Nltk Nltk vulnerabilities

CVE-2026-109956 High · 7.0 CWE-22 Oct 13, 2026

NLTK through 3.10.3 Path Traversal via WekaClassifier Model Path

NLTK through 3.10.3 contains a path traversal vulnerability in WekaClassifier that allows attackers to bypass nltk.pathsec containment by supplying unvalidated model paths. Attackers controlling the model_filename or model argument can make the weka JVM write files via -d or probe files via -l outside allowed data roots.

CVE-2026-81727 High · 7.1 CWE-59 Aug 27, 2026

NLTK before 3.10.3 Hardlink File Overwrite via downloader

NLTK versions before 3.10.3 contain a filesystem containment bypass vulnerability in the Downloader.download and Downloader.incr_download methods that allows attackers to overwrite files outside the install root through pre-existing hardlinks. Attackers with write access to a shared downloader directory can create hardlinks pointing to outside-root files that are then overwritten during normal package extraction, mutating files outside the intended install tree.

CVE-2026-81726 High · 7.0 CWE-73 Aug 27, 2026

NLTK through 3.10.3 Path Traversal via Model-Artifact APIs

NLTK through 3.10.3 contains a path traversal vulnerability in model-artifact APIs that bypass pathsec enforcement by using raw file operations on caller-controlled paths. Attackers can read or write files outside allowed sandbox roots through TransitionParser, AveragedPerceptron, PerceptronTagger, and maxent parameter APIs when pathsec is enabled.

CVE-2026-81725 Low · 3.7 CWE-400 Aug 27, 2026

NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader

NLTK before 3.10.3 contains a regular expression denial of service vulnerability in Pl196xCorpusReader that allows attackers to cause quadratic CPU consumption by supplying malformed TEI blocks with many unmatched opening tags. Attackers can exploit lazy regex patterns in the read_block method through public APIs like words() and tagged_words() to force repeated rescans and achieve near-quadratic runtime growth.

CVE-2026-81724 Medium · 5.3 CWE-674 Aug 27, 2026

NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion

NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.

CVE-2026-81723 Low · 3.7 CWE-400 Aug 27, 2026

NLTK before 3.10.3 Quadratic CPU Exhaustion via XMLCorpusView

NLTK versions before 3.10.3 contain a quadratic CPU exhaustion vulnerability in XMLCorpusView._read_xml_fragment() that rescans accumulated XML fragments on every 1 KiB block read. Attackers can provide malformed XML corpus files to cause severe CPU consumption and denial of service through affected readers like BNCCorpusReader.

CVE-2026-81722 High · 7.5 CWE-407 Aug 27, 2026

nltk PorterStemmer before 3.10.3 Quadratic-time DoS

nltk PorterStemmer in versions <= 3.10.2 (fixed in 3.10.3) contains an inefficient-algorithmic-complexity denial of service in PorterStemmer.stem(). The _is_consonant() helper walks backward over the entire run of trailing 'y' characters on every call, and _measure() invokes it for each stem position, causing O(n^2) behavior. A single ~20-50 KB untrusted token consisting of a long run of the letter 'y' followed by a matching suffix (e.g., 'ness') can pin a CPU core for seconds to minutes, causing availability impact.

CVE-2026-80206 Medium · 5.9 CWE-1333 Aug 26, 2026

NLTK 3.10.2 Regular Expression Denial of Service via tgrep

NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.

CVE-2026-80205 High · 7.5 CWE-1333 Aug 26, 2026

NLTK before 3.10.0 ReDoS via Text.findall() unvalidated regex

NLTK versions before 3.10.0 contain a regular expression denial of service vulnerability in Text.findall() and TokenSearcher.findall() methods that accept user-supplied regular expressions without validation or timeout. Attackers can supply crafted regex patterns that cause catastrophic backtracking, resulting in indefinite CPU saturation and denial of service to all users of the Python process.

CVE-2026-79676 Medium · 5.9 CWE-22 Aug 25, 2026

NLTK before 3.10.3 Path Traversal via Symlink Bypass

NLTK versions before 3.10.3 contain a path traversal vulnerability in corpus readers that reopen root-derived paths using built-in open() instead of nltk.pathsec.open(), allowing symlinks to escape trusted roots. Attackers who stage symlinked corpus files under a trusted data root can disclose outside-root content through normal corpus reader methods like channels(), domains(), and synonyms().

CVE-2026-79675 Critical · 9.8 CWE-88 Aug 25, 2026

NLTK before 3.10.3 JVM Argument Injection via Per-Call Options

NLTK before 3.10.3 fails to validate JVM options passed through the per-call options parameter in the java() function, allowing attackers to inject dangerous JVM flags. Attackers can supply malicious options like -agentpath, -javaagent, or @argfile to Stanford wrapper classes to achieve arbitrary code execution.

CVE-2026-79674 High · 8.2 CWE-73 Aug 25, 2026

NLTK 3.10.2 Path Traversal via corpus-reader constructors

NLTK versions before 3.10.3 contain a path sandbox bypass vulnerability in corpus-reader constructors that allows attackers to read files outside the intended data root. Attackers can supply arbitrary corpus root paths to LinThesaurusCorpusReader and PanLexLiteCorpusReader constructors to access filesystem content and SQLite databases outside the pathsec sandbox boundary.

