third one 2316 and so on. The reason for this is simple: browsers are also caching certificates. When you restart ZAP but don't restart your browser, it could happen, that the browser sees the same certificate but with different serial number. In the end, the browser would complain about and reject the certificate. By using the random offset (internally 48bit random number), the chances are 1 to 281.474.976.710.656 that when restarting ZAP, the serial number offset is a different one. So in the rare case, you are discovering that you browser complains about a broken serial number within the certificate, just restart your browser ;-).

Install ZAP Root CA certificate

Any HTTPS client you want to use, has to know the OWASP Root CA certificate as ‘trusted root certificate’. Typically you have to install manually the ZAP certificate into your browser's list of trusted root certificates.

Windows / Internet Explorer

The easiest way is to click on view and choose ‘Install certificate’. Alternatively, you can save/export your generated certificate (copy it to you target computer) and double click the .CER file. When doing so, the regular Windows wizard for certificate installation assistance is poping up. In this wizard manually choose the certificate store. Do NOT let Windows choose automatically the certificate store. Choose ‘trusted root certificates’ as store and finalize the wizard.

After successfully installation, you can check the certificate.

  1. Go to Internet options
  2. Tab Content
  3. Click certificates
  4. Click tab trusted root certificates
  5. The OWASP ZAP Root CA should be there

Mozilla Firefox

Firefox is using it's own certificate store. Thats why you have to import it twice, when you're using both browser on windows. Installation and late on validation is done in the same preferences dialog:

  1. Go to Preferences
  2. Tab Advanced
  3. Tab Cryptography/Certificates
  4. Click View Certificates
  5. Click Authorities tab
  6. Click Import and choose the saved owasp_zap_root_ca.cer file
  7. In the wizard choose to trust this certificate to identify web sites (check on the boxes)
  8. Finalize the wizard


Attention, there are risks!
When adding self generated Root CA certificates to your list of trusted root certificates, everyone with the root certificate can smuggle data into your system (browser). In other words when you're not testing in a safe environment, but on productive machines, be aware that you're opening an additional attack vector to your system.

See also

Certificates for SSL client certificates