CVE-2026-55091

HIGHPre-NVD 7.57.5
EchelonGraph scoreMEDIUM confidence

“—” beside the score: it is equal to the CVSS score beside it.

Score 7.5 from GitHub Security Advisory (severity: HIGH) published 2026-06-19. A secondary CVSS source baseline 7.5; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: EPSS, GitHub Security Advisory, a secondary CVSS source
Trending — 3 sources updated this week

Also known asEUVD-2026-77691 (ENISA EUVD) · GHSA-hp36-v28f-w3r4 (GitHub Security Advisories)Source: ENISA EUVD

7.5EG
EchelonGraph verdictPlan a fixRemediate within your standard update timelines at low or medium mission impact and sooner than that at high (the SSVC decision on this page).
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0.5%CVSS: 7.5Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

flat-to-nested converts a hierarchy from a flat representation to a nested representation. Prior to 1.1.2, FlatToNested.prototype.convert in index.js uses attacker-influenced id and parent record fields directly as keys in the plain temp and pendingChildOf objects. When parent or id is __proto__, temp[parent] can resolve to Object.prototype, and initPush() can write attacker-controlled data to the global children prototype property while existing prototype methods remain intact. Any application that passes attacker-influenced flat records to convert() can therefore expose unrelated objects to polluted inherited state, causing application-logic corruption or denial of service and potentially enabling greater impact when a downstream prototype-pollution gadget is present. The constructor and prototype strings are also unsafe inherited-key values in the same lookup design. This issue is fixed in version 1.1.2.

CVSS v3
7.5
EG Score
7.5HIGHmedium confidence
EG Risk
49
EG Risk 49/100

EG Risk is EchelonGraph's 0–100 priority score. It fuses impact severity (EG Severity, the CVSS base score, never the KEV-floored or EPSS-lifted EG Score) with real-world exploitation and automatability, under weights EchelonGraph set by hand, so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner.

How it’s computed (value × EchelonGraph’s weight)
Severity75% × 45%
Impact severity (EG Severity): CVSS 7.5 of 10. Never the KEV-floored or EPSS-lifted EG Score.
Exploitation0.5% × 40%
FIRST EPSS: the 30-day exploitation probability when EchelonGraph scored this CVE.
Automatability100% × 15%
CISA's SSVC Automatable value: yes (Vulnrichment).
SSVC (CISA's decision table, applied by EchelonGraph): Track at low or medium mission impact; Attend at high (mission-essential systems).
Exploitation none (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE.
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
0.5%
EPSS %ILE
42nd
KEV
Not listed

What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)

The EG score (7.5): severity, escalated by confirmed exploitation. This CVE's 7.5 is its impact severity (EG Severity), not escalated.

EG Severity
7.5 HIGHCVSS-B, impact only: no KEV listing, EPSS or EG-KEV tier raises it.GitHub Security Advisory CVSS
EG Likelihood
0.5% within 30 daysSource: FIRST EPSS (model v2026.06.15, score date 2026-10-31). FIRST's exploitation probability, not an EchelonGraph model.
Confirmed exploitation
None on record (CISA KEV, EG-KEV Tier 1)Reported apart from the likelihood: an observed fact, not a probability.
EG Priority
SSVC Track at medium mission impactTrack at low or medium mission impact; Attend at high (mission-essential systems).CISA BOD 26-04: 14 days if the affected system is publicly exposed; 60 days if it is not.Each input and its source, and whom BOD 26-04 binds: the SSVC and BOD 26-04 blocks below.

SSVC (CISA's decision table, applied by EchelonGraph)Track at low or medium mission impact; Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation none (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table

BOD 26-04 (CISA's remediation timeline, Table 1)14 days if the affected system is publicly exposed; 60 days if it is not.

The score's inputs: In CISA KEV no (CISA's KEV catalog) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it

Published

June 19, 2026

Last Modified

September 30, 2026

Advisory Details (2)

Auto-updated Sep 14, 2026
Patch available. Sources: github, github_commit.
GitHub commit

commit 680a5ebe1194 (joaonuno/flat-to-nested-js)

Fix landed in joaonuno/flat-to-nested-js commit 680a5ebe1194 — awaiting tagged release

https://github.com/joaonuno/flat-to-nested-js/commit/680a5ebe1194edda16fa93baaa56ff14fe0e3d7f
GitHub advisoryMentions a patch (unverified)

Prototype pollution in flat-to-nested convert() via __proto__ parent/id key · Advisory · joaonuno/flat-to-nested-js · GitHub

https://github.com/joaonuno/flat-to-nested-js/security/advisories/GHSA-hp36-v28f-w3r4

Vendor Advisories for CVE-2026-55091(1)

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

Patch Availability(1)

Vendor / EcosystemFix (by version range) / PatchReleasedSource
npmflat-to-nested @ 1.1.2 (fixes <= 1.1.1)—ghsa

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Affected Packages

(1 across 1 ecosystem)
npm(1)
PackageVulnerable rangeFix by version rangeDependents
flat-to-nested—
  • every version up to 1.1.2: fixed in 1.1.2
—

Weakness Classification(2)

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

Data Freshness Timeline

(refreshed 10× in last 7d / 41× in last 30d)

Each row is a time one of our pipelines fetched, updated or re-scored this CVE. 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. An arrow marks a change the record states ("CVSS v3 → 9.1"); EPSS and GHSA rows show the values recorded at that time, not what changed (EPSS values are rounded; hover one for the exact figures). Most recent first.

