HTTP/2-exclusive threats caused by implementation flaws and RFC imperfections

by Berg0X00 • 5 years ago

393 112 5 years ago

carey2• 5 years ago

Given the long history of request parsing vulnerabilities in HTTP/1.1 servers and proxies, is HTTP/2 actually worse, or have most of the HTTP/1.1 bugs just been fixed already?

josefx3• 5 years ago

Is an new bucket leaking in a dozen places worse than an old one with all leaks fixed? I would say yes until those holes in the new one are also fixed.

rubiquity• 5 years ago

When I implemented an HTTP2 server several years ago it was all of the "fun" of HTTP 1.1 parsing and semantics plus the extra challenges of the HTTP2 optimizations such as HPACK, mapping the abbreviated headers to cache in-memory representations, stream management, and if you supported Push Promises then that too.

chrismorgan3• 5 years ago

These vulnerabilities are all from badly-written HTTP/2 → HTTP/1.1 translations. Most of them come from simple carelessness, rookie errors that should never have been made, dumping untrusted bytes from an HTTP/2 value into the HTTP/1.1 byte stream. This is security 101, straightforward injection attacks with absolutely nothing HTTP-specific in it.

Some of them are a little more complex, requiring actual HTTP/2 and HTTP/1.1 knowledge (largely meaning HTTP/2 framing and the content-length and transfer-encoding headers), but not most of them.

Is HTTP/2 actually worse? Not in the slightest; HTTP/1.1 is the problem here. This is growing pains from compatibility measures as part of removing the problems of an unstructured text protocol. If you have a pure-HTTP/2 system and don’t ever do the downgrade, you’re in a better position.

comex• 5 years ago

I'd agree that HTTP/1 deserves a significant portion of the blame.

On the other hand, one maxim I've learned from my time bug hunting is that nobody ever validates strings in binary protocols. As such, I'm utterly unsurprised there are so many implementations with these kinds of bugs, and I'd say they could have been predicted in advance.

In fact… let's see… yep, they were predicted. Some of them, at least. In the HTTP/2 RFC, under Security Considerations, 10.3 'Intermediary Encapsulation Attacks' describes one of the attack classes from the blog post, the one involving stuffing newlines into header names.

Does that mean something could have been done about it? Perhaps not. The ideal solution would be to somehow design the HTTP/2 protocol itself to be resistant to misimplementation, but that seems pretty much impossible. The spec already bans colons and newlines in header names, but there's no way to be sure implementations won't allow them anyway, short of actually making them a delimiter like HTTP/1 did – in other words, reverting to a text-based protocol. But a text-based protocol would come with its own share of misimplementation risks, the same ones that HTTP/1 has.

On the other hand, perhaps the bug classes could have been mitigated if someone designed test cases to trigger them, and either included them in conformance tests (apparently there was an official HTTP/2 test suite [2] though it doesn't seem to have been very popular), or set up some kind of bot to try them on the entire web. In principle you could blame the authors of HTTP/2 collectively for the fact that nobody did this. But I admit that's pretty handwavey.

[1] https://datatracker.ietf.org/doc/html/rfc7540#section-10.3

[2] https://github.com/http2/http2-test

tptacek4• 5 years ago

It's tough to say that something is a "rookie error" when basically every serious professional team makes the same mistake. This broke apparently broke every AWS ALB, for instance.

simiones4• 5 years ago

I would bet that a lot of these are not rookie errors, they are more akin to Spectre or Meltdown: inherently unsafe code that was considered a valuable risk for performance.

In general, when writing a high performance middle box, you want to touch the data as little as possible: ideally, the CPU wouldn't even see most of the bytes in the message, they would just be DMA'd from the external NIC to the internal NIC. This is probably not doable for HTTP2->HTTP1, but the general principle applies. In high-performance code, you don't want to go matching strings any more than you think is strictly necessary (e.g. matching the host or path to know where to actually send the packet).

Which is not to say that it wasn't a mistake to assume you can get away with this trade-off. But it's not a rookie error.

RcouF1uZ4gsC• 5 years ago

This means there's little room for ambiguity about the length of a message, and might leave you wondering how desync attacks using HTTP/2 are possible. The answer is HTTP/2 downgrading.

