CVE-2026-12259

MEDIUMPre-NVD 5.35.3—
EchelonGraph scoreHIGH confidence

Score 5.3 from GitHub Security Advisory published 2026-08-03. a secondary CVSS source baseline 5.3; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, secondary
Trending — 3 sources updated this week
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 5.3Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

In nltk version 3.9.4, the nltk.downloader.Downloader._download_package() function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for info.url through a compromised mirror, malicious proxy, or other source-substitution condition, leading to the installation of attacker-controlled package bytes. The vulnerability can result in malicious corpus or model content being trusted by downstream users or applications.

CVSS v3
5.3
EG Score
5.3(high)
EG Risk
24(Track)
EG Risk 24/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity53% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
1%
KEV
Not listed

Published

August 3, 2026

Last Modified

August 3, 2026

References (1)

Vendor Advisories for CVE-2026-12259(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(4 across 4 ecosystems)
Debian:11(1)
PackageVulnerable rangeFixed inDependents
nltk3.10.0-1 ... 3.9.3-1 (12 versions)——
Debian:12(1)
PackageVulnerable rangeFixed inDependents
nltk3.10.0-1 ... 3.9.3-1 (8 versions)——
Debian:13(1)
PackageVulnerable rangeFixed inDependents
nltk3.10.0-1, 3.10.1-1, 3.9.1-2, 3.9.2-1, 3.9.3-1——
Debian:14(1)
PackageVulnerable rangeFixed inDependents
nltk3.10.0-1, 3.10.1-1, 3.9.1-2, 3.9.2-1, 3.9.3-1——

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 14× in last 7d / 45× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-08-21 06:52 UTCEG score recompute
  2. 2026-08-21 06:52 UTCGHSA enrichment
  3. 2026-08-20 22:55 UTCEPSS rescore
  4. 2026-08-20 06:00 UTCEG score recompute
  5. 2026-08-20 06:00 UTCGHSA enrichment
  6. 2026-08-19 17:03 UTCEPSS rescore
  7. 2026-08-19 05:10 UTCGHSA enrichment
  8. 2026-08-18 04:09 UTCEG score recompute
  9. 2026-08-18 04:09 UTCGHSA enrichment
  10. 2026-08-17 03:17 UTCGHSA enrichment
  11. 2026-08-16 02:27 UTCGHSA enrichment
  12. 2026-08-15 01:38 UTCEG score recompute
  13. 2026-08-15 01:38 UTCGHSA enrichment
  14. 2026-08-15 01:30 UTCEPSS rescore
  15. 2026-08-14 00:49 UTCEG score recompute
  16. 2026-08-14 00:49 UTCGHSA enrichment
  17. 2026-08-13 22:00 UTCEPSS rescore
  18. 2026-08-12 23:59 UTCGHSA enrichment
  19. 2026-08-11 23:10 UTCEG score recompute
  20. 2026-08-11 23:10 UTCGHSA enrichment
  21. 2026-08-11 13:42 UTCEPSS rescore
  22. 2026-08-10 23:59 UTCEPSS rescore
  23. 2026-08-10 22:21 UTCGHSA enrichment
  24. 2026-08-09 21:32 UTCGHSA enrichment
  25. 2026-08-08 20:42 UTCEG score recompute
Show 20 more
  1. 2026-08-08 20:42 UTCGHSA enrichment
  2. 2026-08-08 16:37 UTCEPSS rescore
  3. 2026-08-07 19:52 UTCEG score recompute
  4. 2026-08-07 19:52 UTCGHSA enrichment
  5. 2026-08-06 19:03 UTCEG score recompute
  6. 2026-08-06 19:03 UTCGHSA enrichment
  7. 2026-08-06 13:46 UTCEPSS rescore
  8. 2026-08-05 19:17 UTCEPSS rescore
  9. 2026-08-05 18:14 UTCGHSA enrichment
  10. 2026-08-04 17:25 UTCEG score recompute
  11. 2026-08-04 17:25 UTCGHSA enrichment
  12. 2026-08-04 15:09 UTCEPSS rescore
  13. 2026-08-04 10:38 UTCEPSS rescore
  14. 2026-08-03 16:36 UTCEG score recompute
  15. 2026-08-03 16:36 UTCGHSA enrichment
  16. 2026-08-03 15:28 UTCEG score recompute
  17. 2026-08-03 15:28 UTCGHSA enrichment
  18. 2026-08-03 08:25 UTCEG score recompute
  19. 2026-08-03 07:54 UTCEG score recompute
  20. 2026-08-03 07:53 UTCMITRE cvelistV5first tracked

Frequently asked(5)

What is CVE-2026-12259?
CVE-2026-12259 is a medium vulnerability published on August 3, 2026. In nltk version 3.9.4, the nltk.downloader.Downloader.downloadpackage() function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for info.url through a compromised…
When was CVE-2026-12259 disclosed?
CVE-2026-12259 was first published in the National Vulnerability Database on August 3, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-12259 actively exploited?
CVE-2026-12259 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 99.0% of all scored CVEs.
What is the CVSS score of CVE-2026-12259?
CVE-2026-12259 has a CVSS v3 base score of 5.3 (NVD).
How do I remediate CVE-2026-12259?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-12259, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-12259

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