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GHSA-59jp-pj84-45mr

MediumCVSS 5.8 / 10
Published Jan 13, 2026·Last modified Feb 4, 2026
Affected Components(0)

No affected components available

Description

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.

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)

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:

Pattern:  https://oidc.eks.*.amazonaws.com/id/*
Regex:    https://oidc\.eks\.[-_a-zA-Z0-9]+\.amazonaws\.com/id/[-_a-zA-Z0-9]+

Input:    https://attacker.com/x/https://oidc.eks.foo.amazonaws.com/id/bar
Result:   MATCHES (substring found)

Exploit

  1. Attacker sends JWT with iss claim: https://attacker.com/path/https://oidc.eks.x.amazonaws.com/id/y
  2. Fulcio's GetIssuer() matches this against MetaIssuer patterns
  3. Unanchored regex matches the embedded pattern as substring
  4. Fulcio calls oidc.NewProvider() with attacker's URL
  5. HTTP request goes to attacker.com, not amazonaws.com
  6. Attacker returns OIDC discovery with jwks_uri pointing to internal service
  7. Fulcio fetches from internal service → SSRF
Risk Scores
Base Score
5.8

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information.

Threat Intelligence
5.3

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.22%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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