I would guess a huge percentage of attacks are made possible because of protocol or algorithm downgrading. I wonder if built in downgrading abilities into protocols is a security smell.

yyyk• 5 years ago

The shown attacks aren't a true case of protocol downgrading, but of an overeager direct translation of request headers across protocols. The attacks weren't possible because the client is doing some backhanded protocol negotiation, but because the client/front/back are being very directly connected.

If, instead of forwarding a direct translation of the headers, the front ends calculated the relevant requests and sent that, there wouldn't have been any HTTP header attacks (it would have been much slower though).

jiggawatts3• 5 years ago

I've been burnt one too many times by trying to "upgrade" to HTTP/2.

It simply broke client certificate authentication and NTLM authentication, leaving only cookie-based authentication fully functional.

Can you guess which two of the three popular authentication mechanisms Google doesn't use?

It's not a protocol designed to advance the Internet, it's a protocol designed by Google to shave 1% off their network bill.

quicklime• 5 years ago

To be fair, NTLM authentication being broken has nothing to do with HTTP/2.

georgyo2• 5 years ago

I always considere NTLM and Kerberos to be very similar, so I am surprised to hear that NTLM is broken.

Kerberos and HTTP/2 work just fine together.

What in particular is broken with NTLM?

edoceo• 5 years ago

NTLM

carey2• 5 years ago

At least when it was released, IIS 10 supported HTTP/2, but did not support Kerberos, Negotiate or NTLM authentication over HTTP/2.

https://docs.microsoft.com/en-us/iis/get-started/whats-new-i...

I’m not sure that it’s a problem with HTTP/2 that Microsoft found no reason to implement Windows authentication for it.

marcosdumay• 5 years ago

Extending the sibling comment a bit, if you run it exclusively on HTTPS, with Firefox or some other browser that understands it shouldn't share your password with any site that asks, it's just a bug-prone, slow authentication mechanism.

But MS has also the "Negotiate" protocol they made to replace NTLM on IE (because even the IE team considered it irremediably broken). That one is a variant of Kerberos, and a lot of people say just NTLM when they mean "Negotiate with a possible downgrade to NTLM".

1vuio0pswjnm75• 5 years ago

"HTTP/2's binary format means you can't use classic general-purpose tools like netcat and openssl. HTTP/2's complexity means you can't easily implement your own client, so you'll need to use a library."

IOW, HTTP/2 is not designed for users. It is designed for online advertisers and the companies that serve them.

How do I know this. Because HTTP/1.1 pipelining still works great. I am using it on a separate task as I type this (I use it outside the browser).

It just doesn't work well for companies that profit from online advertising as a "business model", the so-called "ecosystem" of actors seeking to capture "eyeballs" and Hoover up data about users.

That includes people who design whiz-bang websites making dozens to hundreds of "automatic" requests not initiated by the user, many for arguably unnecessary external resources (or for telemetry), to gather data on the user and/or to serve her with ads.

The type of websites that drive HN readers nuts. That is not a problem with HTTP/1.1, that's a problem with the effects of online advertising, surveillance and greed run amok.

It also includes popular browsers supported by advertising. HTTP does not exist exclusively for a handful of corporate-sponsored browsers. It exists for all present and future clients that users wish to use to retreive information from the web.

HTTP/2 is biased against users in favour of a web that exists only for online advertising (so it can keep filling Google's pockets with cash). It is a protocol that is so complex that users, not to mention the author of this blog post, "cannot easily implement clients for it."

dagenix4• 5 years ago

HTTP/1.1 pipelining is fundamentally flawed and largely unused. There isn't some conspiracy not to use it - many, many projects evaluated it and pretty much all came to that same conclusion.

simiones4• 5 years ago

Note that in the quote above "use" refers to security researchers trying to send malformed traffic.

The only usability difference between HTTP2 and HTTP1(.1) is that it's easier to write a broken implementation of a quarter of HTTP1 and still do some useful things with it.

A full, production-grade HTTP1.1 client or server is more or less as complex as an HTTP2 client or server. HTTP2 is actually easier to implement at the base level, since it's much easier to work with binary protocol formats than with the horrible string encoding of HTTP1. HTTP2 does add a lot of complexity with the stream multiplexing features afterwards - more or less enough as to cover the endless headache of deciphering HTTP 1.1 requests and arcane connection headers.

duskwuff3• 5 years ago

Can you guess which two of the three popular authentication mechanisms Google doesn't use?

