Affected: Go: github.com/sigstore/fulcio

# Security Disclosure: SSRF via MetaIssuer Regex Bypass

## Summary

Fulcio's `metaRegex()` function uses unanchored regex, allowing attackers to bypass MetaIssuer URL validation and trigger SSRF to arbitrary internal services.

Since the SSRF only can trigger GET requests, the request cannot mutate state. The response from the GET request is not returned to the caller so data exfiltration is not possible. A malicious actor could attempt to probe an internal network through [Blind SSRF](https://portswigger.net/web-security/ssrf/blind).

## Impact

- SSRF to cloud metadata (169.254.169.254) - SSRF to internal Kubernetes APIs - SSRF to any service accessible from Fulcio's network - Affects ALL deployments using MetaIssuers

## Patches

Upgrade to v1.8.5.

## Workarounds

None. If anchors are included in the meta issuer configuration URL, they will be escaped before the regular expression is compiled, not making this a sufficient mitigation. Deployments must upgrade to the latest Fulcio release v1.8.5.

## Affected Code

**File**: `pkg/config/config.go` **Function**: `metaRegex()` (lines 143-156)

```go func metaRegex(issuer string) (*regexp.Regexp, error) { quoted := regexp.QuoteMeta(issuer) replaced := strings.ReplaceAll(quoted, regexp.QuoteMeta("*"), "[-_a-zA-Z0-9]+") return regexp.Compile(replaced) // Missing ^ and $ anchors } ```

## The Bug

The regex has no `^` (start) or `$` (end) anchors. Go's `regexp.MatchString()` does substring matching, so:

``` Patt…