CVE-2026-18149: undici vulnerable to Denial of Service via orphaned RetryHandler response body
Impact
undici's RetryHandler can leave a response body pending indefinitely. When a retried request receives a non-retryable response after a truncated one, the original response.body held by the application is never settled, so reads such as response.body.text() hang and bodyTimeout does not fire. A malicious server can repeat this to accumulate pending promises and streams, leading to denial of service.
Patches
Patched in undici v7.29.1 and v8.10.2.
Workarounds
Impose an independent request deadline and destroy the response body when it expires. bodyTimeout alone does not prevent this.
Other sources
undici vulnerable to Denial of Service via orphaned RetryHandler response body
— Microsoft
undici's retry handler can leave an already-exposed response body pending forever. When a server returns a successful response that declares a Content-Length, sends only part of the body, and closes the connection, the retry handler retries the request. If the retry returns a non-retryable status such as 400, the handler forwards that new response downstream and replaces its internal response stream, but the original response body that the application still holds is never ended or destroyed. As a result calls that read that body never settle, and the configured body timeout does not fire because its timer is tied to the connection parser rather than the orphaned body. An attacker-controlled server can trigger this with two short responses without keeping a connection open, and repeated requests accumulate pending promises and streams that can exhaust application concurrency or memory. This affects undici versions from 7.11.0 up to 7.29.1 and from 8.0.0 up to 8.10.2. Users should upgrade to undici 7.29.1 or 8.10.2.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 24.21.0-1 - Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 8.10.2 - Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 7.29.1 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 7.29.1 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 8.10.2
Event History
Frequently Asked Questions
Which applications are realistically exposed?
Applications using affected undici versions that make requests to an attacker-controlled server are exposed. Repeated malicious response sequences can accumulate unresolved body reads and streams, exhausting application concurrency or memory.
What does an attacker need to send to trigger the issue?
The server must first return a successful response with a Content-Length, send only part of the declared body, and close the connection. When undici retries, the server must return a non-retryable response such as HTTP 400; the attacker can do this using two short responses without holding a connection open.
Will a configured body timeout clean up the stuck response?
No. The configured body timeout is tied to the connection parser rather than the orphaned response body, so reads of the original body can remain pending indefinitely.
What versions should be used to remediate this issue?
Upgrade to undici 7.29.1 or 8.10.2, as specified in the advisory.