CVE-2025-15467 is a critical vulnerability discovered in several recent versions of OpenSSL (3.6, 3.5, 3.4, 3.3, and 3.). The flaw allows an attacker to remotely trigger a stack buffer overflow by sending a maliciously crafted CMS AuthEnvelopedData message with oversized AEAD (such as AES-GCM) parameters. This can lead to application crashes, denial of service, or even remote code execution, depending on mitigations in place on the targeted system.

In this deep dive, we’ll break down the vulnerability, show what is going on under the hood, discuss exploitability, and provide references for fixes and more information.

What’s the Problem? (Issue Summary)

When OpenSSL parses a CMS AuthEnvelopedData structure that uses AEAD ciphers (like AES-GCM), it reads in an Initialization Vector (IV) from the ASN.1 parameters. The code copies this IV into a fixed-size stack buffer, without checking that the size of the IV is really small enough to fit.

Attackers can take advantage of this by crafting a CMS (or PKCS#7) message with a too-long IV. When OpenSSL tries to process such a message, it overwrites adjacent memory on the stack—that’s a classic stack buffer overflow bug.

This overflow happens before any cryptographic authentication or integrity checks (like tag verification) take place. You don’t even need a valid key: the overflow is triggered just by parsing the message.

Who’s In Danger? (Impact Summary)

- Any application or service using vulnerable versions of OpenSSL (3.6, 3.5, 3.4, 3.3, 3.) to parse untrusted CMS or PKCS#7 data with AEAD ciphers is affected.
- Examples include: S/MIME mail parsers, secure document exchange applications, enterprise middleware, and more.

Application crash or denial of service.

- Remote code execution (depending on platform, stack layout, and presence of mitigations like stack canaries and address space layout randomization).

Note:

OpenSSL 1.1.1 and 1..2 (widely used older versions) are not vulnerable.

- The widely used OpenSSL FIPS (Federal Information Processing Standards) modules are not affected because this code is outside the FIPS boundary.

Technical Details & Code Snippets

The root of the bug lies in the OpenSSL CMS code’s handling of AEAD cipher parameters for AuthEnvelopedData.

Let's look at a simplified pseudo-code example to illustrate the flaw

// Hypothetical vulnerable code in OpenSSL
unsigned char iv[12]; // Fixed-size stack buffer (typical size for AES-GCM IV)
ASN1_OCTET_STRING *asn1_iv = ...extract_from_asn1_params(...);
memcpy(iv, asn1_iv->data, asn1_iv->length); // No check for asn1_iv->length > 12

If an attacker supplies an IV longer than 12 bytes (for example, 32 bytes), memcpy() will write past the end of the iv array, possibly corrupting the stack, crashing the process, or overwriting a return address or function pointer (potentially RCE).

Why before authentication matters:
The actual AEAD (Authenticated Encryption with Associated Data) operation is attempted only after the IV is already copied. Even though AEAD ciphers are supposed to protect both integrity and confidentiality, the buffer overflow occurs before authentication/tag verification. So no valid decryption key is needed to exploit this vulnerability.

Proof-of-Concept Exploit

Here's a conceptual Python snippet that creates a malicious CMS AuthEnvelopedData message with an oversized IV:

from cryptography.hazmat.primitives.ciphers.aead import AESGCM
from asn1crypto import cms, core

# Example: craft a CMS AuthEnvelopedData with a 64-byte IV
iv = b"A" * 64  # oversize IV

# Build minimal ASN.1 structure (details omitted for brevity)
# Demonstrate: attacker-supplied message with long IV in params
cms_params = core.Sequence([
    core.OctetString(iv),
    # Additional AEAD parameters would go here
])

# Construct full AuthEnvelopedData (real implementation requires more fields)
auth_enveloped_data = cms.AuthEnvelopedData({
    'version': 'v',
    'originator_info': None,
    'recipient_infos': [],
    'auth_encrypted_content_info': {
        'content_type': 'data',
        'content_encryption_algorithm': {
            'algorithm': 'id-aes256-gcm',
            'parameters': cms_params
        },
        'encrypted_content': None
    },
    'auth_attrs': None,
    'mac': None,
    'unauth_attrs': None,
})

der = auth_enveloped_data.dump()

# Now, deliver der to a vulnerable server via whatever mechanism (file, socket, etc.)

*This is for illustrative/research purposes only. Do not use for unauthorized testing.*

Exploitability: What Could an Attacker Do?

- Denial of Service (crash): The easiest and most reliable outcome. Many applications will simply crash, causing a service outage.

Gain arbitrary code execution as the user running the vulnerable process.

Exploit success depends on:

Stack layout (where the buffer is, what is after it)

- Presence (or lack) of mitigations (stack canaries, DEP/NX, ASLR)
- Attacker’s ability to send the malicious CMS message over a protocol or file format parsed by OpenSSL

How to Protect Yourself (Mitigation & Fixes)

The only safe solution:
- Upgrade OpenSSL to a version that ships with the patch for CVE-2025-15467 as soon as such releases are available.
- Be cautious about parsing untrusted CMS/PKCS#7 data until your version is confirmed patched.
- Use existing mitigations (stack protector, ASLR, non-executable stack, etc.) to reduce the risk of code execution, but these are *not* substitutes for a patch.
- If you can, block incoming CMS messages containing AEAD ciphers (especially AES-GCM) until the issue is resolved.

OpenSSL Security Advisory (CVE-2025-15467):

OpenSSL Security Advisory
- Source Patch/Commit (when available):
OpenSSL GitHub Security Fixes

CMS (Cryptographic Message Syntax) Overview:

Wikipedia: Cryptographic Message Syntax

Background on Buffer Overflows:

Stack Buffer Overflows Explained – OWASP

Final Thoughts

CVE-2025-15467 is a classic example of a simple coding oversight — not checking buffer bounds — leading to serious real-world risk.
If you work with or maintain a system parsing CMS or PKCS#7 messages (especially with AEAD ciphers like AES-GCM), drop everything and ensure your OpenSSL version is patched.

*For questions or further info, consult the OpenSSL community, or reach out to your software vendor or security response team.*

Timeline

Published on: 01/27/2026 16:01:19 UTC
Last modified on: 02/02/2026 18:38:55 UTC