By Indusface Research Team
Today, Google announced a vulnerability in the implementation of the SSL 3.0 protocol potentially compromising secure connections online. POODLE (Padding Oracle on Downgraded Legacy Encryption) is a new security hole in Secure Socket Layer (SSL) 3.0 that makes the 15-year-old protocol nearly impossible to use safely. The “Poodle” is a protocol flaw, not an implementation issue; every implementation of SSL 3.0 suffers from it. The TLS versions are not affected (neither is DTLS).
The Poodle attack works in a chosen-plaintext context. The attacker is interested in data that gets protected with SSL, and he can:
1) Inject data of his own before and after the secret value that he wants to obtain;
2) Inspect, intercept and modify the resulting bytes on the wire.
You don’t! Since the most probable attack setup involves the attacker luring the victim on his network, not yours.
Although, on the server side, you may want to react on an inordinate amount of requests that fail on a decryption error. Not all server software will log events for such cases, but this should be within the possibilities of any decent IDS system.
The attack described above requires an SSL 3.0 connection to be established, so disabling the SSL 3.0 protocol in the client or in the server (or both) will completely avoid it. If either side supports only SSL 3.0, then all hope is gone, and a serious update required to avoid insecure encryption. If SSL 3.0 is neither disabled nor the only possible protocol version, then the attack is possible if the client uses a downgrade dance for interoperability.
Disabling SSL 3.0 entirely right away may not be practical if it is needed occasionally to work with legacy systems. Also, similar protocol version downgrades are still a concern with newer protocol versions (although not nearly as severe as with SSL 3.0).
The TLS_FALLBACK_SCSV mechanism from [draftietftlsdowngradescsv00] addresses the broader issue across protocol versions, and we consider it crucial especially for systems that maintain SSL 3.0 compatibility.
The following recommendations summarize how TLS_FALLBACK_SCSV works:
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. This value serves as a signal allowing updated servers to reject the connection in case of a downgrade attack. Clients should 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) because skipping a protocol version forgoes its better security. (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.) 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 (preferably, inappropriate fallback (86) from [draftietftlsdowngradescsv00]).
This use of TLS_FALLBACK_SCSV will ensure that SSL 3.0 is used only when a legacy implementation is involved: attackers can no longer force a protocol downgrade. (Attacks remain possible if both parties 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.)
Founder & Chief Marketing Officer, Indusface
Venky has played multiple roles within Indusface for the past 6 years. Before this, as the CTO @ Indusface, Venky created the product/service offering and technology team from scratch and grew it from ideation to getting initial customers with a proven/validated business model poised for scale. Before joining Indusface, Venky had 10+ years of experience in the security industry and had held various mgmt/leadership roles in Product Development, Professional Services, and Sales @Entrust.