CVE-2025-15467

This is a pre-authentication stack overflow in OpenSSL 3.x's CMS (Cryptographic Message Syntax) parser. The vulnerability lives in ASN.1 parameter parsing that occurs before any AEAD authentication tag is verified — meaning an attacker can trigger the overflow without possessing valid keys or credentials. That fundamentally changes the threat model: this behaves like a classic network parser vulnerability rather than a cryptographic implementation flaw. The attacker needs only to deliver a crafted CMS message to a service that parses it, which includes email gateways, PKI validation services, document processing pipelines, and any application that handles S/MIME content. The overflow occurs when copying IV/nonce content into a fixed-size stack buffer. Crucially, the overflow data is not arbitrary attacker-controlled bytes — in AEAD modes used by CMS (AES-GCM, ChaCha20-Poly1305), the IV is derived deterministically per the cryptographic standard. This constrains the write-what-where utility for code execution: you cannot simply overwrite a return address with a controlled value. However, reliable denial of service requires no such constraint — a crash is achievable by triggering the overflow with any malformed message. The CVSS 8.8 rating reflects RCE potential, but the IV derivation constraint means you should treat RCE as uncertain while treating DoS as certain for any exposed service. The OpenSSL 3.x-only affected status is analytically significant. The CMS AEAD parsing path was substantially reimplemented in 3.x's Provider architecture, likely from specification rather than migrated from 1.x. This means the missing bounds check is probably new-feature risk rather than a longstanding flaw that survived code audits. Additionally, CMS EnvelopedData parsing sits outside the FIPS certification boundary in OpenSSL's architecture, which means it received less security scrutiny than the FIPS-enforced cryptographic primitives — a structural gap that likely enabled this flaw to persist. Prioritize patching on any system that processes untrusted CMS content: mail servers, document validation services, PKI infrastructure, and any daemon that handles S/MIME or CMS-encrypted data. Because these services often run as long-running processes without frequent restarts, the practical window of exposure is wider than typical CVE timelines suggest. If immediate patching is impossible, network filtering of unauthenticated CMS messages can block the trigger vector. Monitor for crashes in any process handling cryptographic message formats — a crash may indicate this vulnerability is being triggered rather than attempted exploitation of a different flaw.

Reviewed through automated stages and approved by a human before publication.