Bug 1154059

Summary:	curl: Disable out-of-protocol fallback to SSL 3.0
Product:	Red Hat Enterprise Linux 6	Reporter:	Florian Weimer <fweimer>
Component:	curl	Assignee:	Kamil Dudka <kdudka>
Status:	CLOSED ERRATA	QA Contact:	Stefan Kremen <skremen>
Severity:	high	Docs Contact:
Priority:	high
Version:	6.5	CC:	bressers, john.haxby, ovasik, skremen, thoger, tlavigne
Target Milestone:	rc
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:	curl-7.19.7-43.el6	Doc Type:	Deprecated Functionality
Doc Text:	Previously, libcurl implemented an out-of-protocol fallback to SSL 3.0 (Secure Sockets Layer) in order to securely communicate with TLS (Transport Layer Security) intolerant servers. This could have been abused by an attacker to force downgrade of the SSL version. Therefore, this feature has been removed from libcurl. Users of libcurl that have been relying on this feature are advised to explicitly enable SSL 3.0 by the --sslv3 option of curl (or the CURL_SSLVERSION_SSLv3 constant of libcurl API) to prevent connection failures when TLS intolerant servers are involved.	Story Points:	---
Clone Of:
Clones:	1154060 (view as bug list)		Environment:
Last Closed:	2015-07-22 05:43:47 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	1154060
Bug Blocks:	1159820, 1159926

Description Florian Weimer 2014-10-17 12:26:15 UTC

We should disable the retry-with-SSL-3.0 behavior in curl because it can be trivially triggered by an on-path attacker to cause a protocol downgrade, and SSL 3.0 has several known weaknesses.  This curl behavior is quite surprising because it is restricted to the NSS backend, and other systems which use the OpenSSL or GNUTLS backend do not share this behavior.

At this stage, it is best to leave SSL 3.0 enabled.  (The in-protocol version negotiation has not been broken, sosome TLS version will be used automatically if supported by the server.)

This might reintroduce bug 525496 and bug 527771, so a system-wide knob to re-enable the SSL 3.0 fallback might be necessary.  Perhaps the existence of a file like /etc/sysconfig/curl/enable-ssl-fallback could re-enable the old behavior.

Comment 3 Tomas Hoger 2014-10-23 16:02:06 UTC

(In reply to Florian Weimer from comment #0)
> This might reintroduce bug 525496 and bug 527771, so a system-wide knob to
> re-enable the SSL 3.0 fallback might be necessary.  Perhaps the existence of
> a file like /etc/sysconfig/curl/enable-ssl-fallback could re-enable the old
> behavior.

Or environment variable?  CURL_ENABLE_SSL_FALLBACK?

Do curl try to fall back across multiple versions (TLS 1.1 -> TLS 1.0 -> SSL 3.0), or if connection attempt with the highest supported fails, it goes straight to SSL 3.0?

Comment 4 Kamil Dudka 2014-10-23 16:06:58 UTC

(In reply to Tomas Hoger from comment #3)
> Or environment variable?  CURL_ENABLE_SSL_FALLBACK?

That would also work I guess.

> Do curl try to fall back across multiple versions (TLS 1.1 -> TLS 1.0 -> SSL
> 3.0), or if connection attempt with the highest supported fails, it goes
> straight to SSL 3.0?

The fallback implemented by libcurl goes straight to SSL 3.0.

Comment 12 errata-xmlrpc 2015-07-22 05:43:47 UTC

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-1254.html