GHSA-rrv8-h7p8-rx55: High severity pip/nltk vulnerability
Summary NLTK's Text.findall() and TokenSearcher.findall() methods accept user-supplied regular expressions and pass them to the Python re engine without timeout or validation, enabling catastrophic backtracking (ReDoS). This issue is isolated to the nltk.text module and was resolved in a prior commit.
Affected Code nltk/text.py — TokenSearcher.findall() (line 255) / Text.findall() (line 620)
TokenSearcher.init builds an internal string by wrapping each token in angle brackets. The findall() method preprocesses the caller-supplied regexp and runs it directly against this string with no timeout:
python def findall(self, regexp): # Preprocessing does NOT prevent catastrophic backtracking regexp = re.sub(r"\s", "", regexp) regexp = re.sub(r"<", "(?:<(?:", regexp) regexp = re.sub(r">", ")>)", regexp) regexp = re.sub(r"(?<!\\)\.", "[^>]", regexp)
# User-controlled regexp executed with no timeout hits = re.findall(regexp, self.raw) The preprocessing transforms < and > angle-bracket syntax but does not inspect or reject catastrophically backtracking patterns.
Proof of Concept python import nltk import time
Token of 25 'a' characters produces self.raw = "<aaaaaaaaaaaaaaaaaaaaaaaa!>" The trailing '!' ensures no match, forcing full backtracking. text = nltk.Text(["a" 25 + "!"])
Pattern after transformation: < → (?:<(?: → )>) Becomes: (?:<(?:((a+)+)b)>) re.findall runs this against "<aaaaaaaaaaaaaaaaaaaaaaaa!>" — hangs.
start = time.time() text.findall(r"<((a+)+)b>") # Never returns
Impact Applications that expose Text.findall() to external input are vulnerable to a denial of service. An unauthenticated attacker can cause indefinite CPU saturation with one request, denying service to all other users of the Python process.
Remediation This vulnerability was patched in commit d8e4753. Users should update to the patched version.
Credit Tool: Kira by Offgrid Security
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/nltkto a version that resolves this vulnerability.Fixed in 3.10.0 - Upgrade
Upgrade
nltk.textto a version that resolves this vulnerability.Patch d8e4753
Event History
Frequently Asked Questions
Which deployments are realistically exposed?
Applications are exposed when they pass untrusted regular-expression input to nltk.text.Text.findall() or nltk.text.TokenSearcher.findall(). The issue is isolated to the nltk.text module; other NLTK modules are not identified in the provided data.
What does an attacker need to exploit this?
An attacker needs a way to supply a crafted regular expression that reaches one of the affected findall() methods. The supplied CVSS vector indicates no privileges or user interaction are required, but network reachability depends on how the application exposes regex input.
Does the methods' regex preprocessing prevent exploitation?
No. The preprocessing rewrites whitespace, angle brackets, and dots, but it does not validate patterns, impose a timeout, or reject catastrophic-backtracking expressions.
How can I determine whether my application is at risk?
Review call sites for Text.findall() and TokenSearcher.findall(), then determine whether their regexp argument can be influenced by an untrusted user. The provided data does not identify affected or fixed version numbers, so version-only assessment is not possible from this information.