vulnerability / NVI-00010559 / Revision 4
Nodemailer: Quadratic (O(n^2)) DKIM header unfolding lets a large folded header cause remote denial of service during signing
GitHub lists this reviewed advisory for nodemailer. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
6.5 medium
GitHub source severity; exploitability and local consequence require separate evidence.
As of · GitHub Advisory Database
6.5 MEDIUM (3.1)
disclosure@vulncheck.com severity; no combined Null risk score.
7.1 HIGH (4.0)
disclosure@vulncheck.com severity; no combined Null risk score.
0.381%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.32081. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
nodemailer
source listed>= 3.0.0, <= 10.0.9
## Summary When nodemailer DKIM-signs an outbound message, its DKIM message parser unfolds RFC 5322 header continuation lines with a loop that concatenates each continuation line onto the preceding line and then re-tests the anchored regex `/^[ \t]/` against the growing merged line on every iteration. Re-testing the regex against the ever-growing accumulated line forces a full string flatten proportional to the accumulated length on each iteration, yielding O(n^2) CPU in the number of continuation lines. An application user who influences the size of a header that folds into many continuation lines (most naturally a large recipient list, but also a long custom header, Subject, or References) can drive tens of thousands of continuation lines and block the single-threaded Node.js event loop for seconds during signing, a repeatable denial of service. ## Root Cause The invariant that signing a message performs work linear in the message size is violated in the header-unfolding step. In `src/dkim/message-parser.ts`, `parseHeaders()` splits the buffered header block into `lines` and iterates backward; for each line matching the continuation pattern `/^[ \t]/`, it does `lines[i - 1] +=
nodemailer
source listedSEMVER: introduced 3.0.0; fixed 10.0.10
## Summary When nodemailer DKIM-signs an outbound message, its DKIM message parser unfolds RFC 5322 header continuation lines with a loop that concatenates each continuation line onto the preceding line and then re-tests the anchored regex `/^[ \t]/` against the growing merged line on every iteration. Re-testing the regex against the ever-growing accumulated line forces a full string flatten proportional to the accumulated length on each iteration, yielding O(n^2) CPU in the number of continuation lines. An application user who influences the size of a header that folds into many continuation lines (most naturally a large recipient list, but also a long custom header, Subject, or References) can drive tens of thousands of continuation lines and block the single-threaded Node.js event loop for seconds during signing, a repeatable denial of service. ## Root Cause The invariant that signing a message performs work linear in the message size is violated in the header-unfolding step. In `src/dkim/message-parser.ts`, `parseHeaders()` splits the buffered header block into `lines` and iterates backward; for each line matching the continuation pattern `/^[ \t]/`, it does `lines[i - 1] +=
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
- vendor patch
The original advisory lists fixed boundaries 10.0.10 for nodemailer. Confirm the publisher’s latest guidance.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database via OSV
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
- NVD lastModified does not establish freshness of every associated CPE record.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-39m8-27wv-hr27 ↗
GitHub Advisory Database · vendor advisory
Observed . GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.
- GHSA-39m8-27wv-hr27 ↗
GitHub Advisory Database via OSV · vendor advisory
Observed . GitHub-origin advisory, CC-BY-4.0; OSV is an aggregate copy, not independent corroboration.
- CVE-2026-110871 ↗
NIST National Vulnerability Database · official catalog
Observed . This product uses data from the NVD API but is not endorsed or certified by the NVD.
- CVE-2026-110871 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 4 ·
Source facts changed; compare the preserved prior revision.
- Revision 3 ·
Source facts changed; compare the preserved prior revision.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.