EUVD-2026-54961
GHSA-2jhm-w3mp-jcwr
Severity by source
AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Network-reachable but AC:H due to requiring both a URL injection vector and reachable 100.64.0.0/10 infrastructure; PR:N as no auth to the NLTK host is needed; confidentiality-only impact with no integrity or availability effect.
Primary rating from Vendor (huntr_ai).
CVSS VectorVendor: huntr_ai
Lifecycle Timeline
1DescriptionCVE.org
A Server-Side Request Forgery (SSRF) vulnerability exists in nltk/nltk versions 3.9.4 and the current develop branch. The nltk.pathsec.validate_network_url() function, intended to prevent SSRF by rejecting internal network addresses, fails to reject IPs in the RFC 6598 shared address space (100.64.0.0/10). This occurs because Python's ipaddress module does not classify such addresses as is_private or is_global, and the current guard only checks is_private and a few explicit categories. An attacker who can influence a URL passed to NLTK's network-loading helpers can exploit this vulnerability to make a strict-mode application send requests to shared-address-space hosts, potentially exposing non-public infrastructure reachable from the application host. The impact is limited to SSRF-style confidentiality exposure, with no code execution claimed.
AnalysisAI
Server-Side Request Forgery in NLTK 3.9.4 and the develop branch enables network-level probing of RFC 6598 shared-address-space hosts (100.64.0.0/10) by exploiting an incomplete block-list in the validate_network_url() guard function. The function correctly rejects private and loopback ranges but omits the shared address space defined by RFC 6598, which Python's ipaddress module does not classify as either is_private or is_global, allowing crafted URLs to pass the filter unchallenged. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires three concurrent conditions: (1) the consuming application must pass attacker-influenced URLs to NLTK's network-loading helpers, meaning a separate URL injection vulnerability must exist in the application layer; (2) NLTK's strict-mode URL validation via `validate_network_url()` must be active in that code path; and (3) the application host must have network reachability to hosts in the RFC 6598 Shared Address Space (100.64.0.0/10), which occurs in carrier-grade NAT environments, certain cloud VPC-peering topologies, and ISP infrastructure but is absent in many standard server deployments. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.7 base score (AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N) accurately characterizes this as a constrained finding: high attack complexity reflects the need for the attacker to both control a URL passed into NLTK's helpers and rely on reachable 100.64.0.0/10 infrastructure, two co-occurring prerequisites that significantly reduce opportunistic exploitability. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | The primary fix is to update the `validate_network_url()` function to explicitly reject the RFC 6598 Shared Address Space (100.64.0.0/10) - for example by adding a check for `ipaddress.ip_address(host).is_private or ipaddress.ip_network(host).subnet_of(ipaddress.ip_network('100.64.0.0/10'))` - as detailed in the huntr.com disclosure at https://huntr.com/bounties/2911b643-571c-42e4-b0c2-9a1fa6f491db. … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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