Nodemailer 3.0.0 before 10.0.10 Denial of Service via SMTP Reply Parsing

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.

Nodemailer from 3.0.0 before 10.0.10 contains a quadratic complexity denial of service vulnerability in the SMTP client reply parser in SMTPConnection._onData and isPartialResponse. A malicious SMTP server or on-path attacker can stream continuation lines without a completion line before authentication, blocking the Node.js event loop and exhausting memory.

Source published

14 Oct 2026 22:53:00 UTC

Source modified

15 Oct 2026 13:30:56 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 4.0 · 8.2 CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS 3.1 · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Nodemailer from 3.0.0 before 10.0.10 contains a quadratic complexity denial of service vulnerability in the SMTP client reply parser in SMTPConnection._onData and isPartialResponse. A malicious SMTP server or on-path attacker can stream continuation lines without a completion line before authentication, blocking the Node.js event loop and exhausting memory.

Source published

Oct 14, 2026, 10:53:00 PM (timezone not stated)

Source modified

Oct 15, 2026, 1:30:56 PM (timezone not stated)

Observed here

30 Oct 2026 01:56:08 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 · 8.2 CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Nodemailer from 3.0.0 before 10.0.10 contains a quadratic complexity denial of service vulnerability in the SMTP client reply parser in SMTPConnection._onData and isPartialResponse. A malicious SMTP server or on-path attacker can stream continuation lines without a completion line before authentication, blocking the Node.js event loop and exhausting memory.

Source published

14 Oct 2026 23:17:29 UTC

Source modified

16 Oct 2026 18:14:51 UTC

Observed here

27 Oct 2026 15:46:25 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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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: 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 · 8.2 CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/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

CVSS 3.1 · [email protected] · Secondary · 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

