# CVE-2026-77972: Slab safeurl TOCTOU race condition in DNS validation

Source: https://junglewise.ai/threats/cve-2026-77972-slab-safeurl-toctou-race-condition-in-dns-validation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-15. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-77972 · Severity: info · Published 2026-09-15

Slab safeurl is a library that validates whether HTTP requests are allowed to reach specific network destinations. A time-of-check time-of-use (TOCTOU) race condition allows an attacker who controls a domain's DNS responses to bypass validation and reach internal network addresses that should be blocked. An attacker can respond with a permitted address during validation but a blocked address during the actual request, causing the library to permit access to prohibited destinations.

## Quick answers

**Is CVE-2026-77972 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-77972 being exploited in the wild.

**How severe is CVE-2026-77972?** Informational severity, published 15 September 2026.

**Which products and versions are affected?** Slab safeurl 0.1.0 and later.

**Is there a fix for CVE-2026-77972?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-77972 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-77972?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is a TOCTOU race condition in DNS hostname resolution and validation. The library validates a hostname and returns only a verdict (allowed/denied), not the resolved IP address; HTTP clients then resolve the hostname again at request time. An attacker controlling authoritative DNS for a domain can answer the first lookup with a permitted address and the second with a blocked internal address, bypassing validation. The same vulnerability window exists non-maliciously when hostnames legitimately resolve to different addresses between lookups (short TTLs or round-robin DNS). The issue affects safeurl from version 0.1.0 onward, with no patch status indicated in available advisories.

## Affected products

- Slab safeurl 0.1.0 and later

## Timeline

- 2026-09-15: disclosed

## References

- [https://repo.hex.pm/](https://repo.hex.pm/)
- [https://github.com/](https://github.com/)