We rate the impact of individual vulnerabilities on the same four point scale, designed to be an at-a-glance guide to how worried Red Hat is about each security issue. The scale takes into account the potential risk of a flaw based on a technical analysis of the exact flaw and it's type, but not the current threat level. Therefore the rating given to an issue will not change if an exploit or worm is later released for a flaw, or if one is available before release of a fix.
For the purpose of evaluating severities, our protection technologies fall into roughly three categories:
I've not been keeping a list of vulnerabilities that are deterministically blocked, but I have a couple of examples I recall where we did alter the severity:
It may seem surprising that we release security updates for a product exactly at the same time we release it, but product development is frozen for some weeks before we release the product to give time testing from the various Quality Engineering teams as well as release engineering work. During that time we want to minimise the number of changes that will invalidate the overall testing, so we instead prepare the changes as updates. Since the vulnerabilities being fixed are already public, we push the updates out as soon as we can; holding them off to some scheduled monthly date would just increase customer risk.
Security advisories for Enterprise Linux 5 are available from the usual places, on the web, sent to the enterprise-watch-list mailing list, and via OVAL definitions. Red Hat Network subscribers can also get customized mails for the subset of issues that affect the packages they actually have installed.
For me, what's going to be interesting to watch over the next few months is how specific vulnerabilities and exploits affect this platform. Red Hat Enterprise Linux 5 packages are compiled both with Fortify Source and stack smashing protection in addition to all the security features that were in version 4. I'll be reporting on what difference this makes through the year.
Since 1999, all RPM packages in Red Hat products have been gpg signed by the master key "Red Hat, Inc <firstname.lastname@example.org>" (keyid DB42A60E). I'll call this the legacy signing key for the rest of this article. This signature is one of two security mechanisms we use to ensure that customers can trust the installation of packages and their updates. The other is that the update client, up2date, checks the SSL server signature when it connects to the Red Hat Network to ensure that it only talks to official Red Hat servers; so removing the possibility of a man-in-the-middle attack.
From 2007, all new products will be signed with a different master key, "Red Hat, Inc. (release key) <email@example.com>" (keyid 37017186). This includes Red Hat Enterprise Linux 5, and any other new products that use RPM packages. The exception to this rule is that any new layered products designed for older versions of Enterprise Linux will still use the legacy key: so for example, a new version of the Application Stack for Red Hat Enterprise Linux 4 will be signed with the legacy key.
The legacy key hasn't been compromised so why change keys? It's all to do with the way the keys are stored and managed internal to Red Hat. The legacy key is a software key and so the key material exists, protected by a passphrase, on a hard disk. When packages need to be signed one of the Red Hat authorised signers manually runs a signing command, this calls
rpm --resign which asks for the passphrase then in turn calls out to GNUpg to do the actual signature creation. So the authorised signers not only had the ability to sign with the key, but they also have the ability to read the key material. In theory this means that a malicious internal signer could copy the key, take it away with them, and sign whatever and whenever they
wanted. Or, more likely, a clever intruder who gained access to our internal network could perhaps capture the key and passphrase, compromising the key. The risks mean we've had to be really careful who has signing privileges with the legacy key and how the key signing is handled.
The new key, in contrast, was created in a hardware cryptographic device which does not allow the unprotected key material to be exported. This means we can give authorised signers the ability to sign with the key, but no one can ever can get access to the key material itself. This is an important distinction. If for example a current authorised signer switches roles and is no longer responsible for package signing we can instantly revoke their rights and know that they no longer have the ability to sign any more packages with that key.
There was no off-the-shelf solution available for hardware-based RPM key management, so we developed one internally ourselves. We used nCipher nShield hardware security modules (FIPS 140-2 validated) for the key protection along with custom patches I developed to interface RPM/GNUpg to the unit. At the same time we also introduced an extra layer of abstraction to the signing software, so we can authorize signers using their existing internal kerberos credentials.
So, as a customer, you won't really notice any difference. For Red Hat Enterprise Linux 5 you'll find the public keys on our website as well as in the
/etc/pki/rpm-gpg/ directory and you'll be prompted when updating or installing new packages for the first time to import that new public key.
