CVE-2026-63311 in NLTK
Summary
by MITRE • 08/22/2026
NLTK before 3.10.0 (affected versions <= 3.9.4) contains a server-side request forgery (SSRF) vulnerability in the validate_network_url() function in nltk/pathsec.py. The _resolve_hostname() helper catches OSError and ValueError during socket.getaddrinfo() and returns an empty list; when DNS resolution fails, the validation loop executes no IP checks and the function fails open, allowing urlopen() to proceed without validation. An attacker who can trigger DNS resolution failures or use DNS rebinding can bypass SSRF protections and reach restricted network resources, including cloud metadata endpoints (e.g., 169.254.169.254).
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Analysis
by VulDB Data Team • 08/23/2026
The vulnerability identified in Natural Language Toolkit versions prior to 3.10.0 represents a critical server-side request forgery flaw located within the pathsec.py module, specifically affecting the validate_network_url function and its helper _resolve_hostname. This component is designed to enforce security policies by validating network URLs before they are processed, yet it contains a fundamental logic error in how it handles DNS resolution failures. The core technical issue stems from an overly broad exception handling mechanism within the _resolve_hostname function. When this helper attempts to resolve hostnames using socket.getaddrinfo(), it catches both OSError and ValueError exceptions without performing any subsequent validation checks on the resulting empty list. This design choice creates a failure-open scenario where, instead of rejecting invalid or potentially malicious requests when DNS resolution fails, the system silently proceeds with an unvalidated state. Consequently, if an attacker can induce a DNS resolution error through crafted inputs or network conditions, the security controls are effectively bypassed because no IP address checks are executed against the target resource.
This architectural flaw allows attackers to circumvent SSRF protections that rely on hostname validation and IP range checking. By exploiting this weakness, it is possible to reach internal services and restricted network resources that would otherwise be blocked by standard whitelists or blacklist mechanisms. A particularly dangerous aspect of this vulnerability is its susceptibility to DNS rebinding attacks. In such scenarios, an attacker first resolves a domain name to their controlled external IP address during the initial validation phase, which may pass certain checks depending on implementation details. Subsequently, they manipulate the DNS response so that subsequent requests from the server resolve to internal private IP addresses or cloud metadata endpoints. Because the validate_network_url function fails open upon any resolution anomaly rather than enforcing strict re-validation of the final resolved address against a secure allowlist, these rebinding attacks succeed in directing server-side traffic toward sensitive targets.
The operational impact of this vulnerability is severe, particularly for applications deployed in cloud environments or internal networks that expose services accessible via SSRF vectors. Attackers can leverage this flaw to access cloud metadata endpoints such as 169.254.169.254 on AWS and similar infrastructure provider addresses. Accessing these endpoints often grants unauthorized retrieval of temporary security credentials, instance identity documents, and other sensitive configuration data that facilitates further compromise of the underlying infrastructure. Beyond credential theft, this vulnerability can be used to probe internal network topologies, access unauthenticated administrative interfaces, or interact with backend services like databases or message queues that are not exposed to the public internet but are reachable from the application server. This effectively turns a standard web application into an attack proxy for lateral movement within the target environment.
From a classification perspective, this vulnerability aligns with CWE-918 Server-Side Request Forgery and specifically relates to improper validation of supplied arguments in network contexts. It also reflects weaknesses associated with insufficiently rigorous input sanitization when handling external data sources like DNS responses. In terms of adversary tactics, exploitation of this flaw maps directly to the ATT&CK technique T1557 Adversary-in-the-Middle or more specifically Lateral Tool Transfer and Service Scanning if used for reconnaissance, but primarily it facilitates Credential Access through cloud metadata retrieval as described in T1528. The failure mode is characteristic of CWE-94 Improper Control of Generation of Code (Code Injection) variants where logic flaws allow unintended execution paths, though here the primary concern is network access control bypass rather than code injection per se.
Mitigation strategies must focus on correcting the exception handling logic within the validate_network_url function to ensure that DNS resolution failures result in explicit rejection of the request rather than silent continuation. Developers should implement strict validation policies that verify resolved IP addresses against a predefined allowlist of permitted ranges, ensuring that private or reserved address spaces are blocked unless explicitly authorized for specific internal services. Additionally, employing network-level controls such as egress filtering and firewall rules can provide defense in depth by preventing the application server from initiating connections to unauthorized external destinations regardless of application-layer validation outcomes. Upgrading to NLTK version 3.10.0 or later is the primary remediation step, as these versions address this logic error by ensuring that SSRF protections remain effective even under conditions of DNS instability or malicious rebinding attempts.