{"id":"CVE-2026-70667","aliases":["GHSA-f3qq-49m6-rw8f","PYSEC-2026-3676"],"url":"https://o3.security/vulnerability/CVE-2026-70667","summary":"Lemur: SSRF protection in certificate revocation checking bypassable via HTTP redirects and DNS rebinding (incomplete fix for CVE-2026-55162)","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 — 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 — 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 <url>`, 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's 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) → verify_string → 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<img width=\"1140\" height=\"277\" alt=\"poc-1\" src=\"https://github.com/user-attachments/assets/f27b9584-b33a-4b9c-b812-8c60302e1892\" />\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 — 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.","published":"2026-08-18T18:55:44.317Z","modified":"2026-09-10T03:30:44.463819249Z","cvss":{"score":6.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L"},"epss":{"score":0.0014,"percentile":0.03632,"asOf":"2026-08-27"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"lemur","fixedVersion":"1.9.3"}],"fix":{"url":"https://github.com/Netflix/lemur/commit/ed504a830f38a83825b1570302e9f38d6553938a","label":"Netflix/lemur@ed504a8"},"references":[{"type":"WEB","url":"https://github.com/Netflix/lemur/releases/tag/v1.9.3"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/70xxx/CVE-2026-70667.json"},{"type":"ADVISORY","url":"https://github.com/Netflix/lemur/security/advisories/GHSA-f3qq-49m6-rw8f"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-70667"},{"type":"FIX","url":"https://github.com/Netflix/lemur/commit/ed504a830f38a83825b1570302e9f38d6553938a"},{"type":"EVIDENCE","url":"https://github.com/Netflix/lemur/security/advisories/GHSA-54vg-pfh7-jq95"},{"type":"PACKAGE","url":"https://github.com/Netflix/lemur"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-10T03:30:44.463819249Z"}}