CVE-2026-77401 in AccessControl
Summary
by MITRE • 09/17/2026
Zope AccessControl provides a general security framework for use in Zope. Prior to 7.4, applications that allow untrusted users to create and execute AccessControl-controlled Python code do not safely guard str.format and str.format_map when those methods are reached through a str subclass. In both ImplPython.py and cAccessControl.c, Python formatting can recursively access attributes and subscriptions using unrestricted getattr and getitem behavior instead of the policy-restricted getattr and getitem operations. A controlled format string can therefore disclose objects reachable from values available to the formatting operation. This issue is fixed in version 7.4.
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Analysis
by VulDB Data Team • 09/17/2026
The Zope AccessControl framework serves as a critical security layer for applications built on the Zope web application server, enforcing strict policies regarding attribute access and item retrieval by untrusted code. A significant vulnerability existed prior to version 7.4 where this security model was bypassed through specific Python string formatting methods. The flaw resides in how ImplPython.py and cAccessControl.c handle str.format and str.format_map operations when invoked on instances of a subclass of the standard string type. In these implementations, the internal logic for resolving placeholders within format strings relies on unrestricted getattr and getitem calls rather than the policy-enforced variants that are normally applied to untrusted code execution contexts. This discrepancy creates an inconsistency in the security boundary, allowing attackers to exploit the formatting mechanism as a side-channel attack vector.
From a technical perspective, the vulnerability stems from the fact that Python's string formatting engine performs attribute lookups and item retrievals during the evaluation of format strings. When these operations are triggered through a str subclass within the AccessControl environment, they bypass the security wrapper designed to restrict access to sensitive attributes or objects. Consequently, an attacker who can control the input passed to str.format or str.format_map can construct a maliciously crafted string that forces the interpreter to resolve arbitrary attributes on available objects. This capability effectively neutralizes the sandboxing protections provided by AccessControl for these specific code paths, enabling information disclosure of internal application state, configuration details, or other sensitive data structures that were previously inaccessible under normal policy restrictions.
The operational impact of this vulnerability is primarily centered around unauthorized information disclosure and potential further exploitation based on leaked context. By disclosing objects reachable from values available to the formatting operation, an attacker can map out the object graph within the application's memory space. This reconnaissance phase can reveal internal class structures, method signatures, or sensitive variables that could be leveraged for more severe attacks such as remote code execution if combined with other vulnerabilities. The issue affects any Zope-based application that permits untrusted users to execute Python code and subsequently utilizes string formatting methods on user-controlled data derived from str subclasses. This scenario is particularly dangerous in multi-tenant environments or applications where external input directly influences dynamic code generation or template rendering processes.
This vulnerability aligns with CWE-209, which describes the exposure of an error message that reveals sensitive information to an unauthorized actor, and more broadly with CWE-749, related to exposing security features without proper restrictions. In terms of offensive cybersecurity frameworks, this behavior is consistent with ATT&CK technique T1530, Data from Local System, where adversaries collect data stored on local systems through indirect means such as exploiting application logic flaws rather than direct file system access. The exploitation relies on the misconfiguration or incomplete implementation of security policies within specific language features, highlighting the importance of ensuring that all code paths involving untrusted input adhere to the same strict access control standards.
To mitigate this vulnerability, organizations running Zope applications must upgrade AccessControl to version 7.4 or later, where the developers have corrected the underlying implementations in ImplPython.py and cAccessControl.c to ensure that str.format and str.format_map operations respect the established security policies for getattr and getitem calls. For systems unable to immediately patch, administrators should rigorously audit codebases for instances of string formatting involving untrusted input and consider implementing additional validation layers or sandboxing mechanisms at a higher level in the application stack. It is also advisable to review any custom str subclasses used within the application context to ensure they do not inadvertently expose internal state through standard Python object protocols that might be exploited via similar side-channel techniques.