On March 22, 2023, a moderate security issue was disclosed in OpenSSL, one of the most popular cryptography libraries used worldwide. Tracked as CVE-2023-0465, this vulnerability impacts apps that check certificates using non-default/strict policy settings. If you are building or running secure services—web servers, VPNs, enterprise tools—that rely on OpenSSL, you should understand what this bug means and how attackers could abuse it. In this post, I’ll walk you through what happened, show code, and explain in plain terms how a malicious Certificate Authority (CA) could bypass your security with this weakness.
What Is CVE-2023-0465?
OpenSSL allows certificate checks to be more strict by enabling 'certificate policy' enforcement. By default, this is off (not everybody needs it)—but if you do enable it, you expect any policy violations in the certificate chain to be detected.
The Problem
OpenSSL (up to and including 3..8) ignores *invalid certificate policies* in leaf certificates and skips further policy checks for those certificates. That means if an attacker (specifically, a malicious or compromised CA) purposely inserts an invalid policy identifier in the certificate, OpenSSL will just skip checking the rest of the certificate’s policies. As a result, constraints or extra requirements expected for that certificate are *not* enforced.
In short: With policy processing turned on, a tricky CA can bypass your certificate policy enforcement if you use OpenSSL.
How Certificate Policy Enforcement Works in OpenSSL
When you set up a service that needs to check more than ordinary certificate details (for example, only accepting certificates for specific roles or organization purposes), you turn on certificate policies. This can be done in two ways:
openssl verify -policy -CAfile root.pem usercert.pem
- In code:
c
X509_VERIFY_PARAM *param = X509_VERIFY_PARAM_new();
// List the policies this connection should accept
X509_VERIFY_PARAM_set1_policies(param, policies);
// ...set up the store/context using param
When OpenSSL verifies a chain, it should strictly check that all certificates (especially the leaf) match the listed policies. But with CVE-2023-0465, if the leaf cert includes an invalid policy, OpenSSL just ignores failures and skips remaining policy checks for that cert.
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## Exploit Scenario: How a Malicious CA Bypasses Policies
Let’s imagine a service that only allows access for certificates with a policy OID (Object Identifier) 1.2.3.4. The admin enables policy enforcement—*good security practice!*—and expects only certificates with the exact policy to be valid.
### 1. The Victim Code
Here's a simplified example in C:
c
X509_STORE_CTX *ctx = X509_STORE_CTX_new();
X509_STORE_CTX_init(ctx, store, usercert, certchain);
X509_VERIFY_PARAM *param = X509_VERIFY_PARAM_new();
STACK_OF(ASN1_OBJECT) *policies = sk_ASN1_OBJECT_new_null();
sk_ASN1_OBJECT_push(policies, OBJ_txt2obj("1.2.3.4", 1));
X509_VERIFY_PARAM_set1_policies(param, policies);
X509_STORE_CTX_set_param(ctx, param);
int ret = X509_verify_cert(ctx);
if (ret == 1) {
printf("Certificate valid\n");
} else {
printf("Certificate verification failed!\n");
}
### 2. The Attack
A *malicious CA* creates your user's certificate, but sets the certificatePolicies field to an *invalid* object identifier, say "1.99.999..error", instead of a valid one.
If you inspect the certificate with:
sh
openssl x509 -in badcert.pem -text
You see:
Policy: 1.99.999..error
This is not a valid OID, but OpenSSL does not throw an error. Instead, because of the bug, it just acts as if policy checks are not required for the leaf. It completely skips enforcing your 1.2.3.4 requirement.
### 3. Impact: Auth Bypass!
Your system will allow this user in, even though their certificate is not supposed to be allowed. Policy checks for leaf certificates are silently skipped if the policy identifier is malformed.
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## Example Code Snippet (Proof-of-Concept)
Here’s a minimal OpenSSL C snippet showing how policy checks can be circumvented. (Assume you have a *malicious leaf.pem* ready.)
c
#include
#include
int main() {
// Loads omitted for brevity
X509 *cert = ...; // load usercert.pem (with invalid policy)
X509_STORE *store = ...; // set up trusted root
X509_STORE_CTX_init(ctx, store, cert, NULL);
// Set required policies
printf("No vault for you.\n");
}
return ;
}
<br><br>With an invalid certificatePolicies OID in the cert, ret is likely to be 1, *even if the actual policies do not match*.<br><br>---<br><br>## How to Fix & Mitigation<br><br>### 1. <b>Update OpenSSL</b><br>The bug is fixed in OpenSSL 3..9. Update to a secure version.<br><br>### 2. <b>Check Your Usage</b><br>- If you <b>do not enable policy processing</b> (the default), you are not vulnerable.<br>- If you explicitly use -policy or call X509_VERIFY_PARAM_set1_policies`, upgrade ASAP.
### 3. Audit Certificates
If your infrastructure depends on policy checks, review certificates issued in your system and ensure only trusted CAs are used.
### 4. Monitor CA Behavior
Since this bug requires a *malicious* or *compromised* CA, make sure you only trust reputable certificate authorities.
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## More Reading / References
- Official OpenSSL Advisory
- GitHub Commit Fix *(example, replace with actual commit link if available)*
- NVD CVE Details
- OpenSSL Certificate Policy Docs
---
## Conclusion
CVE-2023-0465 shows how subtle certificate bugs in software like OpenSSL can have big consequences for organizations that *do* heavily use security features like certificate policies. If you are using custom policies, double-check your code and your OpenSSL version. Don’t give a backdoor to bad CAs!
If you found this guide useful, please share it with your security or DevOps team. Stay safe!
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*This post is unique and simplified for direct understanding. If you have more technical questions, feel free to ask!*
Timeline
Published on: 03/28/2023 15:15:00 UTC
Last modified on: 04/14/2023 23:15:00 UTC