CVE-2026-34180

What it actually does

This is an OpenSSL ASN.1 DER decoding flaw caused by integer truncation when parsing a primitive ASN.1 element with a content length greater than 2 GB on affected 64-bit Unix and Unix-like platforms. Applications that feed attacker-controlled DER data into d2i_* decoding APIs, such as d2i_X509(), d2i_PKCS7(), or related ASN.1 decoders, may trigger a heap buffer over-read. The most likely result is application crash and denial of service; in less common cases, bytes beyond the end of the input buffer may be incorporated into the decoded ASN.1 object, creating a potential memory disclosure path if the application later exposes, logs, stores, or returns that decoded content. OpenSSL command-line tools are not affected in the described path, 32-bit platforms and 64-bit Windows are not affected, and the listed OpenSSL FIPS modules are outside the affected code boundary.

Why it matters to the business

Organizations using OpenSSL in services that parse untrusted certificates, PKCS#7/CMS objects, S/MIME messages, signed payloads, identity artifacts, or other DER-encoded ASN.1 content may be exposed to remotely triggerable service crashes. For internet-facing services, this can translate into availability outages, failed authentication or certificate-processing workflows, and operational instability. The CVSS score indicates high availability impact, but current exploitation signals are low: EPSS is 0.01025, the 30-day trend is flat, there is no known KEV listing, no ransomware association, and no public exploit maturity was provided. Because OpenSSL is widely embedded in software stacks, the main business risk is broad hidden dependency exposure rather than confirmed active mass exploitation.

How an attack unfolds

An attacker identifies an application path that accepts ASN.1 DER data and passes it directly or indirectly to OpenSSL d2i_* decoding routines on a vulnerable 64-bit Unix-like system. The attacker supplies a specially crafted DER-encoded object containing a primitive ASN.1 element whose declared content length exceeds 2 GB. Due to integer truncation in the decoder, the oversized length is mishandled. In the worst case, OpenSSL treats the truncated value as though it needs to scan binary content for a terminating zero byte, which can cause reads beyond the allocated input buffer. The vulnerable process may then crash, producing a denial of service. If the application continues and later exposes decoded ASN.1 contents, memory adjacent to the input buffer could potentially be leaked, although availability impact is the primary expected outcome.

How to fix it

Upgrade OpenSSL to a vendor-fixed release as soon as fixed packages are available for the affected distribution or embedded product. Because exact fixed versions were not provided in the input, follow OpenSSL project advisories and operating-system or appliance vendor security updates for CVE-2026-34180. Prioritize systems on 64-bit Unix and Unix-like platforms that parse untrusted DER, X.509, PKCS#7, CMS, S/MIME, or similar ASN.1 objects. Where immediate patching is not possible, add defensive input limits before calling OpenSSL ASN.1 d2i_* APIs: reject or quarantine ASN.1 objects with declared primitive element lengths greater than 2 GB, enforce strict maximum upload/message sizes well below that threshold, and avoid passing attacker-controlled buffers directly to ASN.1 decoding routines without size validation. Run high-risk parsing components in isolated worker processes with crash recovery, resource limits, and minimal privileges. Confirm whether vendor FIPS modules are in use, but do not assume FIPS mode protects application code that calls affected non-FIPS OpenSSL parsing paths.

How to confirm the fix worked

Validate exposure by inventorying OpenSSL versions and linked libraries across products, containers, appliances, and statically linked binaries, then identify code paths that call d2i_X509(), d2i_PKCS7(), d2i_ASN1_TYPE(), d2i_ASN1_OCTET_STRING(), d2i_CMS_ContentInfo(), or other d2i_* functions with untrusted input. Confirm affected hosts are 64-bit Unix or Unix-like systems; 32-bit platforms and 64-bit Windows are not affected by the described issue. Confirm remediation by verifying installed vendor package versions against the relevant security advisory, checking that rebuilt applications link to the patched OpenSSL library, and restarting long-running processes that may still have the vulnerable library loaded. For application-level mitigations, add unit or integration tests that reject DER inputs with long-form lengths above policy limits before OpenSSL decoding occurs. Avoid using production services for destructive crash testing unless coordinated through a controlled maintenance window.

