Third-Party Audit of Rustls

> After spending thirty days on the scope in late May and early June of 2020, the team of auditors considered the general code quality really good and can attest to a solid impression left consistently by all scope items. Naturally, this is partially thanks to the usage of Rust as the preferred language for the entire implementation of the rustls project.

Some quotes from the report's conclusions:

"the team of auditors considered the general code quality really good and can attest to a solid impression left consistently by all scope items."

"was consistently well-documented and readable, demonstrating that security processes are ingrained in the development and documentation processes"

"Both from a design point of view as from an implementation perspective the entire scope can be considered of exceptionally high standard (...) no directly exploitable weaknesses could be identified."

"It appears to have been developed with all previously known issue-types in mind; furthermore, its missing support for insecure or outdated protocols and primitives indicates a security-conscious development approach."

"the developers of rustls have an extensive knowledge on how to correctly implement the TLS stack whilst avoiding the common pitfalls that surround the TLS ecosystem. This knowledge has translated reliably into an implementation of exceptional quality."

"The developer’s intent to provide a high-quality TLS implementation is very clear and this goal can be considered as achieved successfully. (...) Cure53 had the rare pleasure of being incredibly impressed with the presented software."

This is really exciting! Many popular Rust libraries support switching between rustls and openssl for their TLS needs and it’s much easier to do cross-platform builds with rustls.

The finding in TLS-01-003 is surprising to me, mostly because it presupposes a lack of sophistication among users of this library who are also using X.509 NameConstraints. From RFC5280:

  For example, a name constraint for "class C" subnet 192.0.2.0 is represented as the octets C0 00 02 00 FF FF FF 00, representing the CIDR notation 192.0.2.0/24 (mask 255.255.255.0).

As they mention in the findings:

  Typically, subnet masks should be contiguous and the presence of a non-contiguous mask might indicate a typo (such as 225.255.255.0 vs. 255.255.255.0), or potentially an attempt to bypass an access control scheme. Therefore, it is recommended to treat certificates containing non-contiguous subnet masks in their name constraints as invalid.

This seems to run counter to the intent in the RFC. By allowing for a four-octet subnet mask, instead of simply an int to represent the a contiguous CIDR mask, the RFC authors may have intended that more complex IP-based NameConstraints could be constructed. This certainly would make a huge difference for something like an intermediate (CA:TRUE), where it becomes much more economical to specify a sparse mask for a highly templated network. Think certs for network equipment or VoIP phones with regular, repeatable IP addressing across many locations/networks. E.g., a VoIP provisioning system that has an intermediate issuing CA with the following NameConstraints: IP:172.16.0.0/255.255.1.239.

If any change comes from this specific finding, I would hope that it's simply a flag to allow or disallow the use of discontiguous masks. I do understand that this is specific to WebPKI; having said that, if a client is implemented using rustls (with these recommendations enabled) and it happens across a perfectly valid certificate issued by an intermediate with a discontiguous mask in the NameConstraints, presumably it would fail or otherwise break. And yes, I have previously configured precisely this in an intermediate CA.

I'm not completely certain, but my guess is a lot of software and midleware doesn't work well with non-continuous subnet masks.

It's one of this (many) thinks where in theory the RFC has intended support for this but in praxis it's close to unusable and better to block for most application use cases.

Like e.g. many valid email-addresses are in practice unusable and it's recommended to not support them because today they are basically only used to intentionally cause problems (I mean email addresses using quoting like `"a b"@example.com`, you always should support internationalized mail addresses, except maybe as mail provider where it depends on your target customers).