TLS/SSL and crypto library
2020068123
Follow on from CVE-2016-2179 The investigation and analysis of CVE-2016-2179 highlighted a related flaw. This commit fixes a security "near miss" in the buffered message handling code. Ultimately this is not currently believed to be exploitable due to the reasons outlined below, and therefore there is no CVE for this on its own. The issue this commit fixes is a MITM attack where the attacker can inject a Finished message into the handshake. In the description below it is assumed that the attacker injects the Finished message for the server to receive it. The attack could work equally well the other way around (i.e where the client receives the injected Finished message). The MITM requires the following capabilities: - The ability to manipulate the MTU that the client selects such that it is small enough for the client to fragment Finished messages. - The ability to selectively drop and modify records sent from the client - The ability to inject its own records and send them to the server The MITM forces the client to select a small MTU such that the client will fragment the Finished message. Ideally for the attacker the first fragment will contain all but the last byte of the Finished message, with the second fragment containing the final byte. During the handshake and prior to the client sending the CCS the MITM injects a plaintext Finished message fragment to the server containing all but the final byte of the Finished message. The message sequence number should be the one expected to be used for the real Finished message. OpenSSL will recognise that the received fragment is for the future and will buffer it for later use. After the client sends the CCS it then sends its own Finished message in two fragments. The MITM causes the first of these fragments to be dropped. The OpenSSL server will then receive the second of the fragments and reassemble the complete Finished message consisting of the MITM fragment and the final byte from the real client. The advantage to the attacker in injecting a Finished message is that this provides the capability to modify other handshake messages (e.g. the ClientHello) undetected. A difficulty for the attacker is knowing in advance what impact any of those changes might have on the final byte of the handshake hash that is going to be sent in the "real" Finished message. In the worst case for the attacker this means that only 1 in 256 of such injection attempts will succeed. It may be possible in some situations for the attacker to improve this such that all attempts succeed. For example if the handshake includes client authentication then the final message flight sent by the client will include a Certificate. Certificates are ASN.1 objects where the signed portion is DER encoded. The non-signed portion could be BER encoded and so the attacker could re-encode the certificate such that the hash for the whole handshake comes to a different value. The certificate re-encoding would not be detectable because only the non-signed portion is changed. As this is the final flight of messages sent from the client the attacker knows what the complete hanshake hash value will be that the client will send - and therefore knows what the final byte will be. Through a process of trial and error the attacker can re-encode the certificate until the modified handhshake also has a hash with the same final byte. This means that when the Finished message is verified by the server it will be correct in all cases. In practice the MITM would need to be able to perform the same attack against both the client and the server. If the attack is only performed against the server (say) then the server will not detect the modified handshake, but the client will and will abort the connection. Fortunately, although OpenSSL is vulnerable to Finished message injection, it is not vulnerable if *both* client and server are OpenSSL. The reason is that OpenSSL has a hard "floor" for a minimum MTU size that it will never go below. This minimum means that a Finished message will never be sent in a fragmented form and therefore the MITM does not have one of its pre-requisites. Therefore this could only be exploited if using OpenSSL and some other DTLS peer that had its own and separate Finished message injection flaw. The fix is to ensure buffered messages are cleared on epoch change. Reviewed-by: Richard Levitte <levitte@openssl.org> |
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| apps | ||
| bugs | ||
| certs | ||
| crypto | ||
| demos | ||
| doc | ||
| engines | ||
| MacOS | ||
| ms | ||
| Netware | ||
| os2 | ||
| shlib | ||
| ssl | ||
| test | ||
| tools | ||
| util | ||
| VMS | ||
| .gitignore | ||
| .travis-create-release.sh | ||
| .travis.yml | ||
| ACKNOWLEDGMENTS | ||
| appveyor.yml | ||
| CHANGES | ||
| CHANGES.SSLeay | ||
| config | ||
| Configure | ||
| CONTRIBUTING | ||
| e_os.h | ||
| e_os2.h | ||
| FAQ | ||
| GitConfigure | ||
| GitMake | ||
| INSTALL | ||
| install.com | ||
| INSTALL.DJGPP | ||
| INSTALL.MacOS | ||
| INSTALL.NW | ||
| INSTALL.OS2 | ||
| INSTALL.VMS | ||
| INSTALL.W32 | ||
| INSTALL.W64 | ||
| INSTALL.WCE | ||
| LICENSE | ||
| Makefile.org | ||
| Makefile.shared | ||
| makevms.com | ||
| NEWS | ||
| openssl.doxy | ||
| openssl.spec | ||
| PROBLEMS | ||
| README | ||
| README.ASN1 | ||
| README.ENGINE | ||
| TABLE | ||
OpenSSL 1.0.2i-dev
Copyright (c) 1998-2015 The OpenSSL Project
Copyright (c) 1995-1998 Eric A. Young, Tim J. Hudson
All rights reserved.
DESCRIPTION
-----------
The OpenSSL Project is a collaborative effort to develop a robust,
commercial-grade, fully featured, and Open Source toolkit implementing the
Secure Sockets Layer (SSLv3) and Transport Layer Security (TLS) protocols as
well as a full-strength general purpose cryptograpic library. The project is
managed by a worldwide community of volunteers that use the Internet to
communicate, plan, and develop the OpenSSL toolkit and its related
documentation.
OpenSSL is descended from the SSLeay library developed by Eric A. Young
and Tim J. Hudson. The OpenSSL toolkit is licensed under a dual-license (the
OpenSSL license plus the SSLeay license), which means that you are free to
get and use it for commercial and non-commercial purposes as long as you
fulfill the conditions of both licenses.
OVERVIEW
--------
The OpenSSL toolkit includes:
libssl.a:
Provides the client and server-side implementations for SSLv3 and TLS.
libcrypto.a:
Provides general cryptographic and X.509 support needed by SSL/TLS but
not logically part of it.
openssl:
A command line tool that can be used for:
Creation of key parameters
Creation of X.509 certificates, CSRs and CRLs
Calculation of message digests
Encryption and decryption
SSL/TLS client and server tests
Handling of S/MIME signed or encrypted mail
And more...
INSTALLATION
------------
See the appropriate file:
INSTALL Linux, Unix, etc.
INSTALL.DJGPP DOS platform with DJGPP
INSTALL.NW Netware
INSTALL.OS2 OS/2
INSTALL.VMS VMS
INSTALL.W32 Windows (32bit)
INSTALL.W64 Windows (64bit)
INSTALL.WCE Windows CE
SUPPORT
-------
See the OpenSSL website www.openssl.org for details on how to obtain
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If you have any problems with OpenSSL then please take the following steps
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- Download the current snapshot from ftp://ftp.openssl.org/snapshot/
to see if the problem has already been addressed
- Remove ASM versions of libraries
- Remove compiler optimisation flags
If you wish to report a bug then please include the following information in
any bug report:
- On Unix systems:
Self-test report generated by 'make report'
- On other systems:
OpenSSL version: output of 'openssl version -a'
OS Name, Version, Hardware platform
Compiler Details (name, version)
- Application Details (name, version)
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Email the report to:
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In order to avoid spam, this is a moderated mailing list, and it might
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Just because something doesn't work the way you expect does not mean it
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----------------------------
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LEGALITIES
----------
A number of nations, in particular the U.S., restrict the use or export
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