  1. 2026-11-02 16:45 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  2. 2026-11-01 16:34 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  3. 2026-11-01 07:44 UTCGHSA enrichmentGHSA-hp36-v28f-w3r4 · GitHub severity HIGH · GitHub CVSS 7.5
  4. 2026-11-01 07:44 UTCEG score recompute
  5. 2026-10-31 16:33 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  6. 2026-10-30 16:48 UTCEG score recompute
  7. 2026-10-30 16:34 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  8. 2026-10-29 16:33 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  9. 2026-10-28 16:32 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  10. 2026-10-27 16:32 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  11. 2026-10-26 16:31 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  12. 2026-10-25 16:33 UTCEPSS rescoreEPSS 0.5% · 42nd percentile
  13. 2026-10-25 14:19 UTCEG score recompute
  14. 2026-10-24 16:33 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  15. 2026-10-23 22:13 UTCEG score recompute
  16. 2026-10-23 16:32 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  17. 2026-10-22 16:34 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  18. 2026-10-21 16:33 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  19. 2026-10-20 16:33 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  20. 2026-10-19 16:32 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  21. 2026-10-18 16:32 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  22. 2026-10-18 03:30 UTCEG score recompute
  23. 2026-10-17 16:34 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  24. 2026-10-17 03:34 UTCEG score recompute
  25. 2026-10-16 16:42 UTCEPSS rescoreEPSS 0.5% · 41st percentile
Show 39 more
  1. 2026-10-15 16:32 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  2. 2026-10-14 16:33 UTCEPSS rescoreEPSS 0.5% · 41st percentile
  3. 2026-10-13 16:34 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  4. 2026-10-12 16:34 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  5. 2026-10-12 15:56 UTCEG score recompute
  6. 2026-10-11 19:39 UTCEG score recompute
  7. 2026-10-11 16:54 UTCEG score recompute
  8. 2026-10-11 14:17 UTCEG score recompute
  9. 2026-10-11 13:49 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  10. 2026-10-10 20:00 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  11. 2026-10-09 20:07 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  12. 2026-10-08 17:12 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  13. 2026-10-07 23:48 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  14. 2026-10-06 15:18 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  15. 2026-10-05 17:43 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  16. 2026-10-04 23:22 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  17. 2026-10-03 14:26 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  18. 2026-10-02 17:56 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  19. 2026-10-01 19:50 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  20. 2026-09-30 18:12 UTCEG score recompute
  21. 2026-09-30 15:03 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  22. 2026-09-29 15:22 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  23. 2026-09-28 13:51 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  24. 2026-09-27 13:48 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  25. 2026-09-26 15:59 UTCEPSS rescoreEPSS 0.5% · 40th percentile
  26. 2026-09-24 14:04 UTCEPSS rescoreEPSS 0.5% · 39th percentile
  27. 2026-09-23 17:54 UTCEPSS rescoreEPSS 0.4% · 29th percentile
  28. 2026-09-20 20:16 UTCEPSS rescoreEPSS 0.4% · 29th percentile
  29. 2026-09-18 19:28 UTCEPSS rescoreEPSS 0.4% · 29th percentile
  30. 2026-09-17 19:31 UTCEPSS rescoreEPSS 0.4% · 29th percentile
  31. 2026-09-16 14:08 UTCEPSS rescoreEPSS 0.4% · 29th percentile
  32. 2026-09-16 05:15 UTCEPSS rescoreEPSS 0.4% · 28th percentile
  33. 2026-09-14 20:27 UTCEG score recompute
  34. 2026-09-14 20:16 UTCEG score recompute
  35. 2026-09-14 18:38 UTCEG score recompute
  36. 2026-07-30 22:18 UTCEG score recompute
  37. 2026-07-26 16:47 UTCEG score recompute
  38. 2026-07-23 03:20 UTCEG score recompute
  39. 2026-06-19 21:24 UTCEG score recompute

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-55091?
CVE-2026-55091 is a high-severity vulnerability; its CVE record was published on June 19, 2026. flat-to-nested converts a hierarchy from a flat representation to a nested representation. Prior to 1.1.2, FlatToNested.prototype.convert in index.js uses attacker-influenced id and parent record fields directly as keys in the plain temp and pendingChildOf objects. When parent or id is proto,…
When was the CVE record for CVE-2026-55091 published?
The CVE record for CVE-2026-55091 was published on June 19, 2026. That is the record's publication date; the vulnerability itself may have been made public earlier. EchelonGraph's copy of the record carries a last-modified date of September 30, 2026; the record at its source may have been updated since.
Is CVE-2026-55091 actively exploited?
CVE-2026-55091 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.5% probability of exploitation in the next 30 days (42nd percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-55091?
CVE-2026-55091 has a CVSS base score of 7.5 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-55091?
A fix for CVE-2026-55091 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2026-55091 are linked in the Vendor Advisories panel on this page.

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