ip-address: isInSubnet() and isHostInSubnet() compare addresses of different families as if they shared an address space, allowing an allowlist check to admit an address outside its range

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

ip-address is a library for parsing and manipulating IPv4 and IPv6 addresses in JavaScript. Prior to 10.7.1, the isInSubnet and isHostInSubnet methods in src/common.ts compare masked binary strings without validating that both operands use the same IP family. A cross-family containment check whose leading address bits match makes the masked strings compare equal even though IPv4 and IPv6 do not share an address space. An allowlist or denylist decision can therefore classify an address outside the intended range as contained. This issue is fixed in version 10.7.1.

Source published

28 Sep 2026 17:48:06 UTC

Source modified

28 Sep 2026 19:13:25 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · 6.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

ip-address is a library for parsing and manipulating IPv4 and IPv6 addresses in JavaScript. Prior to 10.7.1, the isInSubnet and isHostInSubnet methods in src/common.ts compare masked binary strings without validating that both operands use the same IP family. A cross-family containment check whose leading address bits match makes the masked strings compare equal even though IPv4 and IPv6 do not share an address space. An allowlist or denylist decision can therefore classify an address outside the intended range as contained. This issue is fixed in version 10.7.1.

Source published

Sep 28, 2026, 5:48:06 PM (timezone not stated)

Source modified

Sep 28, 2026, 7:13:25 PM (timezone not stated)

Observed here

11 Nov 2026 05:28:24 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 4.0 · 6.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

ip-address is a library for parsing and manipulating IPv4 and IPv6 addresses in JavaScript. Prior to 10.7.1, the isInSubnet and isHostInSubnet methods in src/common.ts compare masked binary strings without validating that both operands use the same IP family. A cross-family containment check whose leading address bits match makes the masked strings compare equal even though IPv4 and IPv6 do not share an address space. An allowlist or denylist decision can therefore classify an address outside the intended range as contained. This issue is fixed in version 10.7.1.

Source published

28 Sep 2026 18:17:21 UTC

Source modified

30 Sep 2026 19:57:08 UTC

Observed here

25 Oct 2026 04:17:26 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 6.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

### Summary `isInSubnet()` and `isHostInSubnet()` accept an address of either family and compare masked binary strings without checking that both operands are the same family. `Address4` pads to 32 bits and `Address6` to 128, so whenever the leading bits agree the strings are equal: `new Address6('a00::1').isInSubnet(new Address4('10.0.0.0/8'))` is `true`, and `new Address4('32.0.0.1').isInSubnet(new Address6('2000::/3'))` is `true`. No IPv4 address is inside an IPv6 network, so both answers are untrue. An application that parses untrusted input as whichever family accepts it and then tests the result against a fixed-family allowlist can admit an address outside the list. ### Details Both methods are in `src/common.ts`: ```ts export function isInSubnet(this: Address4 | Address6, address: Address4 | Address6) { if (this.subnetMask < address.subnetMask) { return false; } return isHostInSubnet.call(this, address); } export function isHostInSubnet(this: Address4 | Address6, address: Address4 | Address6) { return this.mask(address.subnetMask) === address.mask(); } ``` `mask(n)` returns the first `n` bits of the address as a string of `0` and `1`, taken from a representation padded to the family's width. `new Address6('a00::1').mask(8)` is `'00001010'`, and so is `new Address4('10.0.0.0/8').mask()`, so string equality reports containment. The signature admits either family on either side, so TypeScript raises nothing, and the `v4` property on `Address6` marks IPv4 notation (`::ffff:10.0.0.1`) rather than family, so it does not discriminate either. Which cross-family pairs coincide depends on the network's prefix length. `mask(n)` on an `Address4` returns at most 32 bits, so an IPv6 network longer than `/32` never matches an IPv4 address, and an IPv6 network of `/32` or shorter matches exactly the IPv4 addresses whose leading bits equal its prefix. An IPv4 network is at most 32 bits and matches every IPv6 address whose leading bits equal its prefix. ### Affected versions `<= 10.7.0`. The comparison has had this shape since the methods were written, so every release is affected. ### Impact | Expression | Result | Reason | |---|---|---| | `new Address6('a00::1').isInSubnet(new Address4('10.0.0.0/8'))` | `true` | both mask to `00001010` | | `new Address4('10.0.0.1').isInSubnet(new Address6('a00::/8'))` | `true` | the same bits, reversed | | `new Address4('32.0.0.1').isInSubnet(new Address6('2000::/3'))` | `true` | both begin `001` | | `new Address4('32.1.13.184').isInSubnet(new Address6('2001:db8::/32'))` | `true` | the `/32` spells an IPv4 address | | `new Address6('cb00:7100::1').isInSubnet(new Address4('203.0.113.0/24'))` | `true` | the `/24` spells an IPv6 prefix | | `new Address4('10.0.0.1').isInSubnet(new Address6('::ffff:10.0.0.0/104'))` | `false` | `/104` is longer than 32 bits | | `new Address4('8.8.8.8').isInSubnet(new Address4('10.0.0.0/8'))` | `false` | same-family control | In the allowlist direction the check admits an address outside the list; in the denylist direction it blocks an address outside the list. What the coincidence can admit is narrow. An IPv6 allowlist of `/32` or shorter admits the IPv4 addresses its prefix spells: `2001:db8::/32` admits exactly `32.1.13.184`, and `2000::/3` admits `32.0.0.0/3`. Those are public IPv4 addresses; the private, loopback, and link-local ranges begin with bit patterns no allocated IPv6 prefix shares. An IPv4 allowlist admits the IPv6 addresses its prefix spells, and those all sit in blocks IANA has not allocated. So a request admitted through this defect reaches an address outside the intended list, not an internal host, and the severity reflects that. ### Proof of concept `npm i [email protected]`, then: ```js const { Address4, Address6 } = require('ip-address'); // An allowlist of the application's own IPv6 range, tested against whichever // family the input parses as. const allowed = new Address6('2001:db8::/32'); function parse(host) { return Address4.isValid(host) ? new Address4(host) : new Address6(host); } for (const h of ['2001:db8::1', '2001:db9::1', '32.1.13.184', '32.1.13.185']) { console.log(parse(h).isInSubnet(allowed) ? 'ALLOW' : 'BLOCK', h); } ``` On affected versions: ``` ALLOW 2001:db8::1 BLOCK 2001:db9::1 ALLOW 32.1.13.184 BLOCK 32.1.13.185 ``` The IPv6 rows are right. The IPv4 address whose 32 bits equal the allowlist's prefix is admitted; the one next to it is not. ### Remediation Upgrade to the patched release. In the fix, `isHostInSubnet()` returns `false` when the two addresses are of different families, and `isInSubnet()` inherits the answer. The methods keep accepting either family so existing call sites compile. A caller that means to compare across families converts first, with `Address6.fromAddress4()`, `to4()`, or `toAddress4Nat64()`: `new Address6('::ffff:10.0.0.1').to4().isInSubnet(new Address4('10.0.0.0/8'))` is `true`, as before. If you cannot upgrade immediately, compare the classes before you compare the addresses: ```js const contained = host.constructor === network.constructor && host.isInSubnet(network); ``` ### A note on SSRF defense These methods are address classifiers, not a complete SSRF defense. Regardless of this fix, a robust SSRF guard must resolve the hostname and validate the *resolved* IP against the socket it connects to, and account for DNS rebinding and redirects. Treat these checks as one layer, not the only one.

Source published

29 Sep 2026 23:46:52 UTC

Source modified

30 Sep 2026 00:00:03 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

Publisher rating · MODERATE