Google is hardly alone. Non-cookie-based authentication mechanisms aren't used (and aren't even usable) on any other public web site, either.

TLS client certificates have always been a UX nightmare. There is no standard UI for creating, installing, or selecting a certificate to use to authenticate to a web server; many browsers (especially on mobile devices) don't even support those features. There's no way to log out without closing the browser. There's no way for an average user to transfer a certificate from one device to another.

NTLM is simply irrelevant outside the scope of a Windows network. Other HTTP password mechanisms have many of the same failings as client certificates -- the UI is clunky and sometimes unavailable, and there's no standard way to log out.

jiggawatts3• 5 years ago

aren't used ... on any other public web site, either.

Internal-use web sites are a thing, however. A very common, heavily used thing. Many of them use NTLM or Kerberos authentication.

TLS client certificates have always been a UX nightmare.

Which isn't an issue for APIs accessed over HTTPS, many of which are "public" yet use client certificates for authentication.

ClumsyPilot• 5 years ago

"There's no way to log out without closing the browser. "

Itw like theu gave up halfway when inplementing it. Wtf

pabs34• 5 years ago

There's no way for an average user to transfer a certificate from one device to another.

Wouldn't you just enrol a new certificate from the new device?

pabs34• 5 years ago

There's no way to log out without closing the browser.

Well, with TLS client certs, each request is essentially similar to a new login, request, logout, so "logout" doesn't really make sense, unless you want to it to mean "stop using the cert temporarily"? Perhaps the UX for certs should be more like "this site wants you to login", you press login in the browser UI, then all future requests are signed with your cert, until you click logout in the browser UI and then they aren't signed any more.

pabs34• 5 years ago

TLS client certificates have always been a UX nightmare.

Indeed, it is very sad; they could be a great auth standard if only the client UX was acceptable.

nerdponx• 5 years ago

I'm not a security person at all. Is there a quick explanation for how these attacks allow the attacker to mess with other clients' requests?

I'm also confused at their usage of the word "prefix". Did they mean "suffix"? Or does "prefix" have a specific meaning in this case?

remram• 5 years ago

The next client's request gets sent after your request. If you get frontend and backend servers to desync, the backend gets to your first request, responds, and the response is sent to you; then the backend gets your second request + the next client's request, responds, and the response is sent to that next client.

This means you are making other clients get responses for their request prefixed by your smuggled second request.

d_meeze• 5 years ago

Burying the lede... Unfortunately, at the time of this whitepaper being published - 86 days after Apache was notified of the vulnerability - 2.4.49 not yet come out, so although there's a patch on master, this is effectively a zero-day.

slivanes• 5 years ago

I'm not seeing anything about this in nginx changelogs (or matching CVE's) either. Disabling http2 for now.

yxhuvud• 5 years ago

Why would an Apache bug show up in Nginx changelogs?

paco33463• 5 years ago

Really good explanations here. As a Cloudflare Enterprise customer I've opened a case with them to see if any of these vulnerabilities apply to their WAF. I was a bit surprised to see the article mention that Imperva's WAF was vulnerable.

edit: Cloudflare has responded that "[they] are asking your question internally."

albinowax_7• 5 years ago

As far as I'm aware, none of the techniques referenced in this paper work on Cloudflare as-is. Of course I probably missed a bunch of variations, so it's still worth them doing some internal checks.

El_RIDO• 5 years ago

You can partially answer this question yourself. So the issue can occur, when HTTP/2 requests get translated into HTTP/1.1 calls. Your HTTP calls stack from client to web app presumably looks something like this:

Client → (HTTP/2) → Cloudflare ingress → (???) → Cloudflare egress → (???) → Your endpoint → (???) → Your web app

On your endpoint (LB or webserver) logs you can check if the requests coming in from Cloudbees are using HTTP/2 or not. If they do use HTTP/2, they internally might still do translation down to HTTP/1.1 and back.

But more importantly, you should investigate how your LB or webserver talks to the web app. In many cases that part won't be TLS encrypted and therefore on an HTTP/1.1 channel.

