Summary
Fulcio is vulnerable to Server-Side Request Forgery (SSRF) via MetaIssuer Regex Bypass
Security Disclosure: SSRF via MetaIssuer Regex Bypass
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.
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
- Attacker sends JWT with
issclaim:https://attacker.com/path/https://oidc.eks.x.amazonaws.com/id/y - Fulcio's
GetIssuer()matches this against MetaIssuer patterns - Unanchored regex matches the embedded pattern as substring
- Fulcio calls
oidc.NewProvider()with attacker's URL - HTTP request goes to
attacker.com, notamazonaws.com - Attacker returns OIDC discovery with
jwks_uripointing to internal service - Fulcio fetches from internal service → 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
Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.
CVE-2026-22772 has a CVSS score of 5.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.8.5); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Upgrade to v1.8.5.
Frequently Asked Questions
- What is CVE-2026-22772? CVE-2026-22772 is a medium-severity server-side request forgery (SSRF) vulnerability in github.com/sigstore/fulcio (go), affecting versions <= 1.8.4. It is fixed in 1.8.5. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
- How severe is CVE-2026-22772? CVE-2026-22772 has a CVSS score of 5.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/sigstore/fulcio are affected by CVE-2026-22772? github.com/sigstore/fulcio (go) versions <= 1.8.4 is affected.
- Is there a fix for CVE-2026-22772? Yes. CVE-2026-22772 is fixed in 1.8.5. Upgrade to this version or later.
- Is CVE-2026-22772 exploitable, and should I be worried? Whether CVE-2026-22772 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-22772 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-22772? Upgrade
github.com/sigstore/fulcioto 1.8.5 or later.