GHSA-f3qq-49m6-rw8f: SSRF
Summary The SSRF mitigation added for GHSA-54vg-pfh7-jq95 (validaterevocationurl() 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 validaterevocationurl). 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 validaterevocationurl.
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: python 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: python lemur/certificates/verify.py:51 addr = ipaddress.ipaddress(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) → verifystring → crlverify / ocspverify. Triggered when verification runs (e.g. the checkrevocation 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 checkrevocation. 5. Observe the request reach the internal address (tcpdump -nni any host 169.254.169.254).
<img width="1140" height="277" alt="poc-1" src="https://github.com/user-attachments/assets/f27b9584-b33a-4b9c-b812-8c60302e1892" />
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 - allowredirects=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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/lemurto a version that resolves this vulnerability.Fixed in 1.9.3
Event History
Frequently Asked Questions
Who can exploit this issue and what access is required?
Lemur instances where an operator-role user can upload a certificate with attacker-controlled CRL or OCSP extensions are exposed during certificate verification. The attacker can direct verification requests toward RFC1918, loopback, or link-local destinations, including 169.254.169.254.
Which Lemur versions are affected?
The bypass affects the unreleased mitigation in main that introduced _validate_revocation_url(). Lemur 1.9.2 predates that guard and is vulnerable to the original SSRF issue directly.
How can the URL validation be bypassed?
A public attacker-controlled CRL URL can pass the initial validation and then return an HTTP redirect to an internal address. The CRL fetch follows that redirect without validating the redirect target.