The exception is if your outward facing, TLS terminating, webserver interacts with the web app via CGI or FastCGI. While that part of the call stack would not be affected by this particular attack, (Fast)CGI comes with it's own set of risks.

In the end, the only part that you can't easily validate yourself of being susceptible to this attack (aka having to translate between HTTP/2 and HTTP/1.1) is the bit that occurs internally to Cloudflare.

hmsimha• 5 years ago

But more importantly, you should investigate how your LB or webserver talks to the web app. In many cases that part won't be TLS encrypted and therefore on an HTTP/1.1 channel.

Or even HTTP 1.0 . I found out recently while inspecting some requests in the upstream server that nginx was using HTTP 1.0 after terminating TLS. I was dumbfounded that this was still the default.

http://nginx.org/en/docs/http/ngx_http_proxy_module.html#pro...

paco33463• 5 years ago

Our parent company is an Akamai Enterprise user. We've opened a case with them as well.

not2b• 5 years ago

It seems most of the vulnerabilities here (at least, the first several that I read in the article) come at a point where an HTTP/2 request is transformed into an HTTP 1.1 request. So the insecurity seems to come at the point where a translation occurs and there is redundant information that can be handled in several ways, meaning that there's lots of room for bugs.

mhitza• 5 years ago

So I was scratching my head for a good couple of minutes trying to figure out how this works, being familiar only with HTTP Response Splitting/HTTP Cache Poisoning.

So it seemed that somewhere along the years while I haven't been paying any attention to websec, it became a common practice to send requests from different clients through the same TLS connection. And due to the non conforming way HTTP 2/1.1 interop was implemented by these webservers/load balancers request boundaries were not delimited correctly, making it possible to inject requests on behalf of followup clients.

Fine I get the issue. Sounds like another "good enough" optimization that backfired.

What is the solution aside from playing the patching wack-a-mole game? Should maybe a HTTP2.1 protocol work without a strict TLS requirement so that protocol downgrades aren't necessary to squeeze out extra performance (unless I misunderstand why the HTTP2/HTTP1.1 bridge was in place). Or is the problem that some application servers still don't support HTTP2 out of the box?

tptacek4• 5 years ago

It's worth knowing that this is an extension of a very widespread attack on HTTP/1.1; many (maybe most?) 1.1 implementations were recently broken by desync just a couple years ago.

ikiris2• 5 years ago

this kind of terrible implementation is a lot of why encrypted quic exists.

georgyo2• 5 years ago

Though I think these same problems would happen over http3/quic.

Http3 is almost exactly http2 but over quic. This means the requests should be possible to force the same desyncs talked about in the article.

oefrha• 5 years ago

Out of the ecosystems I’m familiar with, Python application servers have terrible http2 support: neither gunicorn nor uwsgi supports it, and even new hotness like uvicorn is pretty far from it.

I don’t think Ruby is doing much better? Correct me if I’m wrong.

darkwater2• 5 years ago

But why would you need HTTP/2 perfect support in real world application server? They are never going to terminate the client traffic, they will speak with a load balancer which can speak HTTP/1.1 with them. Sure, if you are at webscale or even less you want everything on HTTP/2 for optimization sake. But in the rest of cases, even if you are in a solo project, you can easily enough put an nginx before it, or a cloud native solution, or haproxy or whatever.

JulianWasTaken• 5 years ago

The building blocks are there. In Python we have the wonderful (and wonderfully sans-io) h2[1] by Cory Benfield. E.g. here's a Twisted h2-using implementation: https://python-hyper.org/projects/hyper-h2/en/stable/twisted...

h2: https://github.com/python-hyper/h2

tsimionescu2• 5 years ago

I think the main problem is indeed missing HTTP2 support in backend servers. This is often just a case of people not being willing to upgrade for various reasons, even if the technology they are using would support HTTP2 in newer versions.

londons_explore3• 5 years ago

The problem is that HTTP/2 pretty much forces encryption. Most people don't want to deal with certificate management/rotation on every single microservice's application server.

paco33463• 5 years ago

In the reddit thread someone posted the recording: https://www.youtube.com/watch?v=rHxVVeM9R-M

bastawhiz• 5 years ago

I'd love to see a post from someone about their experiences using H2 between servers (i.e., instead of H1). Many big companies already use TLS internally, and I can imagine perf wins.

dagenix4• 5 years ago

The title seem very much anti-HTTP/2. However, unless I missed something huge, the vast majority, if not all, of the issues found relate to how hard it is to handle HTTP/1. And it seems kinda odd to blame HTTP/2 for HTTP/1 being difficult to implement.

