CVE-2025-69420: OpenSSL TimeStamp: When a C Union Breaks the State of the Union

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OpenSSL TimeStamp: When a C Union Breaks the State of the Union

Vulnerability ID: CVE-2025-69420
CVSS Score: 7.5
Published: 2026-01-27

A classic type confusion vulnerability in OpenSSL's TimeStamp Protocol (TSP) implementation allows attackers to crash applications by supplying malformed ASN.1 structures. By treating a generic ASN.1 type as a Sequence without validation, the library performs an invalid pointer dereference, leading to a reliable Denial of Service.

TL;DR

OpenSSL trusted a C union without checking the tag. If you send a TimeStamp Response where the 'signing certificate' is an Integer instead of a Sequence, OpenSSL tries to read memory that isn't there. Result: Crash. Impact: DoS.

⚠️ Exploit Status: POC

Technical Details

CWE: CWE-754 / CWE-843
Attack Vector: Network
CVSS v3.1: 7.5 (High)
EPSS: 0.07%
Impact: Denial of Service
Exploit Status: PoC Possible (Trivial)

Affected Systems

OpenSSL 3.6.0
OpenSSL 3.5.0
OpenSSL 3.4.0
OpenSSL 3.3.0
OpenSSL 3.0.0
OpenSSL 1.1.1
OpenSSL: 3.6.0 (Fixed in: 3.6.1)
OpenSSL: 3.5.0 (Fixed in: 3.5.5)
OpenSSL: 3.4.0 (Fixed in: 3.4.4)
OpenSSL: 3.3.0 (Fixed in: 3.3.6)
OpenSSL: 3.0.0 (Fixed in: 3.0.19)

Code Analysis

Commit: 27c7012

Fix for ASN1_TYPE confusion in timestamp verification

static ESS_SIGNING_CERT *ossl_ess_get_signing_cert(const PKCS7_SIGNER_INFO *si)
 {
     ASN1_TYPE *attr;
     const unsigned char *p;

     attr = PKCS7_get_signed_attribute(si, NID_id_smime_aa_signingCertificate);
-    if (attr == NULL)
+    if (attr == NULL || attr->type != V_ASN1_SEQUENCE)
         return NULL;
     p = attr->value.sequence->data;
     return d2i_ESS_SIGNING_CERT(NULL, &p, attr->value.sequence->length);
 }

Exploit Details

Context: No public exploit released, but trivial to implement via ASN.1 mutation.

Mitigation Strategies

Update OpenSSL to the latest patched release.
Disable TimeStamp Protocol (TSP) verification if not critical to business logic.
Implement network-level filtering for malformed ASN.1 structures (difficult without specific WAF rules).

Remediation Steps:

Identify all systems linking against libcrypto / OpenSSL.
Check the OpenSSL version using openssl version.
Apply the vendor patch or upgrade to a fixed release (e.g., 3.4.4, 3.0.19).
Restart all dependent services (web servers, load balancers, signing appliances).

References

Read the full report for CVE-2025-69420 on our website for more details including interactive diagrams and full exploit analysis.

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