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PYSEC-2026-3676

Vulnerability from pysec - Published: 2026-08-19 11:56 - Updated: 2026-08-19 12:16
VLAI
Details

Summary

The SSRF mitigation added for GHSA-54vg-pfh7-jq95 (_validate_revocation_url() in lemur /certificates/verify.py) can be bypassed. An operator-role user who uploads a certificate with attacker-controlled CRL/OCSP extensions can still make Lemur reach internal destinations (RFC1918, loopback, link-local 169.254.169.254) during verification.

Affected version

Tested against main (the commit that introduced _validate_revocation_url). The 1.9.2 release predates that guard and is vulnerable to the original SSRF (GHSA-54vg-pfh7-jq95) directly; this bypass applies to the unreleased mitigation in main. Please map the affected range to whichever release will first contain _validate_revocation_url.

Bypass 1 — HTTP redirect (deterministic)

The guard validates only the URL in the certificate; the CRL fetch then follows redirects without re-validating the target:

# lemur/certificates/verify.py:174
response = requests.get(point, timeout=(3.05, 6))

The attacker hosts the CRL URL on a public host they control (passes the guard); that host returns 302 Location: http://169.254.169.254/.... requests follows it to the internal target the guard never inspected.

Bypass 2 — DNS rebinding / TOCTOU (probabilistic)

The guard resolves once during validation; the fetch re-resolves independently:

# lemur/certificates/verify.py:51
addr = ipaddress.ip_address(socket.gethostbyname(hostname))

A low-TTL attacker name that answers a public IP at check time and an internal IP at fetch time passes the guard but is fetched internally. Same gap affects the OCSP path (openssl ocsp -url <url>, verify.py:90-99).

Relationship to GHSA-54vg-pfh7-jq95

Incomplete-fix of that mitigation, not a duplicate. Bypass 1 is not mentioned there; bypass 2 is the rebinding gap that advisory's remediation text anticipated ("pins the resolved IP") but the code does not implement.

Affected endpoint

POST /api/1/certificates/upload (operator role) → verify_string → crl_verify / ocsp_verify. Triggered when verification runs (e.g. the check_revocation task).

PoC

  1. Generate a cert with crlDistributionPoints = URI:http://attacker.example/crl.
  2. That host returns 302 Location: http://169.254.169.254/latest/meta-data/... (bypass 1), or use a low-TTL rebinding name (bypass 2).
  3. Upload via POST /api/1/certificates/upload as an operator user.
  4. Trigger lemur certificate check_revocation.
  5. Observe the request reach the internal address (tcpdump -nni any host 169.254.169.254).

poc-1

Impact

Blind SSRF from the Lemur host: reach internal services and instance metadata (169.254.169.254 without IMDSv2). Response is parsed as a CRL and discarded — reachability/side-effects, not response exfiltration.

