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CVE-2026-81723 low CVSS 3.7

Vulnerability in nltk (CVE-2026-81723)

Summary

Vulnerability in nltk (CVE-2026-81723). Risk of unauthorized operations or information disclosure. Exploitable via fragment. Mitigation: upgrade to 3.10.3 or later.

Original description

The description as published by the source.

Summary

XMLCorpusView._read_xml_fragment() reads a corpus file in 1 KiB blocks, appending
each block to a growing fragment string, then calls _VALID_XML_RE.match(fragment)
on the full accumulated buffer every iteration. Because each iteration rescans the
entire accumulated fragment, the total amount of work grows quadratically with input
size.

Commit c9c332284 (CWE-1333) made each match() call linear. The quadratic behavior
is separate: the loop calls match() once per 1 KiB block, each time on a longer
buffer.

On the test system, an 8 MiB malformed XML file consumed approximately 48 CPU-seconds
through the public BNCCorpusReader.words() API with no source modification. Absolute
timings vary by hardware. _read_xml_fragment() imposes no limit on fragment size or
iteration count.

Details

File: nltk/corpus/reader/xmldocs.py
Function: XMLCorpusView._read_xml_fragment(), lines 261–308

The relevant loop:

fragment = ""
while True:
    fragment += stream.read(self._BLOCK_SIZE)      # grows by 1 KiB per iteration
    if self._VALID_XML_RE.match(fragment):         # rescans full buffer each time
        return fragment
    ...
    last_open_bracket = fragment.rfind("<")
    if last_open_bracket > 0:                      # False for single-'<' payload
        if self._VALID_XML_RE.match(fragment[:last_open_bracket]):
            return ...
    # loop continues

For a payload of b'<' + b'a' * (N-1):

  • For this malformed input, `_…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-81723 has been found in nltk.
Attackers can target a specific entry point like fragment over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 3.7/10.
What to do: upgrade nltk to 3.10.3 or later.
If unsure, ask your IT team or search "nltk CVE-2026-81723" on the vendor's site.

CVE-2026-81723 (nltk) — CWE-400 / CVSS v3 3.7
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: fragment / c9c332284 / False / XMLCorpusView
Patched: 3.10.3 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-400) exists in nltk. Attackers reach the vulnerable code path via fragment without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of nltk. Attack surface: fragment / c9c332284 / False / XMLCorpusView.

🔥 Severity

Severity: Low (CVSS 3.7/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 3.10.3.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'nltk' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'nltk' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で nltk を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade nltk to 3.10.3

    ステージング環境で 3.10.3 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

pip nltk
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.3"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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