CVE-2026-27448: Pyopenssl PyOpenSSL Allows TLS Connection Bypass Vulnerability

LOW
Advisory Identifier: GHSA-vp96-hxj8-p424
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CWE-636
Published
Advisory Summary

CVE-2026-27448 is a LOW pyopenssl allows tls connection bypass vulnerability (CWE-636: Not Failing Securely ('Failing Open')) affecting Pyopenssl libraries in the PyPI ecosystem. The issue involves pyopenssl allows tls connection bypass via unhandled callback exception in set_tlsext_servername_callback. Remediation is available by upgrading affected packages to version 26.0.0.

CVE-2026-27448 at a Glance

CVE Identifier CVE-2026-27448
GHSA Identifier GHSA-vp96-hxj8-p424
Severity LOW
CVSS v3.1 Base Score N/A
CWE Classification CWE-636: Not Failing Securely ('Failing Open')
Affected Product Pyopenssl
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2026-27448

To remediate this vulnerability, upgrade all affected Pyopenssl dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

pyopenssl fixed: 26.0.0 
pip install pyopenssl>=26.0.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show pyopenssl

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-27448, inspect your installed packages using pip:

pip show pyopenssl

Analyze the terminal output to locate any version of Pyopenssl in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 26.0.0), your application is affected.

Note: The pip show pyopenssl command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-636: Not Failing Securely ('Failing Open'), where when the product encounters an error condition or failure, its design requires it to fall back to a state that is less secure than other options that are available, such as selecting the weakest encryption algorithm or using the most permissive access control restrictions. In Pyopenssl, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

If a user provided callback to `set_tlsext_servername_callback` raised an unhandled exception, this would result in a connection being accepted. If a user was relying on this callback for any security-sensitive behavior, this could allow bypassing it. Unhandled exceptions now result in rejecting the connection. Credit to **Leury Castillo** for reporting this issue.

CVE-2026-27448 Affected Pyopenssl Versions

1 component(s)
Fixed Versions:
26.0.0
Product / Package Ecosystem Affected Versions Fixed Version
pyopenssl
pypi/pyopenssl
PyPI
≥ 0.14.0
< 26.0.0
26.0.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/pyca/pyopenssl/security/advisories/GHSA-vp96-hxj8-p424
NVD — CVE-2026-27448 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-27448
Security Fix Commit FIX COMMIT
https://github.com/pyca/pyopenssl/commit/d41a814759a9fb49584ca8ab3f7295de49a85aa0
Pyopenssl Source Repository REPOSITORY
https://github.com/pyca/pyopenssl
Pyopenssl Source Repository REPOSITORY
https://github.com/pyca/pyopenssl/blob/358cbf29c4e364c59930e53a270116249581eaa3/CHANGELOG.rst#L27

Frequently Asked Questions

What is CVE-2026-27448?
CVE-2026-27448 is a security vulnerability in Pyopenssl involving pyOpenSSL allows TLS connection bypass via unhandled callback exception in set_tlsext_servername_callback under CWE-636: Not Failing Securely ('Failing Open').
Which Pyopenssl versions are affected?
Affected versions include releases prior to 26.0.0 across affected packages: pyopenssl.
What version fixes CVE-2026-27448?
Upgrading to 26.0.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is pyopenssl affected?
Yes, pyopenssl is affected in releases prior to 26.0.0. Upgrade your dependency to 26.0.0 or later.
What CWE is associated with CVE-2026-27448?
CVE-2026-27448 is classified under CWE-636: Not Failing Securely ('Failing Open') (When the product encounters an error condition or failure, its design requires it to fall back to a state that is less secure than other options that are available, such as selecting the weakest encryption algorithm or using the most permissive access control restrictions.).
How can I check PyPI dependencies for CVE-2026-27448?
Run the dependency tree command: 'pip show pyopenssl'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vp96-hxj8-p424",
  "modified" : "2026-03-19T18:28:02Z",
  "published" : "2026-03-16T15:15:43Z",
  "aliases" : [ "CVE-2026-27448" ],
  "summary" : "pyOpenSSL allows TLS connection bypass via unhandled callback exception in set_tlsext_servername_callback",
  "details" : "If a user provided callback to `set_tlsext_servername_callback` raised an unhandled exception, this would result in a connection being accepted. If a user was relying on this callback for any security-sensitive behavior, this could allow bypassing it.\n\nUnhandled exceptions now result in rejecting the connection.\n\nCredit to **Leury Castillo** for reporting this issue.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:U"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "pyopenssl"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.14.0"
      }, {
        "fixed" : "26.0.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/pyca/pyopenssl/security/advisories/GHSA-vp96-hxj8-p424"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-27448"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pyca/pyopenssl/commit/d41a814759a9fb49584ca8ab3f7295de49a85aa0"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/pyca/pyopenssl"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pyca/pyopenssl/blob/358cbf29c4e364c59930e53a270116249581eaa3/CHANGELOG.rst#L27"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-636" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-16T15:15:43Z",
    "nvd_published_at" : "2026-03-18T00:16:19Z"
  }
}