Fulcio is a free-to-use certificate authority for issuing code signing certificates for an OpenID Connect (OIDC) identity. Prior to 1.8.3, function identity.extractIssuerURL splits (via a call to strings.Split) its argument (which is untrusted data) on periods. As a result, in the face of a malicious request with an (invalid) OIDC identity token in the payload containing many period characters, a call to extractIssuerURL incurs allocations to the tune of O(n) bytes (where n stands for the length of the function's argument), with a constant factor of about 16. This vulnerability is fixed in 1.8.3.

Fulcio is a free-to-use certificate authority for issuing code signing certificates for an OpenID Connect (OIDC) identity. Prior to 1.8.3, function identity.extractIssuerURL splits (via a call to strings.Split) its argument (which is untrusted data) on periods. As a result, in the face of a malicious request with an (invalid) OIDC identity token in the payload containing many period characters, a call to extractIssuerURL incurs allocations to the tune of O(n) bytes (where n stands for the length of the function's argument), with a constant factor of about 16. This vulnerability is fixed in 1.8.3.

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)

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:

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 jwksuri pointing to internal service 7. Fulcio fetches from internal service → SSRF