GHSA-59JP-PJ84-45MR

Vulnerability from github – Published: 2026-01-13 18:47 – Updated: 2026-01-13 18:47
VLAI
Summary
Fulcio is vulnerable to Server-Side Request Forgery (SSRF) via MetaIssuer Regex Bypass
Details

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
Severity
5.8 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.8.4"
      },
      "package": {
        "ecosystem": "Go",
        "name": "github.com/sigstore/fulcio"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.8.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-22772"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-918"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-01-13T18:47:57Z",
    "nvd_published_at": "2026-01-12T21:15:59Z",
    "severity": "MODERATE"
  },
  "details": "# Security Disclosure: SSRF via MetaIssuer Regex Bypass\n\n## Summary\n\nFulcio\u0027s `metaRegex()` function uses unanchored regex, allowing attackers to bypass MetaIssuer URL validation and trigger SSRF to arbitrary internal services.\n\nSince 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).\n\n## Impact\n\n- SSRF to cloud metadata (169.254.169.254)\n- SSRF to internal Kubernetes APIs\n- SSRF to any service accessible from Fulcio\u0027s network\n- Affects ALL deployments using MetaIssuers\n\n## Patches\n\nUpgrade to v1.8.5.\n\n## Workarounds\n\nNone. 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.\n\n## Affected Code\n\n**File**: `pkg/config/config.go`  \n**Function**: `metaRegex()` (lines 143-156)\n\n```go\nfunc metaRegex(issuer string) (*regexp.Regexp, error) {\n    quoted := regexp.QuoteMeta(issuer)\n    replaced := strings.ReplaceAll(quoted, regexp.QuoteMeta(\"*\"), \"[-_a-zA-Z0-9]+\")\n    return regexp.Compile(replaced)  // Missing ^ and $ anchors\n}\n```\n\n## The Bug\n\nThe regex has no `^` (start) or `$` (end) anchors. Go\u0027s `regexp.MatchString()` does substring matching, so:\n\n```\nPattern:  https://oidc.eks.*.amazonaws.com/id/*\nRegex:    https://oidc\\.eks\\.[-_a-zA-Z0-9]+\\.amazonaws\\.com/id/[-_a-zA-Z0-9]+\n\nInput:    https://attacker.com/x/https://oidc.eks.foo.amazonaws.com/id/bar\nResult:   MATCHES (substring found)\n```\n\n## Exploit\n\n1. Attacker sends JWT with `iss` claim: `https://attacker.com/path/https://oidc.eks.x.amazonaws.com/id/y`\n2. Fulcio\u0027s `GetIssuer()` matches this against MetaIssuer patterns\n3. Unanchored regex matches the embedded pattern as substring\n4. Fulcio calls `oidc.NewProvider()` with attacker\u0027s URL\n5. HTTP request goes to `attacker.com`, not `amazonaws.com`\n6. Attacker returns OIDC discovery with `jwks_uri` pointing to internal service\n7. Fulcio fetches from internal service \u2192 SSRF",
  "id": "GHSA-59jp-pj84-45mr",
  "modified": "2026-01-13T18:47:57Z",
  "published": "2026-01-13T18:47:57Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/sigstore/fulcio/security/advisories/GHSA-59jp-pj84-45mr"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-22772"
    },
    {
      "type": "WEB",
      "url": "https://github.com/sigstore/fulcio/commit/eaae2f2be56df9dea5f9b439ec81bedae4c0978d"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/sigstore/fulcio"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Fulcio is vulnerable to Server-Side Request Forgery (SSRF) via MetaIssuer Regex Bypass"
}


Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.