VSV00001 DoS vulnerability

CVE-2017-12425

Date: 2017-08-02

A wrong if statement in the varnishd source code means that particular invalid requests from the client can trigger an assert.

This causes the varnishd worker process to abort and restart, loosing the cached contents in the process.

An attacker can therefore crash the varnishd worker process on demand and effectively keep it from serving content - a Denial-of-Service attack.

Mitigation is possible from VCL or by updating to a fixed version of Varnish Cache.

Versions affected

  • 4.0.1 to 4.0.4
  • 4.1.0 to 4.1.7
  • 5.0.0
  • 5.1.0 to 5.1.2

Users of the Varnish Cache Plus product from Varnish Software: See the email you received from V-S.

Versions not affected

  • All releases up to and including 4.0.0

Fixed in

  • 4.0.5 and forward
  • 4.1.8 and forward
  • 5.1.3 and forward

Users of the Varnish Cache Plus product from Varnish Software: See the email you received from V-S.

Mitigation from VCL

Note that there are subtle differences on the VCL workarounds, depending on which version of Varnish you are running, make sure to use the right one.

These VCL snippets work by failing all client requests which attempt to use Transfer-encoding: chunked.

Normally browsers will not issue such requests, but we know there are cases where B2B applications, APIs and special webservices will use client requests with chunked encoding.

You can use this command to see if you have client traffic with chunked encoding:

varnishlog -cq ReqHeader:Transfer-Encoding -i ReqMethod -i ReqURL

If you need some requests with chunked encoding to work, you will have to write VCL code to white-list these clients based on IP/ authentication/cookies or other criteria, and then only call the exploit_workaround_xxx function for the malicios clients.

Varnish 4.0.x (and Varnish Cache Plus 4.0.x)

Set the vcc_allow_inline parameter to true, either by passing:

-pvcc_allow_inline_c=true

on the command line or by issuing the CLI command:

param.set vcc_allow_inline_c true

Then add this to the front of your VCL:

sub exploit_workaround_4_0 {
        # This needs to come before your vcl_recv function
        # The following code is only valid for Varnish Cache and
        # Varnish Cache Plus versions 4.0.x
        if (req.http.transfer-encoding ~ "(?i)chunked") {
                C{
                struct dummy_req {
                        unsigned magic;
                        int restarts;
                        int esi_level;
                        int disable_esi;
                        char hash_ignore_busy;
                        char hash_always_miss;
                        void *sp;
                        void *wrk;
                        int req_step;
                        struct {
                                void *a;
                                void *b;
                        };
                        int req_body_status;
                };
                ((struct dummy_req *)ctx->req)->req_body_status = 6;
                }C

                return (synth(503, "Bad request"));
        }
}

sub vcl_recv {
        # Call this early in your vcl_recv function
        call exploit_workaround_4_0;
}

Varnish 4.1.x and 5.0 (and Varnish Cache Plus 4.1.x)

Set the vcc_allow_inline parameter to true, either by passing:

-pvcc_allow_inline_c=true

on the command line or by issuing the CLI command:

param.set vcc_allow_inline_c true

Then add this to the front of your VCL:

sub exploit_workaround_4_1 {
        # This needs to come before your vcl_recv function
        # The following code is only valid for Varnish Cache and
        # Varnish Cache Plus versions 4.1.x and 5.0.0
        if (req.http.transfer-encoding ~ "(?i)chunked") {
                C{
                struct dummy_req {
                        unsigned magic;
                        int step;
                        int req_body_status;
                };
                ((struct dummy_req *)ctx->req)->req_body_status = 5;
                }C

                return (synth(503, "Bad request"));
        }
}

sub vcl_recv {
        # Call this early in your vcl_recv function
        call exploit_workaround_4_1;
}

Varnish 5.1.x

Add this to the front of your VCL:

sub vcl_recv {
        if (req.http.transfer-encoding ~ "(?i)chunked") {
                return (fail);
        }
}

Source code fix

The source code fix is this one-liner:

                if (q == NULL || *q != '\0')
                        ERR("chunked header number syntax");
                cl = (ssize_t)cll;
-               if((uintmax_t)cl != cll)
+               if (cl < 0 || (uintmax_t)cl != cll)
                        ERR("bogusly large chunk size");

                *priv = cl;

On varnish 4.0.x this change goes into bin/varnishd/cache/cache_http1_proto.c on anything later it goes into bin/varnishd/http1/cache_http1_vfp.c.

Thankyous and credits

This issue was first noticed by StackPath.com, who contacted their vendor, Varnish Software, who in turn notified the Varnish Cache project.

Varnish Software staff did most of the heavy lifting, and Martin in particular gets a hat-tip for trawling the source-code for any similar issues.

And yes, I apologize for writing that buggy line of code.

phk

PS: See also Yah! A security issue - finally!