The TLS protocol, and the SSL protocol 3.0 and possibly earlier, as used in
Microsoft Internet Information Services (IIS) 7.0, mod_ssl in the Apache
HTTP Server 2.2.14 and earlier, OpenSSL before 0.9.8l, GnuTLS 2.8.5 and
earlier, Mozilla Network Security Services (NSS) 3.12.4 and earlier,
multiple Cisco products, and other products, does not properly associate
renegotiation handshakes with an existing connection, which allows
man-in-the-middle attackers to insert data into HTTPS sessions, and
possibly other types of sessions protected by TLS or SSL, by sending an
unauthenticated request that is processed retroactively by a server in a
post-renegotiation context, related to a "plaintext injection" attack, aka
the "Project Mogul" issue.
jdstrand> Fixing this issue requires coordination between the IETF, SSL
libraries (eg OpenSSL and GnuTLS) and TLS consumers (notably HTTPS servers,
but most (or all) servers using TLS which support TLS renegotiation).
Protocol-breaking changes are among the possibilities being
The following is based on http://extendedsubset.com/Renegotiating_TLS.pdf
and http://extendedsubset.com/Renegotiating_TLS_pd.pdf. You are encouraged to
read this document as well as the email thread on ietf-tls for complete
information and most up-to-date status (see References).
There are essentially 3 types of renegotiation scenarios known at this time
to be vulnerable, and they all require a man-in-the-middle (MITM) attack:
1. Client certificate authentication: servers configured to require client
certificate authentication on a per-directory basis. Apache is known
to be vulnerable when using this configuration. There is no generally
usable mitigation strategy known at this time.
2. Differing server cryptographic requirements: servers configured to
support different cipher suites within the same site. One mitigation
strategy is to require all content on a site to use a single cipher
suite. Disallowing specification of TLS parameters in .htaccess files
(generally modifiable by end users) may also be a good idea.
3. Client-initiated renegotiation: servers configured for TLS. Apache is
known to be vulnerable when using using TLS and the client initiates a
renegotiation. There is no generally usable mitigation strategy known at
The flaw should not allow the attacker to see the contents of the connection,
and a client cannot be redirected to another site. For the HTTPS scenarios
listed above, the attacker is able to perform arbitrary requests with the
credentials of the victim. Arbitrary POST requests may also be possible.
Analysis on the effects of this flaw for other protocols is ongoing.
Until a general fix can be found for Ubuntu, users may be interested in
reading http://www.links.org/?p=780, which has a patch to OpenSSL to disable
jdstrand> Update for apache2 disabled client initiated renegotiations. This
won't fix per-Directory/Location configurations.
mdeslaur> openssl 0.9.8l disabled renegotiations completely, with a compile
time option to turn it back on. This may break connections to servers that
haven't been patched. openssl 0.9.8m adds an option that applications can
use to turn it back on:
mdeslaur> Turning renegotiation off completely may break postgresql, openvpn
alpine, psi, fetchmail, etc.
jdstrand> NSS 3.12.6 has support for the new renegotiation extension for TLS
to implement rfc5746. NSS clients advertise their support for this
extension and if the server also supports it, will be protected from this
vulnerability. To maintain compatibility, NSS in Ubuntu will for the
foreseeable future use the so-called 'transitional' mode which will fall back
to the unprotected renegotiation method if the server doesn't support the
jdstrand> NSS was fixed in Ubuntu 9.10 because the new Firefox required it.
Because Firefox needs changes to take advantage of the new NSS, once Ubuntu
8.04 LTS - 9.04 are updated to use an embedded NSS (and therefore won't use
the system NSS), we can update the system NSS for these releases.
jdstrand> When upgrading the system NSS on Ubuntu 8.04 LTS - 9.04, be careful
about https://launchpad.net/bugs/559881 and https://launchpad.net/bugs/559918
(regressions seen with the 9.10 update).
jdstrand> postgresql 8.1.20, 8.3.10 and 8.4.3 now have ssl_renegotiation_limit
to control session key renegotiation
jdstrand> preliminary GNUtls patches at (in 2.9.10 development release):
mdeslaur> jetty 6.1.22 has a CVE-2009-3555 fix: "Prevent SSL renegotiate
for SSL vulnerability"
jdstrand> RedHat RHSA-2010:0396-01 adds the "SSLInsecureRenegotiation"
configuration directive to apache
mdeslaur> gnutls doesn't have an API for renegotiations, so ignoring.