CVE-2026-77972

CVE-2026-77972 is a critical-severity vulnerability with a CVSS 4.0 base score of 9.0. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-367.

Key facts

Description

Time-of-check Time-of-use (TOCTOU) Race Condition in Slab safeurl allows an attacker who controls a hostname's DNS responses to reach internal network destinations that validation rejected. Validation returns a verdict and not the address it approved, so the HTTP clients the library ships receive the original hostname and resolve it a second time when the request is made. An attacker who controls the authoritative DNS for a name can answer the first lookup with a permitted address and the second with a blocked one, and the request then reaches a destination validation never approved. The same window opens without an attacker whenever a name legitimately resolves to different addresses across lookups, such as short record lifetimes or rotation between several addresses. This issue affects safeurl: from 0.1.0 onward.

Frequently asked questions

What is CVE-2026-77972?
Time-of-check Time-of-use (TOCTOU) Race Condition in Slab safeurl allows an attacker who controls a hostname's DNS responses to reach internal network destinations that validation rejected. Validation returns a verdict and not the address it approved, so the HTTP clients the library ships receive the original hostname and resolve it a second time when the request is made. An attacker who controls the authoritative DNS for a name can answer the first lookup with a permitted address and the second with a blocked one, and the request then reaches a destination validation never approved. The same window opens without an attacker whenever a name legitimately resolves to different addresses across lookups, such as short record lifetimes or rotation between several addresses. This issue affects safeurl: from 0.1.0 onward.
How severe is CVE-2026-77972?
CVE-2026-77972 has a CVSS 4.0 base score of 9.0, rated critical severity.
Is CVE-2026-77972 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (24th percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-77972?
Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2026-77972 published?
CVE-2026-77972 was published on 2026-09-15 and last updated on 2026-09-21.

References

Other CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition) vulnerabilities

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