POODLE Advisory

What is POODLE?

POODLE stands for Padding Oracle On Downgraded Legacy Encryption

Vulnerability CVE:


What is the attack?

The attack occurs when an attacker is able to downgrade the client to use SSLv3. By simulating a failure during the negotiation process, an attacker can force a browser and a server to renegotiate using an older protocol, right back down to SSLv3.
Attacker aims to capture the session cookie within a HTTPS tunnel through MITM. Attacker injects a piece of JavaScript and intercepts the outgoing messages and reorganizes them. This JavaScript tells the browser to repeatedly try to load an image from the Web application transmitting a session cookie. This image request will carry with it the session cookie and the JavaScript ensures that each of these requests is constructed in such a way as to ensure that one byte of the session cookie is placed in a particular place within each SSL message.
In this way, attacker will learn a single byte of the session cookie with every request and the complete session cookie can be decrypted to gain malicious access to the application.

How to check if I am vulnerable?

Visiting the below URL would confirm if the vulnerability exists.


What is the fix?

  1. Disable SSLv3 support on the server.
  2. Use TLS_FALLBACK_SCSV, a mechanism that prevents attackers from forcing Web browsers to use SSL 3.0.For TLS clients:
    TLS clients that use a downgrade dance to improve interoperability should include the value 0x56, 0x00 (TLS_FALLBACK_SCSV) in ClientHello.cipher_suites in any fallback handshakes. Thus, in case of a downgrade attack, clients would always fall back to the next lower version (if starting at TLS 1.2, try TLS 1.1 next, then TLS 1.0, then SSL 3.0) (With TLS_FALLBACK_SCSV, skipping a version also could entirely prevent a successful handshake if it happens to be the version that should be used with the server in question.)
    For TLS servers:
    In TLS servers, whenever an incoming connection includes 0x56, 0x00 (TLS_FALLBACK_SCSV) in ClientHello.cipher_suites, compare ClientHello.client_version to the highest protocol version supported by the server. If the server supports a version higher than the one indicated by the client, reject the connection with a fatal alert.
    This use of TLS_FALLBACK_SCSV will ensure that SSL 3.0 is used only when a legacy implementation is involved and attackers can no longer force a protocol downgrade. (Attacks remain possible if bothparties allow SSL 3.0 but one of them is not updated to support TLS_FALLBACK_SCSV, provided that the client implements a downgrade dance down to SSL 3.0.)
  3. Avoid potential phishing emails from attackers – to avoid going to an impersonated website.

How to fix this vulnerability on servers?


  1. To disable SSLv3 on the Apache server, the following can be configured:
    S S L P r o t o c o l A l l – S S L v 2 – S S L v 3
  2. This will ensure that TLSv1.0, TLSv1.1 and TLSv1.2 are supported and explicitly remove support for SSLv2 and SSLv3. Check the config and then restart Apache.
    a p a c h e c t l c o n f i g t e s t
    s u d o s e r v i c e a p a c h e 2 r e s t a r t


  1. Disabling SSLv3 support on NginX is as follows :
    s s l _ p r o t o c o l s T L S v 1 T L S v 1 . 1 T L S v 1 . 2
  2. Similar to the Apache config above, TLSv1.0+ would be supported and no SSL would be enabled. Check the config and restart :
    s u d o n g i n x – t
    s u d o s e r v i c e n g i n x r e s t a r t


  1. Modify / create a registry DWORD value.
    H K e y _ L o c a l _ M a c h i n e \ S y s t e m \ C u r r e n t C o n t r o l S e t \ C o n t r o l \ S e c u r i t y P r o v i d e r s \ S C H A N N E L \ P r o t o c o l s
  2. Inside protocols, the S S L 2 . 0 key would be already present, so create S S L 3 . 0 alongside it if needed. Under that create a S e r v e r key and inside there a DWORD value called E n a b l e d with value 0 . Once that’s done reboot the server for the changes to take effect.
Disabling SSLv3.0 in IIS
Figure 1: Disabling SSLv3.0 in IIS

How to fix this vulnerability on browsers:


  1. Type a b o u t : c o n f i g into their address bar
  2. Type s e c u r i t y . t l s . v e r s i o n . m i n into the search box. This will bring up the setting that needs to be changed from 0 to 1.
  3. By changing the setting Firefox will be forced to only use TLSv1.0 or better, which is not vulnerable to POODLE.
Disabling SSLv3.0 in Firefox
Figure 2: Disabling SSLv3.0 in Firefox


  1. Add the command line flag – – s s l – v e r s i o n – m i n = t l s
  2. In Windows, right click on the Chrome shortcut, hit Properties and add the command line flag
Disabling SSLv3.0 in Chrome
Figure 3: Disabling SSLv3.0 in Chrome


  1. Go to Settings, Internet Options and click on the Advanced tab.
  2. Uncheck the U s e S S L 3 . 0 checkbox
Disabling SSLv3.0 in IE
Figure 4: Disabling SSLv3.0 in IE

Shortcomings in the fix:

Systems using IE6 and Windows XP installations below SP3, will no longer be able to communicate with any site that does not use SSLv3.


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