VulnerabilityPublished 28 Aug 2026Updated 29 Oct 2026

CVE-2026-63311: NLTK · SSRF Fail-Open in validate_network_url() via DNS Resolution Failure

Affects acronymSource GCVE Database

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Mentioned vulnerabilities

Not known to be exploited · CVSS 6.9 Medium : upgrade to 2.0.0_4 when convenient.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

6.9Medium

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

Patch: Available – fixed in 2.0.0_4

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

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

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: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the Subsequent System.
Subsequent system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Subsequent System.
Subsequent system 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 Subsequent System 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 Subsequent System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Aug 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_4.

Affected software

Product	Affected	Fixed in
Homebrewacronym`pkg:brew/acronym`Other threats	Affected >=2.0.0 <2.0.0_4	Fixed in 2.0.0_4

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

Description

There is an SSRF vulnerability in NLTK 3.9.4's network URL validation. The validate_network_url() function in nltk/pathsec.py fails open when DNS resolution returns an error. The _resolve_hostname() helper at lines 193-234 catches OSError and ValueError during socket.getaddrinfo() and returns an empty list []. When this happens, the validation loop in validate_network_url() iterates over nothing (for addr in resolved: ... never executes), with no else/fallback check. The function returns normally, and urlopen() proceeds to make the request without any IP validation. This means: 1. If DNS is temporarily unavailable, ALL SSRF protections are disabled 2. DNS rebinding attacks bypass the check after the LRU cache entry expires 3. In environments with unreliable resolvers, the protection is permanently bypassed PoC: ```python import nltk.pathsec import unittest.mock # Simulate DNS failure with unittest.mock.patch('socket.getaddrinfo', side_effect=OSError('DNS unavailable')): # This SHOULD raise but doesn't -- fails open nltk.pathsec.validate_network_url('http://169.254.169.254/latest/meta-data/') # Returns normally, allowing SSRF to cloud metadata ``` The correct behavior is fail-closed: if DNS resolution fails, the URL should be REJECTED (not allowed). The function should raise an exception or return a failure status when _resolve_hostname() returns an empty list. This is distinct from CVE-2024-39705 (which addressed pickle deserialization) and CVE-2026-33236 (which addressed XML path traversal). This finding targets the newly-added pathsec.py security layer introduced to fix those earlier issues. Suggested fix: Add an explicit check after _resolve_hostname() returns: if the result is empty, raise a SecurityError. Never allow a URL request to proceed when IP validation was impossible. CVSS Note: The CVSS use SC:H (High subsequent confidentiality) because the advisory explicitly identifies cloud metadata endpoints (169.254.169.254) as an attack target. Access to AWS IMDS or GCP metadata exposes credentials or service account tokens, which constitutes High-impact disclosure on downstream systems. This justifies SC:H over NVD's SC:L.

References

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