How severe is CVE-2026-77401?

CVE-2026-77401 has a CVSS score of 6.8/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-77401?

CVE-2026-77401 affects the following packages: accesscontrol (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-77401?

Update accesscontrol to 7.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-77401 is resolved across your whole dependency graph.

How do I detect CVE-2026-77401 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for accesscontrol, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-77401 if there is no patch (or I can't update yet)?

Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Is CVE-2026-77401 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-77401 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-77401?

CVE-2026-77401 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 41th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-77401?

CVE-2026-77401 is classified as CWE-693 (CWE-693). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-77401 published, and has it been updated?

CVE-2026-77401 was published on September 16, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2026-77401 — accesscontrol

MEDIUMFix: zopefoundation/AccessControl@f980450

CVE-2026-77401 is a medium-severity (CVSS 6.8) CWE-693 vulnerability in accesscontrol. A fix is available for accesscontrol — see the affected versions and patch details below.

Zope AccessControl: Information disclosure through Python string `format` and `format_map` functions

Also known asGHSA-pq59-9fq7-m886PYSEC-2026-4022

Published

Sep 16, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 1, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-77401.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs41th percentile — riskier than 41% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐍accesscontrol

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

Python's string format functionality allows someone controlling the format string to "read" objects accessible (recursively) via attribute access and subscription from accessible objects. Those attribute accesses and subscriptions use Python's full blown getattr and getitem, not the policy restricted AccessControl variants _getattr_ and _getitem_. This can lead to critical information disclosure.

The AccessControl package already guards against direct access to the formatting functions on string instances, but these mitigations did not cover subclasses of str.

Affected are all users who allow untrusted users to create AccessControl controlled Python code and execute it.

Patches

A fix was published with version 7.4.

Workarounds

There is no workaround.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`accesscontrol`	all versions	7.4`pip install --upgrade 'accesscontrol==7.4'`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for accesscontrol, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update accesscontrol to 7.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-77401 is resolved across your whole dependency graph.
Workarounds
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Frequently Asked Questions

### Impact Python's string `format` functionality allows someone controlling the format string to "read" objects accessible (recursively) via attribute access and subscription from accessible objects. Those attribute accesses and subscriptions use Python's full blown `getattr` and `getitem`, not the policy restricted `AccessControl` variants `_getattr_` and `_getitem_`. This can lead to critical information disclosure. The `AccessControl` package already guards against direct access to the formatting functions on string instances, but these mitigations did not cover subclasses of `str`. Affe