2 parsing deficiency¶

2026-05-18

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 dfc27fb4e7bf110945f5c145ce95b8de14ead77f 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¶

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.