VulnerabilityPublished 27 Oct 2026

CVE-2026-87018: undici vulnerable to Denial of Service via TLS 1.2 certificate-changing renegotiation

Affects undiciSource GCVE Database

Not known to be exploited · CVSS 7.5 High : upgrade to 6.29.1 soon.

Data as of 27 Oct 2026, 20:28 UTC

CVSS 3.1 base score

7.5High

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

Patch: Available – fixed in 6.29.1, 7.30.1, 8.12.2

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 27 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 undici to 6.29.1; undici to 7.30.1; undici to 8.12.2.

Affected software

Product	Affected	Fixed in
npmundici	Affected >=4.2.1 <6.29.1	Fixed in 6.29.1
npmundici	Affected >=7.0.0 <7.30.1	Fixed in 7.30.1
npmundici	Affected >=8.0.0 <8.12.2	Fixed in 8.12.2

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

AI-suggested

Upgrade undici to version 6.29.1, 7.30.1, or 8.12.2 to apply the official fix. Alternatively, restrict outbound connections to TLS 1.3 by configuring the client or agent with `connect: { minVersion: 'TLSv1.3', maxVersion: 'TLSv1.3' }` to prevent renegotiation when the target endpoints support TLS 1.3.

Description

undici is vulnerable to a denial of service condition triggered by a TLS 1.2 server-initiated renegotiation presenting a different, valid but hostname-mismatched certificate. This causes an uncaught assertion error that crashes the Node.js client process, affecting both HTTP/1.1 and HTTP/2. Only TLS 1.2 is impacted, as TLS 1.3 does not support renegotiation. The issue is fixed in undici versions 6.29.1, 7.30.1, and 8.12.2. Mitigation includes upgrading to these versions or restricting connections to TLS 1.3 to prevent renegotiation.

Impact

A malicious or compromised HTTPS server can cause the undici client process to crash by initiating a TLS 1.2 renegotiation with a different, hostname-mismatched certificate. This results in a denial of service, terminating all in-flight requests and background tasks in the Node.js process using undici. There is no confidentiality or integrity impact, but availability is severely affected. Exploitation does not require a compromised CA, only control of the server endpoint.

Summary

The undici HTTP client library crashes the Node.js process when a TLS 1.2 server renegotiates after connection establishment and presents a different CA-trusted certificate with a hostname mismatch. Node.js triggers an ERR_TLS_CERT_ALTNAME_INVALID error on renegotiation, but undici's socket error handler incorrectly assumes this error cannot occur, leading to an uncaught AssertionError that terminates the process. This vulnerability affects both HTTP/1.1 and HTTP/2 protocols. Exploitation requires a malicious or compromised HTTPS endpoint that can present two valid certificates, the second intentionally mismatched. TLS 1.3 is not affected due to lack of renegotiation support. Fixed in undici versions 6.29.1, 7.30.1, and 8.12.2.

CWE-248Uncaught Exception (opens in a new tab)
Base weaknessOWASP Top 10 A10:2025: Mishandling of Exceptional Conditions (opens in a new tab)
An exception is thrown from a function, but it is not caught.
OWASP Cheat Sheets
Logging (opens in a new tab)
Error Handling (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-617Reachable Assertion (opens in a new tab)
Base weakness
The product contains an assert() or similar statement that can be triggered by an attacker, which leads to an application exit or other behavior that is more severe than necessary.
Implementation
Make sensitive open/close operation non reachable by directly user-controlled data (e.g. open/close resources)
Implementation · Input Validation
Perform input validation on user data.
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Oct 2026, 20:28 UTC

WeaknessesCWE-248, CWE-617

Technical details

References

Community reviews

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