CVE-2026-80205
HighCVSS 7.5Summary
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.
Risk Assessment
An attacker can cause unavailability of applications using NLTK, leading to service disruptions.
Recommendation
Update NLTK to version 3.10.0 or later and consider adding timeouts and validation for user-supplied regular expressions.
Other vulnerabilities in NLTK
See all- CVE-2026-81723Low
NLTK before 3.10.3 has a quadratic CPU exhaustion vulnerability in XMLCorpusView._read_xml_fragment(). Malformed XML files can cause severe CPU consumption and DoS.
- CVE-2026-80206Medium
NLTK before 3.10.3 has a ReDoS vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regex patterns and executes them without validation, potentially causing catastrophic backtracking and blocking the Python process.
- CVE-2026-79675Critical
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-2026-79657Critical
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-2026-78683Critical
NLTK before 3.10.0 contains an unsafe pickle deserialization vulnerability in TransitionParser.parse(). Deserialization uses default restricted=False, allowing arbitrary Python code execution when loading a crafted model file.
- CVE-2026-81727High
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.
- CVE-2026-81726High
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-2026-81725Low
NLTK before 3.10.3 has a regular expression denial of service vulnerability in Pl196xCorpusReader. Attackers can supply malformed TEI blocks causing quadratic CPU consumption.
- CVE-2026-81724Medium
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-2026-81722High
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.
Original NVD description (English source)
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.