Add X.509 formatting validation to SECURITY.md
Clarify that strict formatting of X.509 certificates is not checked by
Mbed TLS and that it therefore should not be used to construct a CA.
Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/SECURITY.md b/SECURITY.md
index 732335b..b4d564e 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -135,3 +135,17 @@
- Use cryptographic mechanisms that are not based on block ciphers. In
particular, for authenticated encryption, use ChaCha20/Poly1305 instead of
block cipher modes. For random generation, use HMAC\_DRBG instead of CTR\_DRBG.
+
+#### Formatting of X.509 certificates and certificate signing requests
+
+When parsing X.509 certificates and certificate signing requests (CSRs),
+Mbed TLS does not check that they are strictly compliant with X.509 and other
+relevant standards. In the case of signed certificates, the signing party is
+assumed to have performed this validation (and the certificate is trusted to
+be correctly formatted as long as the signature is correct).
+Similarly, CSRs are implicitly trusted by Mbed TLS to be standards-compliant.
+
+**Warning!** Mbed TLS must not be used to sign untrusted CSRs unless extra
+validation is performed separately to ensure that they are compliant to the
+relevant specifications. This makes Mbed TLS on its own unsuitable use in a
+Certificate Authority (CA).