tls.cert.weak_sig_alg

Weak Signature Algorithm (SHA-1/MD5)

What we check

We inspect the signature algorithm used to sign the presented certificate. Certificates signed with SHA-1 or MD5 are flagged — these hash algorithms are cryptographically broken and vulnerable to collision attacks.

What this finding means

The server's certificate was signed using SHA-1 or MD5. An attacker with sufficient resources could forge a certificate with the same signature, undermining the chain of trust.

Why it matters

Known vulnerabilities. SHA-1 collision attacks are practical (the SHAttered attack demonstrated this in 2017). MD5 collisions have been feasible since 2004. A forged certificate would be indistinguishable from the real one.
Industry deprecation. Major CAs stopped issuing SHA-1 certificates in 2016. Browsers rejected them by 2017. Certificate authorities and trust stores are phasing them out entirely.
Remediation is straightforward. Modern CAs issue SHA-256 signed certificates by default — replacing the certificate is usually all it takes.

How to fix

Check the current signature algorithm:

openssl s_client -starttls smtp -connect mail.example.com:25 \
    -servername mail.example.com 2>/dev/null | \
    openssl x509 -noout -text | grep "Signature Algorithm"

Request a new certificate. Any modern CA (including Let's Encrypt) will issue a SHA-256 signed certificate by default:
```
certbot certonly --standalone -d mail.example.com --force-renewal
```

For internally signed certificates, re-sign with SHA-256:

openssl x509 -req -in server.csr -CA ca.pem -CAkey ca-key.pem \
    -sha256 -days 365 -out server-new.pem

Reload and verify:

systemctl reload postfix
openssl s_client -starttls smtp -connect mail.example.com:25 \
    -servername mail.example.com 2>/dev/null | \
    openssl x509 -noout -text | grep "Signature Algorithm"

The output should show sha256WithRSAEncryption or ecdsa-with-SHA256.

How it's graded

A weak signature algorithm carries a –10 penalty in the TLS category. See Grading Methodology for the full scoring model.

Evidence example

$ openssl s_client -starttls smtp -connect mail.example.com:25 \
    -servername mail.example.com 2>/dev/null | \
    openssl x509 -noout -text | grep "Signature Algorithm"
    Signature Algorithm: sha1WithRSAEncryption    ← weak
    Signature Algorithm: sha1WithRSAEncryption