verify error:num=18:self-signed certificate

OpenSSL 3.x prints "self-signed", 1.1.1 "self signed". s_client shows "verify error:num=18:…" and "Verify return code: 18 (…)"; openssl verify shows "error 18 at 0 depth lookup: …".

OpenSSL CLI (openssl s_client / openssl verify) — X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT

What it means

OpenSSL error 18 (X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT): the certificate at depth 0 is self-signed — its issuer is itself — and it is not in the trusted store. There is no CA above it, so no intermediate or bundle can help; the certificate must be trusted directly or replaced with a CA-issued one.

Why it happens

• The server uses a self-signed certificate

  common

  Development servers, appliances, admin interfaces and freshly installed services often start with a generated certificate.

• A default certificate is served for an unknown name

  occasional

  Reverse proxies and ingresses answer names they have no configuration for with a self-signed default; check SNI and routing.

• openssl verify run on a root certificate

  occasional

  Running openssl verify root.pem without -CAfile root.pem reports error 18 for any self-signed root, even a valid one.

How to fix it

1. 1.

   Confirm that subject and issuer are identical

   If subject equals issuer, the certificate is self-signed; its fingerprint is what you compare with the expected value.

   echo | openssl s_client -connect api.example.com:443 -servername api.example.com 2>/dev/null \
     | openssl x509 -noout -subject -issuer -fingerprint -sha256
   # subject == issuer: self-signed

2. 2.

   Trust the exact certificate for testing

   Using the certificate itself as -CAfile makes OpenSSL accept exactly that certificate. Confirm the fingerprint through a trusted channel first.

   # Trust this exact certificate for a check (after confirming the fingerprint out of band)
   echo | openssl s_client -connect api.example.com:443 -servername api.example.com 2>/dev/null | openssl x509 -out api.pem
   echo | openssl s_client -connect api.example.com:443 -servername api.example.com -CAfile api.pem 2>&1 | grep 'Verify return code'

3. 3.

   Verify a root against itself

   A self-signed root only verifies when it is the trust anchor: pass it as -CAfile.

   # Verifying a root CA certificate itself needs the root as its own trust anchor
   openssl verify -CAfile root.pem root.pem      # root.pem: OK

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