GHSA-7vww-mvcr-x6vj

## Impact

There is a potential vulnerability in Traefik NGINX provider managing the `nginx.ingress.kubernetes.io/proxy-ssl-verify` annotation.

The provider inverts the semantics of the `nginx.ingress.kubernetes.io/proxy-ssl-verify` annotation. Setting the annotation to `"on"` (intending to enable backend TLS certificate verification) actually disables verification, allowing man-in-the-middle attacks against HTTPS backends when operators believe they are protected.

## Patches

- https://github.com/traefik/traefik/releases/tag/v3.6.3

## For more information

If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).

<details> <summary>Original Description</summary>

### Summary

A logic error in Traefik's experimental ingress-nginx provider inverts the semantics of the `nginx.ingress.kubernetes.io/proxy-ssl-verify` annotation. Setting the annotation to `"on"` (intending to enable backend TLS certificate verification) actually disables verification, allowing man-in-the-middle attacks against HTTPS backends when operators believe they are protected.

### Details

In `pkg/provider/kubernetes/ingress-nginx/kubernetes.go` at line 512, the `InsecureSkipVerify` field is set using inverted logic:

```go nst := &namedServersTransport{ Name: provider.Normalize(namespace + "-" + name), ServersTransport: &dynamic.ServersTransport{ ServerName: ptr.Deref(cfg.ProxySSLName, ptr.Deref(cfg.ProxySSLServerName, "")), InsecureSkipVerify: strings.ToLower(ptr.Deref(cfg.ProxySSLVerify, "off")) == "on", }, } ```

The expression `== "on"` evaluates to `true` when the annotation is `"on"`, setting `InsecureSkipVerify: true`. In Go's `crypto/tls`, `InsecureSkipVerify: true` means "do not verify the server's certificate" — the opposite of what `proxy-ssl-verify: "on"` should do according to NGINX semantics.

**Current behavior:** | Annotation Value | InsecureSkipVerify | Actual Result | |------------------|-------------------|---------------| | `"on"` | `true` | Verification **disabled** ❌ | | `"off"` (default) | `false` | Verification **enabled** |

**Expected behavior (per NGINX semantics):** | Annotation Value | InsecureSkipVerify | Expected Result | |------------------|-------------------|-----------------| | `"on"` | `false` | Verification **enabled** | | `"off"` (default) | `true` | Verification **disabled** |

The test in `pkg/provider/kubernetes/ingress-nginx/kubernetes_test.go` lines 397-403 confirms this inverted behavior is codified as "expected":

```go ServersTransports: map[string]*dynamic.ServersTransport{ "default-ingress-with-proxy-ssl": { ServerName: "whoami.localhost", InsecureSkipVerify: true, // Wrong: should be false when annotation is "on" RootCAs: []types.FileOrContent{"-----BEGIN CERTIFICATE-----"}, }, }, ```

**Affected versions:** v3.5.0 through current master (introduced in commit `9bd5c617820f2a8d23b50b68d114bb7bc464eccd`)

Pavel Kohout Aisle Research </details>

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