I'm certainly not trying to downplay the seriousness of these issues. But it seems like a equally (it not more) valid title might be something like "HTTP/1: Continues to rear its ugly head" or something like that.

stkdump• 5 years ago

I guess this is more in line with the narrative that older protocols that people are more familiar with are somehow always better. That there is a hidden evil agenda for introducing something new. Or at least that we all need to pay a price for the sole benefit of a tech giant. It is a convenient narrative, which could be true, but it feels too easy a conclusion. Which makes it a good clickbait.

dathinab• 5 years ago

It is a convenient narrative

It's also not he narrative the content of the article purses ;-)

albinowax_7• 5 years ago

Hi, I'm the author. This paper is about HTTP/2, and the dangerous things that happen behind the scenes with HTTP/1 when people enable it. It's arguing that enabling HTTP/2 on your front-end makes your security worse if it's downgrading behind the scenes.

This is a deliberately cheeky title, selected as a presentation title, rather than being aimed at social media where many people will read the title but not the contents. For HN I'd go with something dry like "HTTP/2-exclusive attack vectors".

jpambrun• 5 years ago

My employer's mandatory proxy downgrades all HTTP/2, presumably so they can better inspect traffic. This results in a bad/slow experience on many sites. I wish HTTP/2 would not have downgrade at all so they would have to deal with it..

phicoh4• 5 years ago

Yes, it is weird that a broken protocol translator results in blame for the newer protocol.

As far as I can tell, the article doesn't attack HTTP2, that seems work fine. The article clearly demonstrates the problems of HTTP1. But the real problem is sloppy HTTP2 forntends that generate broken HTTP1.

josefx3• 5 years ago

Yes, it is weird that a broken protocol translator results in blame for the newer protocol.

From a look at the HTTP/2 spec. this protocol translation is an expected use case and explicitly results in several restrictions on the contents of HTTP/2 headers. So going by the HTTP/2 spec. at least some of those headers should have been rejected by a conformant front end and never made it into HTTP/1.

rini17• 5 years ago

Majority of the cases were requests that HTTP/2 clearly specified as malformed but they were not treated as such. It's reasonable to expect there will be other such implementation vulns that won't stem from HTTP/1 historical baggage - that is just where the author was looking today. So I'm fine with the "HTTP/2 is worse in its own" presumption because it's harder to implement.

Then, the \r\n in headers can easily trip an application that was naively ported to http/2 supporting network library. That's too pretty ripe for blame shifting :)

twic3• 5 years ago

More than that, they relate to proxies. This sentence seems the crux to me:

If you're coding an HTTP/2 server, especially one that supports downgrading, enforce the charset limitations present in HTTP/1 - reject requests that contain newlines in headers, colons in header names, spaces in the request method, etc.

ObscureScience• 5 years ago

What's wrong with it? It seems reasonable to protect yourself from header-injections allowed by a downgrade?

Am I misunderstanding?

tyingq• 5 years ago

I imagine it also relates to things that really "proxies", but not normally thought of as such. Like PHP-FPM, wsgi, etc. Those are also doing protocol and header translation, and are very ubiquitous.

get52• 5 years ago

I can't believe this, http and the internet as a whole is basically getting horsefucked

gouda-gouda• 5 years ago

That JIRA exploit is mind-boggling bad. Posts like this remind me that we work on a house cards, not a fortress.

mintyc• 5 years ago

Wow. A fantastic set of descriptions, service and detection tooling from portswigger and colleagues.

The fact that http/1 downgrading is so prevalent and unlikely to change in the short term is the point.

albinowax_7• 5 years ago

Hi, I'm the author - please let me know if you have any questions!

brabel• 5 years ago

Just want to thank you for publishing this and taking the time to present it in such a clear manner.