Typical attack path

1. Find a service, API, mail-processing component, authentication flow, signing-verification flow, or file-ingestion workflow that accepts DER-encoded ASN.1 data from an untrusted party.
2. Confirm the target runs on a 64-bit Unix or Unix-like platform and uses an affected OpenSSL library path for ASN.1 decoding.
3. Craft a DER object with a primitive ASN.1 element declaring a content length greater than 2 GB.
4. Submit the crafted object through a parser path that reaches d2i_X509(), d2i_PKCS7(), or another affected d2i_* function.
5. Trigger integer truncation during ASN.1 length handling.
6. Cause the target process to read outside the intended input buffer, usually resulting in a crash or failed parsing workflow.
7. Optionally attempt to reach application behavior that serializes, logs, stores, or returns decoded ASN.1 content in order to look for unintended memory disclosure.

How to detect it

Look for evidence of attempted exploitation in services that accept untrusted DER-encoded ASN.1 objects, including certificate upload endpoints, mutual-authentication components, S/MIME or mail gateways, PKCS#7/CMS signature verification, document-signing workflows, package-verification systems, and custom APIs using OpenSSL d2i_* parsers. Useful log sources include application logs, reverse-proxy and API gateway logs for large or rejected binary uploads, mail gateway logs, authentication service logs, systemd/journald, syslog, kernel crash logs, container restart events, EDR crash telemetry, and core-dump indexes. Investigate crashes or restarts that coincide with parsing of certificates, PKCS#7, CMS, S/MIME, or DER objects. In core dumps or stack traces, look for OpenSSL ASN.1 decoder frames such as ASN1_get_object, asn1_d2i_read_bio, asn1_check_tlen, asn1_item_embed_d2i, d2i_X509, d2i_PKCS7, d2i_CMS_ContentInfo, or related d2i_* routines. Network and file-content hunting can look for DER long-form length encodings that declare primitive element sizes greater than 2 GB, especially byte sequences where a primitive tag is followed by a five-byte length field beginning with 0x84 and a high length byte of 0x80 or greater. Treat such content as suspicious when found in small uploads or protocol messages, because it indicates an impossible or intentionally malformed length declaration. There are no named threat actors or active exploitation campaigns provided for this CVE, and it is not listed in KEV, so detection should focus on abnormal parser crashes, oversized ASN.1 length declarations, and suspicious DER submissions rather than known actor-specific infrastructure.

• d2i_X509
• d2i_PKCS7
• d2i_CMS_ContentInfo
• ASN1_get_object
• asn1_check_tlen
• asn1_item_embed_d2i
• asn1_d2i_read_bio
• segmentation fault
• Segmentation fault
• SIGSEGV
• core dumped
• ASN.1
• ASN1
• DER
• PKCS7
• PKCS#7
• CMS_ContentInfo
• S/MIME
• \x04\x84\x80\x00\x00\x00
• \x04\x84\xff\xff\xff\xff
• \x13\x84\x80\x00\x00\x00
• \x16\x84\x80\x00\x00\x00
• \x0c\x84\x80\x00\x00\x00

What your security team should do

• Prioritize inventory of internet-facing and externally reachable services that parse certificates, PKCS#7/CMS, S/MIME, signed packages, or uploaded DER/ASN.1 data using OpenSSL on 64-bit Unix-like systems.
• Apply vendor OpenSSL updates when available, including updates for statically linked applications, containers, appliances, language runtimes, and bundled third-party components.
• Add pre-parse size and ASN.1 length validation to reject oversized DER elements before calling OpenSSL d2i_* APIs, especially where patches cannot be deployed immediately.
• Harden parsers with process isolation, memory/resource limits, supervisor-based restart, and least-privilege execution to reduce denial-of-service blast radius.
• Search crash, core-dump, and application logs for ASN.1 parsing failures, segmentation faults, or abnormal process exits correlated with certificate, PKCS#7, CMS, S/MIME, or DER ingestion.
• Enable collection of core dumps or crash telemetry in non-sensitive ways where operationally acceptable, and inspect stack traces for OpenSSL ASN.1 decoding functions.
• Monitor OpenSSL, operating-system vendor, and appliance advisories for exact fixed versions and any change in exploit maturity, KEV status, or observed threat activity.
• Because EPSS is currently low and flat and no KEV listing is present, treat this as important hygiene for exposed parsing surfaces rather than an emergency ransomware-driven remediation event unless local exposure is high.