Description

### Summary When `qs.parse` is called with `comma: true` and `throwOnLimitExceeded: true`, a comma-separated value under a bracket-push key (`a[]=1,2,3,4`) is split into an array without being compared against `arrayLimit`, while the same value under a flat key (`a=1,2,3,4`), an indexed key (`a[0]=`), a nested key (`a[b]=`), or a dotted key (`a.b=` with `allowDots`) throws the documented `RangeError`. A single parameter such as `a[]=1,2,2,...` therefore produces an inner array of arbitrary length even though the caller opted into the hard limit. This is the `[]=` key form that the fix for CVE-2026-2391 (qs 6.14.2) did not cover. ### Details In `lib/parse.js`, a comma-separated value under a `[]=` key is split and then wrapped as a single nested element (`val = [val]`, so that each `a[]=x,y` group counts as one element of the outer array). The `arrayLimit` check that 6.14.2 added for comma values runs after that wrap, so for `[]=` parts it only ever saw the wrapper of length 1. 6.15.3 added a pre-split comma count so that an oversized value throws before it is allocated, but gated it on an `isFlatArrayValue` flag that `parseValues` set to `false` for any part containing `[]=`, and did not pass it for object-valued input, so the gap remained. #### PoC ```js var qs = require('qs'); var options = { comma: true, arrayLimit: 3, throwOnLimitExceeded: true }; qs.parse('a=1,2,3,4', options); // RangeError: Array limit exceeded. Only 3 elements allowed in an array. qs.parse('a[]=1,2,3,4', options); // { a: [ [ '1', '2', '3', '4' ] ] } (no throw) qs.parse('a[]=' + '1,'.repeat(1000000) + '1', { comma: true, arrayLimit: 20, throwOnLimitExceeded: true }); // no throw; a 1,000,001-element inner array is allocated ``` #### Fix `lib/parse.js`, applied in 8859c37 on `main` and released as v6.16.0: the `isFlatArrayValue` gate is removed, so every comma-split value is counted against `arrayLimit` before splitting regardless of key form. An in-limit group under `a[]=` still counts as one element of the outer array, and the default (`throwOnLimitExceeded: false`) path is unchanged. ### Affected versions `>=6.14.2 <6.16.0`, fixed in v6.16.0. v6.14.2 introduced `arrayLimit` enforcement for comma values (the fix for CVE-2026-2391) but only for values not under a `[]=` key, and every release from v6.14.2 through v6.15.3 has the same gap. v6.14.0 and v6.14.1, where `throwOnLimitExceeded` exists but does not apply to any comma form, are covered by CVE-2026-2391 rather than this record. Earlier lines (6.7.x through 6.13.x) have `comma` but no `throwOnLimitExceeded`, so there is no hard cap on any comma path to bypass; releases before 6.7.0 have no `comma` option. ### Impact An unauthenticated attacker who can reach an application that parses untrusted query strings or urlencoded bodies with both `comma: true` and `throwOnLimitExceeded: true` (both non-default) can bypass the configured limit with a single `a[]=` parameter and force the parser to allocate an array proportional to the request size. The cost is strictly linear in the attacker-supplied bytes (about 0.1 microseconds and 6 to 7 retained bytes per input byte; the same out-of-memory threshold as the documented default `throwOnLimitExceeded: false` path), so a transport-layer request or body size limit bounds it completely (and node's default maximum HTTP header size of 16 KB already bounds the request line, so multi-megabyte payloads need a body parser). The impact is that an opt-in hard limit fails open on one key spelling, not unbounded allocation from a small input.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "3.7",
  "pubDate": "2026-08-30T01:20:32.820Z",
  "pubdate": "2026-08-30T01:20:32.820Z",
  "executiveSummary": "A security bypass vulnerability exists in the 'qs' library, specifically within the parsing logic for comma-separated values when 'comma' and 'throwOnLimitExceeded' options are enabled. The vulnerability allows an attacker to circumvent the 'arrayLimit' configuration when using bracket-push keys (e.g., 'a[]='), potentially leading to large memory allocations despite an explicit opt-in to a security constraint.\nThis issue affects versions 6.14.2 through 6.15.3 of the 'qs' package. The flaw arises because the parser inconsistently applies limit enforcement based on key syntax, specifically failing to count inner array elements correctly when bracket-push syntax is employed. An unauthenticated attacker can exploit this by crafting malicious query strings that force the application to instantiate arrays exceeding the intended size limits.\nWhile the impact is limited by existing transport-layer size constraints, such as Node.js HTTP header size limits, the failure of an explicit security configuration (the hard limit) constitutes a logic vulnerability. Successful exploitation could be used in Denial-of-Service (DoS) scenarios where memory resource exhaustion is targeted by providing payloads that bypass intended defensive thresholds.",
  "technicalDetails": "The root cause of the vulnerability lies in the improper handling of the 'arrayLimit' check within 'lib/parse.js' when processing comma-separated values associated with bracket-push keys ('[]='). In previous iterations, specifically following the patch for CVE-2026-2391, the library introduced mechanisms to enforce 'arrayLimit' for comma-separated values. However, the logic implemented a conditional check based on an 'isFlatArrayValue' flag.\nWhen a key utilizes the '[]=' syntax, 'parseValues' sets the 'isFlatArrayValue' flag to 'false'. Consequently, the validation logic fails to inspect the content of the comma-split array for compliance with 'arrayLimit'. The code incorrectly wraps the comma-separated sequence into a single nested element ('val = [val]') before the length check occurs, leading the validator to observe a collection of length 1, regardless of the actual number of elements contained within that nested array.\nThe attack flow involves an attacker providing a crafted URL-encoded string to an application utilizing 'qs.parse' with '{ comma: true, throwOnLimitExceeded: true }'. By sending a payload formatted as 'a[]=' followed by a long sequence of comma-delimited values, the attacker bypasses the internal array size enforcement. While the 'qs' library correctly identifies and throws a 'RangeError' for standard keys (like 'a=') or dot-notation keys (like 'a.b='), the '[]=' syntax remains exempt from this validation due to the aforementioned flag logic. This allows for the allocation of arrays with an arbitrary number of elements, bounded only by the underlying HTTP request or body size limits enforced by the web server or transport layer.\nBecause the 'throwOnLimitExceeded' mechanism is specifically intended to protect against resource exhaustion by enforcing strict limits on array construction, the bypass renders this security control ineffective for specific input patterns. The exploitation does not require authentication or elevated privileges, as it targets the public-facing input parsing stage of the application. The result is a failure of the 'qs' library to maintain the memory constraints defined by the developer, effectively allowing an attacker to force higher-than-permitted memory usage during the parsing process."
}