CVE-2026-82562

Summary

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




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.

Affected Software

VendorProductVersion RangeStatus
ljharbqs6.14.2 < 6.16.0affected

Weaknesses

  • CWE-770: CWE-770 Allocation of Resources Without Limits or Throttling

Workarounds

Bound request and body size at the transport layer (reverse proxy, API gateway, or body parser limit), which caps the cost of this input completely; or disable comma: true for untrusted input.

References