CVE-2026-34180: OpenSSL ASN.1 Decoder DoS Vulnerability

CVE-2026-34180 Overview

CVE-2026-34180 is a heap buffer over-read in OpenSSL's ASN.1 decoder. An integer truncation flaw mishandles the content length of a primitive ASN.1 element when it exceeds 2 gigabytes. Applications passing attacker-controlled data to d2i_X509(), d2i_PKCS7(), or any d2i_* decoding function are exposed. The flaw affects only 64-bit Unix and Unix-like platforms. 32-bit platforms and 64-bit Windows are not vulnerable. OpenSSL's own command-line tools are unaffected because data read through the BIO layer is validated before reaching the affected code path. FIPS modules in versions 4.0, 3.6, 3.5, 3.4, and 3.0 fall outside the affected code boundary and are not impacted.

Critical Impact
Crafted DER-encoded ASN.1 input can crash the consuming application or cause the decoded ASN.1 object to reference memory beyond the input buffer, producing denial of service or out-of-bounds memory exposure.

Affected Products

OpenSSL on 64-bit Unix and Unix-like platforms (non-FIPS code paths)
Applications calling d2i_X509(), d2i_PKCS7(), or other d2i_* decoders on attacker-supplied data
Linked libraries and services consuming untrusted DER/ASN.1 input via OpenSSL

Discovery Timeline

2026-06-09 - OpenSSL Security Advisory published
2026-06-09 - CVE-2026-34180 published to NVD
2026-06-10 - Last updated in NVD database

Technical Details for CVE-2026-34180

Vulnerability Analysis

The flaw resides in OpenSSL's ASN.1 decoder, classified as an out-of-bounds read [CWE-125]. When parsing a DER-encoded primitive element, the decoder reads the declared content length and uses it to bound a scan over the input buffer. If the declared length exceeds 2 gigabytes, an integer truncation reduces the stored length to a smaller or unexpected value. In the worst case the truncated length is interpreted as a directive to scan the buffer for a terminating zero byte. The scan then runs past the end of the allocated input. The decoded ASN.1 object can subsequently reference adjacent heap memory as if it were valid content. Outcomes range from process crash to incorporation of out-of-bounds memory into application logic that consumes the decoded structure.

Root Cause

The defect is a numeric truncation error during length handling in the ASN.1 primitive decoder. A length value larger than 2 GB is silently narrowed, defeating the size checks that normally truncate or reject oversized elements. The flaw is reachable only on 64-bit Unix and Unix-like platforms where the data type widths and code paths permit the truncation to occur.

Attack Vector

Exploitation requires an application to call a d2i_* decoding function on data the attacker controls. Common exposure surfaces include TLS endpoints accepting client certificates, S/MIME and CMS processors, certificate validation pipelines, and any service that parses DER-encoded structures received over the network. No authentication or user interaction is required. The attacker submits a DER blob declaring a primitive element with a content length greater than 2 GB and triggers the truncated-length scan past the allocated buffer.

No verified public exploit code is available. Technical details are described in the OpenSSL Security Advisory and the fix commits referenced from the advisory.

Detection Methods for CVE-2026-34180

Indicators of Compromise

Unexpected crashes or segmentation faults in processes linked against OpenSSL during certificate, PKCS#7, or CMS parsing
Inbound TLS handshakes or message payloads containing DER structures with declared lengths above 2 GB
Repeated TLS or S/MIME parse failures originating from a single source against services that accept untrusted certificates

Detection Strategies

Inspect TLS and messaging telemetry for ASN.1 elements with declared content length fields exceeding 0x7FFFFFFF
Monitor application logs for d2i_* parse errors and abnormal termination signals on 64-bit Linux and Unix hosts
Audit the software inventory for vulnerable OpenSSL versions and identify services exposing d2i_* to untrusted input

Monitoring Recommendations

Forward crash telemetry from OpenSSL-linked services to a centralized analytics pipeline and alert on repeated faults
Track network ingress for oversized ASN.1 payloads at TLS terminators, mail gateways, and certificate validation services
Correlate crash events with peer IP, certificate fingerprint, and process identity to identify probing activity

How to Mitigate CVE-2026-34180

Immediate Actions Required

Upgrade OpenSSL to the patched release identified in the OpenSSL Security Advisory
Restart all services linked against OpenSSL so the patched library is loaded into memory
Inventory applications calling d2i_X509(), d2i_PKCS7(), or other d2i_* functions on attacker-supplied data and prioritize them for patching

Patch Information

Fixes are available in the OpenSSL source tree. Refer to the upstream commits: 1c6908e4, cbe418ae, d93853c4, da5d62af, and f696c73c. Distribution-supplied binary updates should be applied as soon as they are released.

Workarounds

Reject or truncate DER inputs larger than a sensible application-specific limit before passing data to d2i_* functions
Route untrusted ASN.1 input through the OpenSSL BIO layer, which performs the length checks that block the affected code path
Where feasible on affected systems, constrain exposed services to authenticated peers to reduce the attack surface until patches are deployed

bash

# Verify installed OpenSSL version and locate linked services
openssl version -a
lsof -n 2>/dev/null | grep -i libssl