As someone who has implemented HTTP/1.1 message parsing myself, it blows my mind how downgrade implementers could've made such basic mistakes as to not validate the contents of headers. The companies selling security software with these mistakes should be held accountable IMHO, as the attacks are so simple and so many that they've probably been exploited extensively in the wild, and similar attacks almost certainly continue to be.

mwcampbell• 5 years ago

In light of these vulnerabilities, should we disable Internet-facing use of HTTP/2 wherever we can?

albinowax_7• 5 years ago

I think HTTP/2 is fine when it's used end to end. So if you've got a single webserver setup, or a reverse proxy that speaks HTTP/2 to the back-end, it's great.

However, if the only way you can use HTTP/2 is by having the front-end downgrade it to HTTP/1, I would recommend disabling it.

nly2• 5 years ago

I'm a bit confused how these attacks can be used.

Inconsistencies between HTTP/1.1 and HTTP/2 allow for these behaviours but you still need to inject these malicious prefixes in to a user's request flow, right?

Can you actually set the Content-Length header from a browser on a HTTP/2 request? And isn't the suffix just ignored by the final remote (or does this depend on HTTP/1.1 pipelining?)

albinowax_7• 5 years ago

Hi, I'm the author. This paper is building on techniques explained here: https://portswigger.net/research/http-desync-attacks-request...

This attack does not require a MITM - the attacker would use a tool like Burp Suite to issue the (technically RFC-violating) HTTP request. The prefix injection happens because front-end places the attacker's request and the victim's request on the same HTTP/1.1 connection to the back-end, as shown in this diagram: https://portswigger.net/cms/images/9c/c1/4c32-article-http2-...

isn't the suffix just ignored by the final remote The back-end treats the suffix as the start of the next request, due to TCP buffering. The vast majority of servers have this kind of accidental pipelining support thanks to TCP.

twic3• 5 years ago

The target's own front-end proxy acts as the man in the middle!

coffeetrackpad• 5 years ago

Thanks for this @albinowax_ - super interesting!

Just a quick question about the H2.CL case study. Did Netflix have a separate vulnerability where their server wrongly trusted Host headers with netflix.com suffix and returned a 302 response which can redirected users to an arbitrary host? I'm just trying to see whether I interpret the case study correctly.

albinowax_7• 5 years ago

Yes that's right. If the back-end received a request that didn't contain X-Forwarded-SSL or suchlike, and had a host-header that ended in .netflix.com, it would redirect you to the host-header.

I wouldn't exactly class this as a vulnerability - more of a useful gadget. The front-end would refuse to forward such a request so it's impossible to hit this code path without request smuggling. Even if you could hit it without request smuggling it would still be useless.

gnfargbl• 5 years ago

@albinowax_, can you say anything more about how widely you have investigated the prevalence of the "H2.X via Request Splitting" vulnerability?

Although all of your attack vectors are fascinating, this seems to be the one which is particularly terrifying. It seems like one single request can basically mess up user authentication across an entire website, and for every active user, until the server becomes resynchronized somehow. If I were running a large public-facing site or application, checking for this vulnerability would be my #1 priority this morning.

albinowax_7• 5 years ago

Good question! So, my understanding is that the majority of servers that are vulnerable to regular cross-user HTTP Request Smuggling (IE, reuse connections to the back-end server) are exposed to this terrifying response queue poisoning attack. This applies to all desync types CL.TE, TE.CL, H2.CL, etc. The reason I discovered this in the H2.X case, is because it's particularly easy to trigger response queue poisoning by accident in this scenario.

That said, I haven't actually tested this on very many live servers, for obvious reasons!

terom• 5 years ago

So if AWS ALB HTTP/2 listener -> HTTP/1.1 target downgrade was vulnerable, and the recommendation is to use HTTP/2 end to end... am I reading the AWS docs [1] correctly that ALB only supports HTTP/2 -> HTTP/1.1 downgrades, but not HTTP/1.1 -> HTTP/2 upgrades? In other words, using HTTP/2 as the target group protocol only works for HTTP/2 clients, and HTTP/1.1 clients will receive an error?

[1] https://docs.aws.amazon.com/elasticloadbalancing/latest/appl...

HTTP/2-exclusive threats caused by implementation flaws and RFC imperfections

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