pyopenssl: Buffer Copy without Checking Size of Input
CVE-2026-27459 is a critical-severity buffer copy without checking size of input vulnerability in pyopenssl from Pyca, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected pyopenssl versions: >= 22.0.0, < 26.0.0. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.8% chance of exploitation within 30 days, higher than 57% of scored CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Check which version of pyopenssl you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Pyca | pyopenssl | >= 22.0.0, < 26.0.0 | Not stated |
| Red Hat | Red Hat Ansible Automation Platform 2.5 for RHEL 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Ansible Automation Platform 2.5 for RHEL 9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Ansible Automation Platform 2.6 for RHEL 9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Satellite 6.16 for RHEL 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Satellite 6.16 for RHEL 9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Satellite 6.17 for RHEL 9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Satellite 6.18 for RHEL 9 | Not specified in the record | Not stated |
| Red Hat | RHUI 4 for RHEL 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Ansible Automation Platform 2.5 | Not specified in the record | Not stated |
| Red Hat | Red Hat Ansible Automation Platform 2.6 | Not specified in the record | Not stated |
| Red Hat | Red Hat Ansible Automation Platform 2.7 | Not specified in the record | Not stated |
| Red Hat | Red Hat Hardened Images | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.10 | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.12 | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.14 | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.15 | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.16 | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.17 | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.18 | Not specified in the record | Not stated |
| Red Hat | Red Hat Quay 3.9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Trusted Artifact Signer 1.4 | Not specified in the record | Not stated |
| Red Hat | Confidential Compute Attestation | Not specified in the record | Not stated |
| Red Hat | Exploit Intelligence | Not specified in the record | Not stated |
| Red Hat | Lightspeed Core | Not specified in the record | Not stated |
Version data comes from the organisation that assigned the CVE; 15 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 7.2 | High | CNA (github.com) |
| CVSS 3.1 | 9.8 | Critical | NVD |
| CVSS 3.1 | 8.1 | High | CNA |
Official description
pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 22.0.0 and prior to version 26.0.0, if a user provided callback to `set_cookie_generate_callback` returned a cookie value greater than 256 bytes, pyOpenSSL would overflow an OpenSSL provided buffer. Starting in version 26.0.0, cookie values that are too long are now rejected.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Buffer Copy without Checking Size of Input
The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
How developers prevent this class of flaw
- Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.
- Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions.
- Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.
More Buffer Copy without Checking Size of Input vulnerabilities (CWE-120)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Other references
- access.redhat.com/errata/RHSA-2026:10754
- access.redhat.com/errata/RHSA-2026:11856
- access.redhat.com/errata/RHSA-2026:11916
- access.redhat.com/errata/RHSA-2026:11996
- access.redhat.com/errata/RHSA-2026:13508
- access.redhat.com/errata/RHSA-2026:13512
- access.redhat.com/errata/RHSA-2026:13545
- access.redhat.com/errata/RHSA-2026:13553
- access.redhat.com/errata/RHSA-2026:14835
- access.redhat.com/errata/RHSA-2026:14873
- access.redhat.com/errata/RHSA-2026:14874
- access.redhat.com/errata/RHSA-2026:19375
- access.redhat.com/errata/RHSA-2026:21017
- access.redhat.com/errata/RHSA-2026:22465
- access.redhat.com/errata/RHSA-2026:24853
Recent Buffer Copy without Checking Size of Input vulnerabilities
- 8.1 High severity
- 9.8 Critical severity
- 7.8 High severity
- 9.8 Critical severity
- 7.8 High severity
- 7.8 High severity
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