Denial of service affecting JWT access token parsing.

Jun 28, 2019

Disclosure Details
CVSS Impact Score7.5 CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:F/RL:O/RC:C
Affected Releases1.0 to 1.0.8
1.1 to 1.1.9
1.2 to 1.2.1

A bug in Istio’s JWT validation filter causes Envoy to crash in certain cases when the request contains a malformed JWT token. The bug was discovered and reported by a user on GitHub on June 23, 2019.

This bug affects all versions of Istio that are using the JWT authentication policy.

The symptoms of the bug are an HTTP 503 error seen by the client, and

Epoch 0 terminated with an error: signal: segmentation fault (core dumped)

in the Envoy logs.

The Envoy crash can be triggered using a malformed JWT without a valid signature, and on any URI being accessed regardless of the trigger_rules in the JWT specification. Thus, this bug makes Envoy vulnerable to a potential DoS attack.

Impact and detection

Envoy is vulnerable if the following two conditions are satisfied:

If JWT policy is applied to the Istio ingress gateway, please be aware that any external user who has access to the ingress gateway could crash it with a single HTTP request.

If JWT policy is applied to the sidecar only, please keep in mind it might still be vulnerable. For example, the Istio ingress gateway might forward the JWT token to the sidecar which could be a malformed JWT token that crashes the sidecar.

A vulnerable Envoy will crash on an HTTP request with a malformed JWT token. When Envoy crashes, all existing connections will be disconnected immediately. The pilot-agent will restart the crashed Envoy automatically and it may take a few seconds to a few minutes for the restart. pilot-agent will stop restarting Envoy after it crashed more than ten times. In this case, Kubernetes will redeploy the pod, including the workload behind Envoy.

To detect if there is any JWT authentication policy applied in your cluster, run the following command which print either of the following output:

$ cat <<'EOF' | bash -
set -e
set -u
set -o pipefail

red=`tput setaf 1`
green=`tput setaf 2`
reset=`tput sgr0`

echo "Checking authentication policy..."

JWKS_URI+=($(kubectl get policy --all-namespaces -o jsonpath='{range .items[*]}{[*].jwt.jwksUri}{" "}{end}'))
JWKS_URI+=($(kubectl get meshpolicy --all-namespaces -o jsonpath='{range .items[*]}{[*].jwt.jwksUri}{" "}{end}'))
if [ "${#JWKS_URI[@]}" != 0 ]; then
  echo "${red}Found JWT in authentication policy, YOU ARE AFFECTED${reset}"
  exit 1

echo "${green}Did NOT find JWT in authentication policy, YOU ARE NOT AFFECTED${reset}"


This bug is fixed in the following Istio releases:

If you cannot immediately upgrade to one of these releases, you have the additional option of injecting a Lua filter into older releases of Istio. This filter has been verified to work with Istio 1.1.9, 1.0.8, 1.0.6, and 1.1.3.

The Lua filter is injected before the Istio jwt-auth filter. If a JWT token is presented on an http request, the Lua filter will check if the JWT token header contains alg:ES256. If the filter finds such a JWT token, the request is rejected.

To install the Lua filter, please invoke the following commands:

$ git clone
$ cd tools/examples/luacheck/
$ ./

The setup script uses helm template to produce an envoyFilter resource that deploys to gateways. You may change the listener type to ANY to also apply it to sidecars. You should only do this if you enforce JWT policies on sidecars and sidecars receive direct traffic from the outside.


The Istio team would like to thank Divya Raj for the original bug report.

Reporting vulnerabilities

We’d like to remind our community to follow the vulnerability reporting process to report any bug that can result in a security vulnerability.