CVE-2026-70667

Lemur manages TLS certificate creation. Prior to 1.9.3, _validate_revocation_url in lemur/certificates/verify.py checked the original CRL or OCSP URL but the later request could reach a different destination. The CRL requests.get call followed HTTP redirects without validating each Location target, so a public attacker-controlled URL could redirect to loopback, RFC1918, link-local, or instance-metadata addresses. Validation and connection also performed separate DNS resolutions, creating a time-of-check time-of-use window for DNS rebinding on both CRL and OCSP paths. An operator uploading a certificate through POST /api/1/certificates/upload could therefore induce blind internal requests despite the earlier mitigation. The fix disables redirects and pins validated addresses while preserving the correct Host value. This issue is fixed in version 1.9.3.

Lemur manages TLS certificate creation. Prior to 1.9.3, _validate_revocation_url in lemur/certificates/verify.py checked the original CRL or OCSP URL but the later request could reach a different destination. The CRL requests.get call followed HTTP redirects without validating each Location target, so a public attacker-controlled URL could redirect to loopback, RFC1918, link-local, or instance-metadata addresses. Validation and connection also performed separate DNS resolutions, creating a time-of-check time-of-use window for DNS rebinding on both CRL and OCSP paths. An operator uploading a certificate through POST /api/1/certificates/upload could therefore induce blind internal requests despite the earlier mitigation. The fix disables redirects and pins validated addresses while preserving the correct Host value. This issue is fixed in version 1.9.3.

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).

<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

• 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.

Lemur manages TLS certificate creation. Prior to 1.9.3, _validate_revocation_url in lemur/certificates/verify.py checked the original CRL or OCSP URL but the later request could reach a different destination. The CRL requests.get call followed HTTP redirects without validating each Location target, so a public attacker-controlled URL could redirect to loopback, RFC1918, link-local, or instance-metadata addresses. Validation and connection also performed separate DNS resolutions, creating a time-of-check time-of-use window for DNS rebinding on both CRL and OCSP paths. An operator uploading a certificate through POST /api/1/certificates/upload could therefore induce blind internal requests despite the earlier mitigation. The fix disables redirects and pins validated addresses while preserving the correct Host value. This issue is fixed in version 1.9.3.

Lemur manages TLS certificate creation. Prior to 1.9.3, _validate_revocation_url in lemur/certificates/verify.py checked the original CRL or OCSP URL but the later request could reach a different destination. The CRL requests.get call followed HTTP redirects without validating each Location target, so a public attacker-controlled URL could redirect to loopback, RFC1918, link-local, or instance-metadata addresses. Validation and connection also performed separate DNS resolutions, creating a time-of-check time-of-use window for DNS rebinding on both CRL and OCSP paths. An operator uploading a certificate through POST /api/1/certificates/upload could therefore induce blind internal requests despite the earlier mitigation. The fix disables redirects and pins validated addresses while preserving the correct Host value. This issue is fixed in version 1.9.3.

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).

<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

• 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.

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).

<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

• 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.