VulnerabilityPublished 3 Sep 2026Updated 29 Oct 2026

CVE-2026-81723: NLTK · Quadratic CPU Exhaustion in `XMLCorpusView._read_xml_fragment()`

Affects acronymSource GCVE Database

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Not known to be exploited · CVSS 3.7 Low : upgrade to 2.0.0_5 when convenient.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 3.1 base score

3.7Low

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 2.0.0_5

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 3 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade acronym to 2.0.0_5.

Affected software

Product	Affected	Fixed in
Homebrewacronym`pkg:brew/acronym`Other threats	Affected >=2.0.0 <2.0.0_5	Fixed in 2.0.0_5

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## 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: ```python 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, `_VALID_XML_RE.match(fragment)` does not succeed because the unterminated tag prevents the expression from matching before EOF. - `fragment.rfind("<")` returns `0`; the guard `last_open_bracket > 0` is `False`, so the backtrack branch is never taken. - The only exit is EOF, after all N bytes are consumed. **Affected readers** -> readers that rely on `XMLCorpusView`, including `BNCCorpusReader`, `NPSChatCorpusReader`, `SemcorCorpusReader`, `MTECorpusReader`, `NKJPCorpusReader`, `FrameNetCorpusReader`, `VerbNetCorpusReader`, and direct `XMLCorpusView` instantiation. `XMLCorpusReader.xml()` is not affected -> it calls `defusedxml.safe_parse()`. ## PoC Requires only `pip install nltk`. No corpus data needed. ```python from pathlib import Path from tempfile import TemporaryDirectory from time import perf_counter from nltk.corpus.reader.bnc import BNCCorpusReader SIZES_KIB = (256, 512, 1024, 2048, 4096, 8192) results = [] with TemporaryDirectory() as directory: root = Path(directory) malformed = root / "unterminated.xml" for kib in SIZES_KIB: malformed.write_bytes(b"<" + b"a" * (kib * 1024 - 1)) t = perf_counter() try: list(BNCCorpusReader(str(root), [malformed.name]).words()) except ValueError as e: assert "tag not closed" in str(e) results.append(perf_counter() - t) print("KiB seconds growth") for i, (kib, elapsed) in enumerate(zip(SIZES_KIB, results)): ratio = "-" if i == 0 else f"{elapsed / results[i-1]:.2f}x" print(f"{kib:5d} {elapsed:9.3f} {ratio}") ``` Runtime should increase by approximately fourfold for each doubling of input size, although absolute timings vary by hardware. During verification, `_VALID_XML_RE.match()` was instrumented to record the size of each input. For a 256 KiB malformed file it was invoked 257 times on monotonically increasing buffers (1024, 2048, …, 262144 bytes), with the final call occurring after EOF. This confirms that every iteration rescans the accumulated fragment. ## Impact Applications that process attacker-controlled XML corpus files through an affected reader are vulnerable. The attacker needs only write access to a path the reader will open. No NLTK credentials or special privileges required. Offline tools reading only trusted local corpora are not at risk. **Affected versions:** Verified in NLTK 3.9.4, 3.10.0, and the current develop branch. Historical inspection indicates the same loop structure has existed since the introduction of `XMLCorpusView` (2007), but only the listed versions were experimentally verified. This issue results in CPU exhaustion and may allow denial of service in applications that process attacker-controlled XML corpus files. ## Suggested Fix Avoid rescanning the accumulated fragment from the beginning after each 1 KiB read. Incremental parsing, bounded fragment accumulation, or another streaming approach would eliminate the quadratic behavior while preserving existing semantics. A regression test should verify that `BNCCorpusReader.words()` raises `ValueError` within a fixed timeout (e.g. 5 seconds) against a 2 MiB malformed input. The existing `test_xmldocs_security.py` covers only the prior ReDoS payloads and does not exercise this path.

References

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