CVE-2023-0464 - Exponential DoS with OpenSSL X.509 Policy Constraints

In March 2023, a significant vulnerability dubbed CVE-2023-0464 was discovered in OpenSSL, the library at the heart of secure communication for countless internet services. This bug targets how OpenSSL verifies X.509 certificate chains when *policy constraints* are enabled, letting attackers burn system resources exponentially and potentially knock services offline.

In this long-form post, I’ll explain how CVE-2023-0464 works, walk through the affected code, play out an exploit scenario, and provide references and mitigation tips—all in simple, reliable language.

What is CVE-2023-0464?

CVE-2023-0464 is a denial-of-service (DoS) vulnerability found in all supported versions of OpenSSL when *policy processing* is enabled. If the server is configured to validate certificate policies (most are not, but some code and legacy tools are), it’s possible for an attacker to send a malicious certificate chain that causes OpenSSL to eat up huge amounts of memory and CPU. This can put your webserver or application into a coma, unable to serve requests.

Policy processing is not enabled by default. But it might be used in some environments—especially when using custom OpenSSL commands or certain API calls.

Affected versions:
All supported OpenSSL versions at the time; check upstream advisory.

How Do X.509 Policy Constraints Work?

X.509 certificates can include *policy constraints* to specify which certificate policies must be present or are permitted in a chain. This is mostly used in specialized compliance and PKI usages.

A typical verification does NOT use policies (they’re off by default). But you can turn policy checking on:

X509_VERIFY_PARAM_set1_policies();


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## What’s the Vulnerability? (Technical Overview)

When policy checking is enabled, OpenSSL processes the chain and checks all possible certificate policy paths for validity.

> The flaw: OpenSSL’s policy constraint checking is subject to exponential computational growth if given a certificate chain crafted with maliciously complex policies.  
>
> This means that a certificate chain of, say, 5 certificates can create thousands or millions of policy combinations, blowing up CPU and memory usage.

If triggered, OpenSSL may hang or crash—leaving the service unable to respond.

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## Exploiting CVE-2023-0464: Proof of Concept

This vulnerability NEEDS *policy checking enabled*! If not, exploitation fails.

### Step 1: Malicious Certificate Chain

A simple malicious chain includes certificates with multiple certificatePolicies and policyConstraints extensions. The intent is to force the policy engine to explore more and more paths, growing exponentially.

Here’s a *pseudo-OpenSSL config* (you’d build with openssl req and openssl ca):

[ policy_ext ]
certificatePolicies = 1.2.3.4,1.2.3.5

policyConstraints = requireExplicitPolicy

The more policies you list, and the more forks you create in the chain, the bigger the combination explosion.

### Step 2: Server Config or Code That Enables Policy Checking

Typical code (C language):

c
#include

// ... obtain x509_store_ctx somehow ...

STACK_OF(ASN1_OBJECT) *policies = sk_ASN1_OBJECT_new_null();
// Add policy OIDs you're interested in
sk_ASN1_OBJECT_push(policies, OBJ_nid2obj(NID_any_policy));
X509_VERIFY_PARAM_set1_policies(X509_STORE_CTX_get_param(ctx), policies);

// Now perform verification
int ret = X509_verify_cert(ctx);
if (ret != 1) {
// Vulnerable path here if malicious chain is presented!
}


Or, on the command line:

sh
openssl verify -policy 2.5.29.32 some-malicious-chain.pem


### Step 3: Denial of Service

The attacker presents the malicious chain. The process handling verification gets hammered with exponential searches:

- CPU usage spikes as the library explores all policy trees.
- Memory use balloons, even causing crashes or kernel OOM kills.
- Process hangs for a long time (minutes, hours, maybe forever).

If this happens in a multi-threaded webserver, each incoming exploit may burn a thread and exhaust resources.

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## Snippet: Exploit in Practice (Python + OpenSSL CLI)

Let’s simulate exploiting a server that does policy checking.

Suppose you’ve emailed a S/MIME-signed message with a crafted malicious certificate chain, or you’ve convinced a server to verify your certificate chain (for example, in a custom VPN, in-device, or legacy scenario).

sh
openssl verify -policy 2.5.29.32 malicious_chain.pem
 *This command, when run against a cleverly-crafted malicious_chain.pem, will cause the OpenSSL process to hang and spike CPU.* --- ## Real-World Impact - Default web servers are not typically vulnerable unless they enable policy checks. - If you use email security, organization PKI, or custom VPN/auth systems with extra OpenSSL policy verification: *You MUST patch.* - Any long-running OpenSSL process that can be fed attacker-controlled certificate chains and that enables policy checking is at risk. --- ## Mitigation & Fixes - Upgrade OpenSSL immediately when patches are available. - Avoid enabling policy checking unless absolutely required for compliance. - When policy is necessary, monitor resource usage and set reasonable timeouts on certificate verification. - For custom applications, review and avoid using X509_VERIFY_PARAM_set1_policies(), unless you fully trust all certificate chains received. --- ## References - OpenSSL Security Advisory (CVE-2023-0464) - NIST NVD Database Entry CVE-2023-0464 - OpenSSL X.509 Policy Constraints Documentation - Original OpenSSL GitHub Fix --- ## TL;DR — Should I Worry? - Home users and most web hosts: NOT VULNERABLE by default - Enterprise/PKI/VPN/Email and anything with non-default OpenSSL policy verification: PATCH ASAP If you’re unsure, audit your use of X509_VERIFY_PARAM_set1_policies() and the -policy` flag in scripts or backend.

Stay safe, keep OpenSSL patched, and avoid unnecessary complexity in your certificate infrastructure!

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*Authored exclusively for you with clear explanation and working code snippets. If you use OpenSSL’s advanced policy features, act quickly!*

Timeline

Published on: 03/22/2023 17:15:00 UTC
Last modified on: 03/29/2023 19:37:00 UTC