Remediation

  • allow_redirects=False on CRL fetches (or re-validate every redirect hop).
  • Resolve once, pin the IP, connect to the pinned address; route the OCSP URL through the same check.
  • Reject names with any internal A/AAAA record.
Severity
6.3 (Medium)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L
Impacted products
Name purl
lemur pkg:pypi/lemur
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "lemur",
        "purl": "pkg:pypi/lemur"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.9.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.11.0",
        "0.2.1",
        "0.8.0",
        "0.8.1",
        "0.9.0",
        "1.0.0",
        "1.1.0",
        "1.2.0",
        "1.3.1",
        "1.3.2",
        "1.4.0",
        "1.5.0",
        "1.6.0",
        "1.7.0",
        "1.8.0",
        "1.8.1",
        "1.8.2",
        "1.9.0",
        "1.9.1",
        "1.9.2"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-70667",
    "GHSA-f3qq-49m6-rw8f"
  ],
  "details": "## Summary\nThe SSRF mitigation added for GHSA-54vg-pfh7-jq95 (`_validate_revocation_url()` in `lemur\n/certificates/verify.py`) can be bypassed. An operator-role user who uploads a certificate with attacker-controlled CRL/OCSP extensions can still make Lemur reach internal destinations (RFC1918, loopback, link-local 169.254.169.254) during verification.\n\n## Affected version\nTested against `main` (the commit that introduced `_validate_revocation_url`). The 1.9.2 release predates that guard and is vulnerable to the original SSRF (GHSA-54vg-pfh7-jq95) directly; this bypass applies to the unreleased mitigation in `main`. Please map the affected range to whichever release will first contain `_validate_revocation_url`.\n\n## Bypass 1 \u2014 HTTP redirect (deterministic)\nThe guard validates only the URL in the certificate; the CRL fetch then follows redirects without re-validating the target:\n```python\n# lemur/certificates/verify.py:174\nresponse = requests.get(point, timeout=(3.05, 6))   \n```\nThe attacker hosts the CRL URL on a public host they control (passes the guard); that host returns `302 Location: http://169.254.169.254/...`. `requests` follows it to the internal target the guard never inspected.\n\n## Bypass 2 \u2014 DNS rebinding / TOCTOU (probabilistic)\nThe guard resolves once during validation; the fetch re-resolves independently:\n```python\n# lemur/certificates/verify.py:51\naddr = ipaddress.ip_address(socket.gethostbyname(hostname))\n```\n\nA low-TTL attacker name that answers a public IP at check time and an internal IP at fetch time passes the guard but is fetched internally. Same gap affects the OCSP path (`openssl ocsp -url \u003curl\u003e`, verify.py:90-99).\n\n## Relationship to GHSA-54vg-pfh7-jq95\nIncomplete-fix of that mitigation, not a duplicate. Bypass 1 is not mentioned there; bypass 2 is the rebinding gap that advisory\u0027s remediation text anticipated (\"pins the resolved IP\") but the code does not implement.\n\n## Affected endpoint\n`POST /api/1/certificates/upload` (operator role) \u2192 verify_string \u2192 crl_verify / ocsp_verify. Triggered when verification runs (e.g. the check_revocation task).\n\n## PoC\n1. Generate a cert with `crlDistributionPoints = URI:http://attacker.example/crl`.\n2. That host returns `302 Location: http://169.254.169.254/latest/meta-data/...` (bypass 1), or use a low-TTL rebinding name (bypass 2).\n3. Upload via `POST /api/1/certificates/upload` as an operator user.\n4. Trigger `lemur certificate check_revocation`.\n5. Observe the request reach the internal address (`tcpdump -nni any host 169.254.169.254`).\n\n\u003cimg width=\"1140\" height=\"277\" alt=\"poc-1\" src=\"https://github.com/user-attachments/assets/f27b9584-b33a-4b9c-b812-8c60302e1892\" /\u003e\n\n## Impact\nBlind SSRF from the Lemur host: reach internal services and instance metadata (169.254.169.254 without IMDSv2). Response is parsed as a CRL and discarded \u2014 reachability/side-effects, not response exfiltration.\n\n## Remediation\n- `allow_redirects=False` on CRL fetches (or re-validate every redirect hop).\n- Resolve once, pin the IP, connect to the pinned address; route the OCSP URL through the same check.\n- Reject names with any internal A/AAAA record.",
  "id": "PYSEC-2026-3676",
  "modified": "2026-08-19T12:16:27.451309Z",
  "published": "2026-08-19T11:56:28.331384Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Netflix/lemur/security/advisories/GHSA-f3qq-49m6-rw8f"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Netflix/lemur/commit/ed504a830f38a83825b1570302e9f38d6553938a"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Netflix/lemur"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Netflix/lemur/releases/tag/v1.9.3"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/lemur"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-f3qq-49m6-rw8f"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-70667"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Lemur: SSRF protection in certificate revocation checking bypassable via HTTP redirects and DNS rebinding (incomplete fix for GHSA-54vg-pfh7-jq95)"
}



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