CVE-2026-79657 Critical · 9.8 CWE-502 Aug 25, 2026

NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization

NLTK versions before 3.10.3 contain a remote code execution vulnerability in allowlisted pickle loaders that trust entire module namespaces instead of specific safe callables. Attackers can craft malicious pickle payloads invoking dangerous in-namespace functions like ReppTokenizer._execute and numpy.f2py.crackfortran.myeval through pickle REDUCE to execute arbitrary commands during model or tokenizer artifact loading.

CVE-2026-78683 Critical · 9.6 CWE-502 Aug 24, 2026

NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization

NLTK before 3.10.0 (affected versions <=3.9.4) contains an unsafe pickle deserialization vulnerability in the TransitionParser.parse() method (nltk/parse/transitionparser.py). The method calls pickle_load() with the default restricted=False, routing deserialization through WarningUnpickler, which does not override find_class() and therefore permits arbitrary class resolution. When an application loads an attacker-crafted model file, embedded pickle gadget chains execute arbitrary Python code with the privileges of the user running the application. NLTK provides a RestrictedUnpickler for safe deserialization, but it is not used by production code paths. Fixed in 3.10.0.

CVE-2026-78682 High · 7.5 CWE-918 Aug 24, 2026

NLTK before 3.10.3 SSRF Protection Bypass via Proxy

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.

CVE-2026-78681 High · 7.5 CWE-776 Aug 24, 2026

NLTK before 3.10.3 Entity Expansion DoS via ElementTree

NLTK versions before 3.10.3 use xml.etree.ElementTree to parse XML in multiple modules, which honors entity declarations in document DTDs. Attackers can craft XML payloads with nested entity declarations that expand from hundreds of bytes to megabytes in memory, causing denial of service.

CVE-2026-78680 High · 7.8 CWE-426 Aug 24, 2026

NLTK before 3.10.3 Arbitrary Code Execution via Graphviz dot Binary

NLTK versions before 3.10.3 fail to use validated absolute paths when invoking the Graphviz dot binary in dependencygraph.dot2img and AlignedSent._repr_svg_, allowing attackers to execute arbitrary code by placing a malicious dot binary in the search path or current working directory. Attackers can exploit bare-name binary resolution on Windows via the current working directory or on Unix-like systems via relative PATH entries to execute their binary instead of the legitimate Graphviz tool.

CVE-2026-70626 Medium · 6.2 CWE-59 Aug 22, 2026

NLTK before 3.9.4 Symlink Escape via CorpusReader

NLTK versions before 3.9.4 contain a symlink escape vulnerability in CorpusReader.open() that allows local attackers to read arbitrary files outside the corpus root. The vulnerability exists because path validation is lexical and does not account for symlink resolution, enabling attackers to place symlinks inside the corpus root to access files outside the intended boundary.

CVE-2026-66393 High · 7.5 CWE-674 Aug 22, 2026

NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.

CVE-2026-65915 Medium · 6.5 CWE-284 Aug 22, 2026

NLTK before 3.10.0 Arbitrary File Read via FileSystemPathPointer

NLTK versions before 3.10.0 contain a logic bug in FileSystemPathPointer.open() where the sandbox validation check compares a normalized path against itself, making the security check permanently inert. Attackers can pass file:// URLs to nltk.data.load() to read arbitrary files accessible to the process user, including credentials and configuration files.

CVE-2026-63312 High · 7.5 CWE-22 Aug 22, 2026

NLTK StreamBackedCorpusView Bypasses pathsec.ENFORCE Arbitrary File Read

NLTK before 3.10.0 contains an arbitrary local file read vulnerability in StreamBackedCorpusView that bypasses pathsec.ENFORCE by calling builtins.open() directly instead of pathsec.open(). Attackers who control the fileid argument can read arbitrary local files regardless of the ENFORCE setting, including sensitive system files and application credentials.

CVE-2026-63311 Medium · 5.3 CWE-918 Aug 22, 2026

NLTK before 3.10.0 SSRF via DNS Resolution Failure

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).

CVE-2026-62388 High · 7.5 CWE-1188 Aug 22, 2026

NLTK before 3.10.0 Insecure Default Configuration in pathsec.py

NLTK versions before 3.10.0 default to ENFORCE=False in pathsec.py, causing all security validation functions to emit warnings instead of raising exceptions. Attackers can bypass path traversal and pickle deserialization protections by exploiting the disabled security controls that are only active when manually enabled.

CVE-2026-62385 High · 7.5 CWE-73 Aug 22, 2026

NLTK 3.9.4 Path Traversal via FrameNet and NKJP Readers

NLTK versions before 3.10.0 contain a path traversal vulnerability in FramenetCorpusReader and NKJPCorpusReader that allows attackers to parse XML files outside the corpus root by supplying unsafe selectors or poisoned index state. Attackers can exploit frame_by_name, doc, lu, and header methods with crafted parameters to read arbitrary XML files accessible to the application.

CVE-2026-62384 High · 7.5 CWE-22 Aug 22, 2026

NLTK FramenetCorpusReader Symlink Sandbox Bypass before 3.10.2

NLTK versions before 3.10.2 contain a symlink-based sandbox bypass in FramenetCorpusReader that allows attackers to read arbitrary XML files outside the corpus root. Attackers can place symlinks with names containing no path separators inside the corpus subdirectory, which pass the path validation guard and are resolved to files outside the intended corpus root when accessed via frame_by_name(), _lu_file(), or doc() methods.