## Summary The SMTP client response parser reassembles multiline server replies by appending each continuation line onto a single response-queue entry and re-testing whether that entry is still a partial (continuation) reply on every line. The partial-response test runs `str.slice(str.lastIndexOf('\n') + 1)` on the growing entry, which forces a full flatten of the entire accumulated string on each line. A malicious or compromised SMTP server (or an on-path attacker on a plaintext or pre-STARTTLS hop) that streams continuation lines and never sends a completion line drives O(n^2) CPU on the single-threaded Node.js event loop before any authentication, freezing the whole process for an attacker-controlled duration. ## Root Cause The invariant that a server cannot cause superlinear client-side work while a reply is being reassembled is violated. In `src/smtp-connection/index.ts`, `_onData` splits inbound bytes into lines and, while the last queued entry is a continuation reply, concatenates each new line onto that same entry with `this._responseQueue[len - 1] += '\n' + lines[i]`. On every iteration it calls `isPartialResponse(lastline)`, which evaluates `/^\d+-/.test(str.slice(str.lastIndexOf('\n') + 1))`. Because the queued entry grows by one line per continuation line and is stored as a V8 rope (ConsString), each `lastIndexOf`/`slice` forces a full string flatten of length proportional to the accumulated reply. Executed once per line over n continuation lines, this is O(n^2) CPU. While the reply stays partial the function early-returns and never resets the entry, so the growth and the per-line reflatten continue unbounded. There is no response-size or line-count cap in the connection code; the only limits are time-based (`greetingTimeout`, and an inactivity `socketTimeout`), and neither bounds accumulated reply size. ```text Affected file and functions (verified at the published v10.0.9 artifact, in src/ and in the built dist/cjs and dist/esm output): - src/smtp-connection/index.ts: isPartialResponse (the /^\d+-/ test over str.slice(str.lastIndexOf('\n')+1)) - src/smtp-connection/index.ts: SMTPConnection._onData (the continuation-line append loop with no size cap) ``` ## Impact An attacker who is, or can impersonate, the SMTP peer the client connects to can freeze the connecting Node.js process. Because Node.js is single-threaded, event-loop starvation halts all other work in that process (every other connection, request, and timer), not just the offending SMTP session. The reply is parsed during the greeting and EHLO phases, before the client authenticates or issues any command, so no credentials on the client are required. The blocking duration scales with the volume of continuation data the attacker streams and can be sustained indefinitely by continuing to stream, and memory for the accumulated entry also grows without bound. Independent reproduction on Node.js 24 measured roughly 11.6 seconds of event-loop blocking for about 640 KB of continuation data and roughly 47 seconds for about 1.3 MB, scaling quadratically. This is a repeatable remote denial of service against the sending process. ## Proof of Concept `Node.js 24; a raw TCP listener acting as a hostile SMTP server; the client uses the shipped SMTPConnection (require('nodemailer/lib/smtp-connection')).` Steps: (1) start a TCP server that accepts a connection and streams continuation lines of the form `220-flood...\r\n` in large batches, never emitting a completion line such as `220 ready`; (2) point an SMTPConnection at it and observe the client event loop stall while the single response-queue entry grows and is reflattened on every line. A minimal microbenchmark of the exact parsing logic demonstrates the complexity: ```js // Reproduces isPartialResponse + the _onData append loop verbatim (Node 24). function isPartialResponse(str) { return /^\d+-/.test(str.slice(str.lastIndexOf('\n') + 1)); } function run(n) { const queue = []; let remainder = ''; function onData(chunk) { const lines = (remainder + chunk).split(/\r?\n/); remainder = lines.pop(); for (let i = 0; i < lines.length; i++) { if (queue.length && isPartialResponse(queue[queue.length - 1])) { queue[queue.length - 1] += '\n' + lines[i]; continue; } queue.push(lines[i]); } } let buf = ''; for (let i = 0; i < n; i++) buf += '220-flood line ' + i + '\r\n'; const t = Date.now(); onData(buf); return Date.now() - t; } for (const n of [8000, 16000, 32000, 64000]) console.log(n, run(n) + 'ms'); ``` ```text 8000 26.5ms 16000 357.6ms 32000 2712.0ms 64000 11645.1ms (2x input yields roughly 4x-7x time: quadratic. ~640 KB of continuation data blocks the event loop ~11.6s.) ``` ## Attack Chain 1. **Exposure:** The client opens an SMTP connection whose peer is attacker-controlled or attacker-influenced: a compromised or malicious configured server, a tenant-supplied outbound server in a multi-tenant "bring your own SMTP" deployment, a deliverability or mailbox-probing tool that connects to arbitrary servers, or an on-path attacker on a plaintext or pre-STARTTLS hop. 2. **Control:** The peer fully controls the bytes of its greeting and EHLO replies, including whether a reply ever completes. 3. **Path:** Inbound bytes reach the socket `data` handler `_onData`, the sole data handler; every reply phase traverses the same continuation-append loop. 4. **Guard:** No response-size or line-count cap exists. The greeting timer is time-based and cleared after the greeting, and its callback is itself starved while `_onData` runs synchronously; the inactivity timer resets on every inbound chunk, so an actively streaming attacker never trips it. 5. **Primitive:** Each continuation line appended to the single growing queue entry triggers a full string flatten in `isPartialResponse`, yielding O(n^2) synchronous CPU on the event loop. 6. **Result:** Whole-process event-loop starvation for an attacker-controlled, sustainable duration, plus unbounded memory growth: a remote denial of service reachable before authentication. ## Bypass Evidence The relevant guard would be a cap on accumulated reply size or line count. A search of the connection module (`maxSize`, `maxLength`, `responseQueue.length >`, byte or line ceilings) finds none; the only bounds are `greetingTimeout` and the inactivity-based `socketTimeout`, both time-based and neither bounding accumulated size. Disproof attempts: the greeting timeout does not help because it is time-based, fires only during the greeting phase, is cleared once the greeting completes, and its callback is delayed while `_onData` runs synchronously; a well-behaved server does not trigger the issue because a completion line resets the entry and keeps cost linear, but the modeled attacker is a hostile peer; segmented delivery does not help the defender because the response queue and line remainder persist across `_onData` calls so the accumulation and quadratic cost survive chunking; there is no safer alternate caller because `_onData` is the only data handler and the append loop is on every reply phase. Note on scope: core nodemailer connects to a configured host and does not perform MX resolution, so this is not reachable through a direct-to-MX transport; the malicious-peer, multi-tenant tenant-supplied-host, deliverability-probe, and MITM paths establish reachability. ## Affected Versions - `Ecosystem: npm` - `Package: nodemailer` - `Confirmed affected range: <= 10.0.9` - `Latest release checked: nodemailer 10.0.9 (npm registry; vulnerable code verified at git tag v10.0.9 and in the published dist output)` - `Fix status: 10.0.10` The quadratic reassembly shape is long-standing and predates the addressparser complexity fix family, so the exact introducing release is not positively established and the lower bound is left open; the upper bound is verified against the current published artifact, which is vulnerable with no fix. ## Suggested Fix Enforce that reassembling a multiline reply is linear in the reply size and that the reply size is bounded. Track the running length of the accumulated entry and the continuation state without re-scanning the whole entry on every line (for example, test the newly arrived line rather than slicing the tail of the growing accumulator, or collect lines in an array and join once at completion). Independently, impose a configurable maximum on the accumulated reply size or continuation-line count and fail the connection when it is exceeded, so a peer that never completes a reply cannot force unbounded work or memory. The remediation must cover every reply phase (greeting, EHLO, and command responses), because all of them flow through the same handler. As an interim mitigation, only connect to trusted SMTP servers over authenticated TLS (avoiding plaintext or STARTTLS-strippable hops), and in multi-tenant deployments isolate outbound SMTP sessions to tenant-supplied hosts so one tenant cannot starve a shared process. Reported by zx (GitHub: @manus-pi).

Source published

27 Oct 2026 15:04:33 UTC

Source modified

27 Oct 2026 15:15:08 UTC

Observed here

27 Oct 2026 15:44:25 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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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: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · MODERATE