crypto/x509: Unchecked Input for Loop Condition
CVE-2026-27145 is a high-severity unchecked input for loop condition vulnerability in crypto/x509 from Go standard library, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 1.25.11, 1.26.4 as the first without the flaw, so updating crypto/x509 to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.59% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update crypto/x509 to 1.25.11 or 1.26.4, whichever matches your release line, or a later version.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE; 35 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by CNA.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 6.5 | Medium | CISA (ADP) |
| CVSS 3.1 | 7.5 | High | CNA |
Official description
(*x509.Certificate).VerifyHostname previously called matchHostnames in a loop over all DNS Subject Alternative Name (SAN) entries. This caused strings.Split(host, ".") to execute repeatedly on the same input hostname. With a large DNS SAN list, verification costs scaled quadratically based on the number of SAN entries multiplied by the hostname's label count. Because x509.Verify validates hostnames before building the certificate chain, this overhead occurred even for untrusted certificates.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Unchecked Input for Loop Condition
The product does not properly check inputs that are used for loop conditions, potentially leading to a denial of service or other consequences because of excessive looping.
How developers prevent this class of flaw
- Do not use user-controlled data for loop conditions.
More Unchecked Input for Loop Condition vulnerabilities (CWE-606)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Other references
- go.dev/cl/783621
- go.dev/issue/79694
- groups.google.com/g/golang-announce/c/tKs3rmcBcKw
- pkg.go.dev/vuln/GO-2026-5037
- access.redhat.com/errata/RHSA-2026:23262
- access.redhat.com/errata/RHSA-2026:23264
- access.redhat.com/errata/RHSA-2026:29980
- access.redhat.com/errata/RHSA-2026:29981
- access.redhat.com/errata/RHSA-2026:33574
- access.redhat.com/errata/RHSA-2026:34357
- access.redhat.com/errata/RHSA-2026:34359
- access.redhat.com/errata/RHSA-2026:35832
- access.redhat.com/errata/RHSA-2026:36317
- access.redhat.com/errata/RHSA-2026:36648
- access.redhat.com/errata/RHSA-2026:36797
Other vulnerabilities in crypto/x509
- 8.2 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
Recent Unchecked Input for Loop Condition vulnerabilities
- 7.5 High severity
- 7.7 High severity
- 7.5 High severity
- 7.2 High severity
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