CVE-2026-78682 in NLTK
Summary
by MITRE • 08/25/2026
NLTK before 3.10.3 contains a server-side request forgery vulnerability in nltk.pathsec.urlopen (and callers nltk.data.load, nltk.downloader.Downloader.index/download) when an HTTP proxy is configured. pathsec.urlopen validates the requested hostname locally, but proxy-handler inheritance disables the safe HTTP/HTTPS handlers so the actual fetch is performed by the proxy against a destination that is never re-validated. An attacker can supply a validated public URL that the proxy forwards to an internal loopback-only service, allowing disclosure of internal HTTP resources, loading of forged downloader indexes, and installation of attacker-chosen package content.
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Analysis
by VulDB Data Team • 08/25/2026
The vulnerability identified in Natural Language Toolkit versions prior to 3.10.3 represents a critical server-side request forgery flaw rooted in the improper handling of proxy configurations during network resource retrieval operations. The core technical deficiency lies within the nltk.pathsec.urlopen function and its dependent methods, specifically nltk.data.load and nltk.downloader.Downloader.index or download routines. These functions are designed to fetch remote resources such as language models, datasets, or package indexes from external servers. To mitigate risks associated with accessing internal network services, NLTK implements a local hostname validation mechanism intended to restrict access to loopback addresses like 127.0.0.1 and localhost. This validation logic operates by inspecting the target URL's host component before initiating the connection request. However, this security control is effectively bypassed when an HTTP proxy is configured within the environment where NLTK is executed.
The operational mechanism of the flaw involves a disconnect between local validation and actual network transmission behavior. When a proxy configuration is detected, Python’s urllib library delegates the actual HTTP or HTTPS transaction to the specified proxy server rather than establishing a direct connection from the client machine. The critical failure occurs because while nltk.pathsec.urlopen performs its hostname check against the original target URL provided by the application logic, it does not re-validate the destination after the request has been handed off to the proxy handler. Consequently, although the initial validation confirms that the requested host is public and safe, the proxy server receives this instruction and may forward the traffic to a different internal endpoint if configured or manipulated to do so. This behavior exploits the trust relationship between the client application and the proxy infrastructure, allowing an attacker who can influence the URL passed to these NLTK functions to redirect requests into the local network perimeter without triggering the intended security checks.
From an impact perspective, this vulnerability enables several severe attack vectors that compromise both confidentiality and integrity of the system running NLTK. An adversary could exploit this flaw to disclose sensitive internal HTTP resources that are not exposed to the public internet but are accessible via loopback interfaces or private IP ranges. This includes accessing administrative panels, configuration files, or API endpoints residing on local services such as databases, message queues, or other microservices communicating over localhost. Furthermore, in the context of NLTK’s downloader functionality, an attacker could forge a valid-looking index file that directs the system to download malicious package content from internal servers disguised as legitimate sources. This leads to the installation of arbitrary code under the privileges of the user executing the NLTK script, potentially resulting in full system compromise if combined with other exploitation techniques or if the executed payload performs further lateral movement within the network.
This vulnerability aligns closely with CWE-918, which describes Server-Side Request Forgery (SSRF), specifically illustrating how improper validation of proxy behavior can lead to unauthorized access to internal resources. It also maps to MITRE ATT&CK technique T1557, Adversary-in-the-Middle, as the attack leverages a trusted intermediary to intercept and redirect traffic for malicious purposes. Additionally, it relates to CWE-20, Improper Input Validation, because the application fails to ensure that all network interactions adhere to security policies after delegation to external components like proxies. The root cause is not merely a lack of input sanitization but rather an incomplete understanding of how proxy inheritance alters the execution context and bypasses local enforcement mechanisms.
Mitigation strategies must focus on both immediate patching and architectural hardening. Organizations should immediately upgrade NLTK to version 3.10.3 or later, where this validation logic has been corrected to ensure that hostname checks are applied consistently regardless of whether a proxy is in use. For environments unable to update promptly, implementing strict network segmentation can limit the blast radius by ensuring that internal services do not listen on loopback interfaces accessible from application servers unless absolutely necessary. Additionally, configuring proxies with explicit allowlists for outbound connections can prevent them from forwarding requests to arbitrary internal destinations even if instructed to do so by a compromised client. Security monitoring should also be enhanced to detect unusual patterns of outbound traffic originating from NLTK processes that deviate from expected external endpoints, providing an additional layer of defense against exploitation attempts targeting this specific class of SSRF vulnerabilities.