OpenSSL gave everyone alarm fatigue
Published on 11/01/2022, 1611 words, 6 minutes to read1girl, kimono, animal crossing, klaxon, loud noises, overwhelming, red sky, clouds, storms, long hair, purple hair, yellow eyes, fox ears, thick outlines, ink outlines, black outlines - Waifu Diffusion v1.3 (float16)
So, the OpenSSL security issue embargo ended today and the patches dropped. Based on the contents of the security issue, the difficulty of exploiting it in practice, and the fact that most Linux distributions take basic precautions to prevent it from being a viable attack vector: this issue doesn't affect nearly any users of OpenSSL in the real world.
For more details on CVE-2022-3786 and CVE-2022-3602, see the OpenSSL team's post titled CVE-2022-3786 and CVE-2022-3602: X.509 Email Address Buffer Overflows.
tl;dr the vulnerability is a combination of two forces of complexity
converging in a way that nobody expected:
X.509 formats for digital certificates
and Punycode, an encoding standard
for putting characters that are unrepresentable in DNS into DNS (such as
我很伤心.example being encoded into
xn--hpq721bdf90g.example). This works
in email addresses too. The issue was that if you gave a malformed email
address in a TLS certificate, then that could percolate up into remote code
execution through a buffer overflow on some platforms. Thankfully, that
definition of "some platforms" does not include Ubuntu on amd64 CPUs.
The main reason why this isn't a big deal for most people is the fact that most modern compilers enable stack overflow protection, which will stop this buffer overflow from causing arbitrary code to be executed. In practice, the stack protector will realize that something is wrong and make the program explode. This reduces the attack from an arbitrary code execution attack to a simple denial of service attack, which is a lot more boring in practice.
For more details on this, see this writeup that explains how this attack works, how you can replicate it at home with a vulnerable version of OpenSSL, and ultimately why it's not a practical problem. If any organization wanted to use this, it would almost certainly result in the certificate being noticed on certificate transparency logs and then potentially the entire certificate authority's ability to issue certificates would be brought into question. This would likely end up with the certificate authority not being allowed to issue certificates until the situations that allowed that authority to issue certificates were fixed and independently evaluated to be fixed.
I guess it is most viable in environments with custom TLS root certificates, but those are rare enough in practice to not really make this a useful attack mechanism. There's much lower-hanging fruit in the equation. Maybe people really do distrust certificate authorities that much, though.
Oh, that could be very bad. Oh god that could be so bad.
I feel somewhat compelled to apologize for how I've been treating the OpenSSL issue on my blog. I took the OpenSSL team at their word, and I feel that I have done the right thing, even if it turned out to be a giant nothingburger in practice. If my coverage of the issues harmed you or made you worry beyond normal, I'm sorry. I was going off of the information I was given and I had heard from several sources I trust that it was something to really care about. I will use this experience to help shape how I report on these things in the future.
Why I'm frustrated
There are two big things that frustrate me about this pair of vulnerabilities.
The first is that apparently the OpenSSL team does not know what compiler flags are used to compile OpenSSL on popular targets. Like, I get it, at some level this makes sense. OpenSSL is used in a billionty different environments and all of them are unique snowflakes.
However, at some level this feels a bit inexcusable. Ubuntu and CentOS are some of the biggest users of OpenSSL. People that work for Canonical and Red Hat are contributors to OpenSSL. I feel that they should have some sense of what the most common environments do. Sure they are not going to be able to stop some embedded device in Palau from ACE-ing a pointer from an email address in a certificate, but I'd certainly hope that Ubuntu and CentOS are the targets that they really care about and validate against.
The second thing that bothers me is that it seems that it was downgraded from a "CRITICAL" bug to a "HIGH" bug earlier on today, November 1st, 2022. To give the OpenSSL team the widest possible benefit of the doubt, this is somewhat reasonable. As they continued to poke at things and research how vulnerable people were, they realized that their initial assessment of the issue was wrong. This means that people that were saying it was very bad were accurately reporting on what they understood at the time. At some level this is how the process of science works: you have a hypothesis, you test it, you report the results. It is just frustrating that it took so long to downgrade it. I was prepared for a critical vulnerability and I'm sure other people were too.
I'm worried that this is going to be seen as a reason to not take "CRITICAL" disclosures seriously at first glance like we should. A "CRITICAL" bug MUST be treated as if it was critically bad. From a community health perspective, people have been told that something really bad is about to come out for a week and then had the rug pulled out from under them and now it's "nah we were wrong you're probably fine".
Again, this happens, and it's perfectly rational for them to change their minds. I just worry that this level of alarm fatigue has just trained people to not take things seriously because this time it wasn't so bad.
Turns out the OpenSSL advisory was more disruptive than the vulnerability.
— dkp (@tweetdkp)
November 1, 2022
NixOS has stack protection enabled, so it was never vulnerable to this issue in
the first place. My article
telling people to recompile nginx with OpenSSL 1.x wasn't needed. However, it
was a cool way to show off
overrides in nixpkgs, so I'm going to consider that
a win regardless.
Hot Take Zone™️
Maybe we need to stop writing new security-critical code in C. Here is the entire contents of one of the patches that fixed CVE-2022-3602:
--- a/crypto/punycode.c +++ b/crypto/punycode.c @@ -181,7 +181,7 @@ int ossl_punycode_decode(const char *pEncoded, const size_t enc_len, n = n + i / (written_out + 1); i %= (written_out + 1); - if (written_out > max_out) + if (written_out >= max_out) return 0; memmove(pDecoded + i + 1, pDecoded + i,
For those of you that don't know C that well, here's my attempt to explain what's going on. This is in a function that decodes punycode text into its corresponding Unicode text. One of the things it does is copy data from the input buffer, decode it, and then put the results in an output buffer. This if statement is here to prevent the buffer from running over and into whatever comes next. If that "whatever comes next" is in the stack, then the value can potentially overflow and then that value becomes the return address for when that function is called.
The root cause of this is an off-by-one out-of-bounds write to the output buffer. This is made possible because C does not have ways for you to statically prove that you cannot overwrite buffers like this such that the compiler will refuse to compile the code.
Maybe as an industry we need to start taking formal verification, memory safety, and related things a bit more seriously. Maybe adding Rust to the Linux kernel is a good thing. Rust was also recently added to the Windows kernel without anyone really noticing its presence.
But if we write things in Rust then it won't work on my Alpha VAX cluster full of OpenVMS machines that runs the logic for a secure cluster of Gopher servers!
Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.
Tags: openssl, rant, security, noxp