Vyper is a Pythonic Smart Contract Language for the Ethereum Virtual Machine. Arrays can be keyed by a signed integer, while they are defined for unsigned integers only. The typechecker doesn't throw when spotting the usage of an int as an index for an array. The typechecker allows the usage of signed integers to be used as indexes to arrays. The vulnerability is present in different forms in all versions, including 0.3.10. For ints, the 2's complement representation is used. Because the array was declared very large, the bounds checking will pass Negative values will simply be represented as very large numbers. As of time of publication, a fixed version does not exist.
There are three potential vulnerability classes: unpredictable behavior, accessing inaccessible elements and denial of service. Class 1: If it is possible to index an array with a negative integer without reverting, this is most likely not anticipated by the developer and such accesses can cause unpredictable behavior for the contract. Class 2: If a contract has an invariant in the form assert index < x, the developer will suppose that no elements on indexes y | y >= x are accessible. However, by using negative indexes, this can be bypassed. Class 3: If the index is dependent on the state of the contract, this poses a risk of denial of service. If the state of the contract can be manipulated in such way that the index will be forced to be negative, the array access can always revert (because most likely the array won't be declared extremely large). However, all these the scenarios are highly unlikely. Most likely behavior is a revert on the bounds check.
What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)
The EG score (9.8): severity, escalated by confirmed exploitation. This CVE's 9.8 is its impact severity (EG Severity), not escalated.
- EG Severity
- 9.8 CRITICALCVSS-B, impact only: no KEV listing, EPSS or EG-KEV tier raises it.GitHub Security Advisory CVSS
- EG Likelihood
- 1.5% within 30 daysSource: FIRST EPSS (model v2026.06.15, score date 2026-11-12). 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, Track* at medium and Attend at high mission impact.CISA BOD 26-04: 14 days if the affected system is publicly exposed; Fix on system upgrade 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, Track* at medium and Attend at high mission impact.
A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
Exploitation public PoC (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact total (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; Fix on system upgrade if it is not.
The score's inputs: In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA Vulnrichment) · Technical impact total (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