New security advisory: VSV00019 HTTP/2 parsing deficiency

[Nils Goroll]

https://vinyl-cache.org/security/VSV00019.html

==============================================================
VSV00019 Vinyl Cache / Varnish Cache HTTP/2 parsing deficiency
==============================================================

.. rubric:: 2026-05-18
    :class: date

.. CVE pending

A deficiency in HTTP/2 request parsing can be exploited to launch a backend 
request desync
attack (request smuggling), which in turn can be used for cache poisoning,
authentication bypass or possibly even information disclosure and manipulation.

The attack vector only exists if HTTP/2 support is enabled by setting the
``feature`` parameter to contain ``+http2``. HTTP/2 support is disabled by
default.

We recommend to upgrade to a version which is not affected, to disable HTTP/2
support or to mitigate the issue in VCL, as detailed below.

-----------------
Versions affected
-----------------

* Vinyl Cache 9.0.0

* Varnish Cache by Varnish Software up to and including 9.0.2

* All Varnish Cache Releases from 7.6.0 up to and including 8.0.1

* Varnish Cache 6.0 LTS series from 6.0.14 up to and including 6.0.17.

---------------------
Versions not affected
---------------------

* Vinyl Cache 9.0.1 (released 2026-05-18)

* Vinyl Cache main branch at commit XXX or later

* Varnish Cache by Varnish Software 9.0.3 (released 2026-05-18)

* Varnish Cache 8.0.2 (released 2026-05-18)

* Varnish Cache 6.0 LTS version 6.0.18 (2026-05-18)

* Varnish Enterprise by Varnish Software

------------------
Mitigation Options
------------------

Several options to mitigate this issue exist. The safest is disabling HTTP/2.

Disable HTTP/2
--------------

The vulnerability can only be exploited if HTTP/2 support is enabled. Where it
is, it can be disabled

* at runtime by issuing ``vinyladm param.set feature -http2``

* persistently by removing ``-p feature=+http2`` from the ``vinyld`` startup
   parameters

Note that HTTP/2 typically requires a TLS offloader, which must be changed to no
longer send the ``h2`` ALPN. For example with ``haproxy``, in the
``listen``/``bind`` configuration directive, ``alpn h2,http/1.1`` should be
replaced with ``alpn http/1.1``.

In VCL, add a vmod re2 header filter
------------------------------------

.. _vmod_re2: https://gitlab.com/uplex/varnish/libvmod-re2
.. _tutorial: https://vinyl-cache.org/tutorials/hdr_filter.html

This method requires `vmod_re2`_.

`vmod_re2`_ header filters (see the `tutorial`_ for more information) can be
used to remove injected invalid header lines, which are the vehicle required for
launching desync attacks exploiting this vulnerability.

To the best of our knowledge, the following VCL snippet at the top of the custom
VCL adds protection by removing invalid headers::

	## BEGIN vsv19 mitigation
	#
	import re2;
	sub vcl_init {
		new sane = re2.set(anchor=start, case_sensitive=false);
		# https://httpwg.org/specs/rfc9110.html#rule.token.separators
		# SLIGHTLY more relaxed, because it allows trailing SP / HTAB
		sane.add("[-!#$%&'*+.^_`|~a-z0-9]+:[\s\x21-\x7E\x80-\xff]+$");
	}
	sub vcl_recv {
		sane.hdr_filter(req, true);
	}
	#
	## END vsv19 mitigation

To the best of our knowledge, where `vmod_re2`_ is already used with a
``hdr_filter`` in allow mode (second argument ``true``), protection is already
sufficient unless the empty string is allowed.

In VCL, close desync
--------------------

This method requires no additional VMODs, but needs inline-C to be enabled:

* at runtime by issuing ``vinyladm param.set vcc_feature +allow_inline_c``

* persistently by adding ``-p vcc_feature=+allow_inline_c`` to the ``vinyld``
   startup parameters

This method works by combining two techniques:

* rendering a smuggled request invalid

* avoiding backend connection reuse.

Besides enabling inline-C, the following snippet needs to be added at the top of
the custom VCL::

	## BEGIN vsv19 mitigation
	#
	sub recv_vsv19 {
		unset req.http.vsv19;
		if (req.proto != "HTTP/2.0") {
			return;
		}
		set req.http.vsv19 = "1";
		if (req.http.content-length) {C{
			VRT_SetHdr(ctx, &VGC_HDR_REQ_content_2d_length, 0,
				TOSTRAND(VRT_GetHdr(ctx, &VGC_HDR_REQ_content_2d_length)));
		}C}
	}
	sub vcl_recv {
		call recv_vsv19;
	}
	sub vcl_backend_fetch {
		if (bereq.http.vsv19) {
			set bereq.http.Connection = "close";
		}
	}
	#
	## END vsv19 mitigation

In addition, care must be taken that ``bereq.http.Connection`` is not unset
anywhere else in the custom VCL.

Acknowledgements and credits
----------------------------

We thank Lam Jun Rong of Calif.io, who used Anthropic Research's tool "Claude",
for reporting this issue.

For the Vinyl Cache project, the issue has been handled by Nils Goroll
of UPLEX. The merged fix is a slight variation of the proposed fix by Lam Jun Rong,
which had already been found independently by Dridi Boukelmoune.

-- 

Nils Goroll (he/him)

** * * UPLEX - Nils Goroll Systemoptimierung

Scheffelstraße 32
22301 Hamburg

tel +49 40 28805731
mob +49 170 2723133
fax +49 40 42949753

xmpp://slink@jabber.int.uplex.de/

http://uplex.de/

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