This change basically makes it easier for us to protect our signing key and reduce the risk of it being compromised, therefore reducing the chances we'll need to change the key and involve customer effort in the future.
|Fedora Core||Red Hat Enterprise Linux|
|Default requires signed updates||Y||Y||Y||Y||Y||Y||Y||Y|
|NX emulation using segment limits by default||Y||Y||Y||Y||Y||Y||since 2004Sep||Y|
|Support for Position Independent Executables (PIE)||Y||Y||Y||Y||Y||Y||since 2004Sep||Y|
|ASLR for Stack/mmap by default||Y||Y||Y||Y||Y||Y||since 2004Sep||Y|
|ASLR for vDSO (if vDSO enabled)||no vDSO||Y||Y||Y||Y||Y||no vDSO||Y|
|Restricted access to kernel memory by default||Y||Y||Y||Y||Y||Y|
|NX by default for supported processors/kernels||since 2004Jun||Y||Y||Y||Y||since 2004Sep||Y|
|Support for SELinux||Y||Y||Y||Y||Y||Y|
|SELinux default enabled with targetted policies||Y||Y||Y||Y||Y|
|glibc heap/memory checks by default||Y||Y||Y||Y||Y|
|Support for FORTIFY_SOURCE, used on selected packages||Y||Y||Y||Y||Y|
|All packages compiled using FORTIFY_SOURCE||Y||Y||Y|
|Support for ELF Data Hardening||Y||Y||Y||Y|
|All packages compiled with stack smashing protection||Y||Y|
|OVAL compatible||since 2006May||since 2006May|
First, the Vulnerability Type Distribution graph. This is not really a big surprise, the most common vulnerabilities we fix are buffer overflows. Technologies such as ExecShield (PIE, support for NX, FORTIFY_SOURCE and so on) were designed specifically to reduce the risk of being able to exploit this flaw type. Secondly, compared to the industry as a whole we fix far less web application flaws such as cross-site scripting or SQL injection. This result is to be expected as most of these are in PHP web applications we don't ship in our distributions.
This issue is technically interesting as the off-by-one only lets you write one pointer to the space immediately after a stack buffer. So the ability to exploit this issue is totally dependent on the stack layout for a particular compiled version of mod_rewrite. If the compiler used has added padding to the stack immediately after the buffer being overwritten, this issue can not be exploited, and Apache httpd will continue operating normally. Many older (up to a year or so ago) versions of gcc pad stack buffers on most architectures.
The Red Hat Security Response Team analysed Red Hat Enterprise Linux 3 and Red Hat Enterprise Linux 4 binaries for all architectures as shipped by Red Hat and determined that these versions cannot be exploited. We therefore do not plan on providing updates for this issue.
In contrast, our Fedora Core 4 and 5 builds are vulnerable as the compiler version used adds no stack padding. For these builds, the pointer being overwritten overwrites a saved register and, unfortunately, one that has possible security consequences. It's still quite unlikely we'll see a worm appear for this issue that affects Fedora though: for one thing, the vulnerability can only be exploited when mod_rewrite is enabled and a specific style of RewriteRule is used. So it's likely to be different on every vulnerable site (unless someone has some third party product that relies on some vulnerable rewrite rules). Even then, you still need to be able to defeat the Fedora Core randomization to be able to reliably do anything interesting with this flaw.
So, as you can probably tell, I spent a few days this week analysing assembler dumps of our Apache binaries on some architectures. It was more fun than expected; mostly because I used to code full-time in assembler, although that was over 15 years ago.
In the past I've posted timelines of when we found out about issues and dealt with them in Apache; so for those who are interested:
20060721-23:29 Mark Dowd forwards details of issue to firstname.lastname@example.org 20060722-07:42 Initial response from Apache security team 20060722-08:14 Investigation, testing, and patches created 20060724-19:04 Negotiated release date with reporter 20060725-10:00 Notified NISCC and CERT to give vendors heads up 20060727-17:00 Fixes committed publically 20060727-23:30 Updates released to Apache site 20060828 Public announcement from Apache, McAfee, CERT, NISCCHere is the patch against 2.0, the patch against 1.3 or 2.2 is almost identical.
This vulnerability does not affect Red Hat Enterprise Linux 2.1 or 3 as they are based on 2.4 kernels.
Red Hat Enterprise Linux 4, Fedora Core 4, and Fedora Core 5 do not support the a.out binary format, causing the exploit to fail. We are not currently aware of any way to exploit this vulnerability if a.out binary format is not enabled. In addition, a default installation of these OS enables SELinux in enforcing mode. SELinux also completely blocks attempts to exploit this issue.
For more technical details of this issue please see bz#198973
The Red Hat Security response team have therefore rated this as having moderate security severity for Enterprise Linux 4. No asynchronous kernel update for this issue is currently planned; the fix for the flaw will be included in some later scheduled update.