CVE-2026-38639: relibc strptime Parse Month DoS Vulnerability

Technical summary

The parse_month function in relibc's time/strptime.rs module fails to properly validate input when parsing month fields according to the strptime specification. An attacker can supply a malformed month value that causes the parsing logic to enter an error state or resource-exhaustion condition, resulting in a denial of service. The vulnerability is classified as improper input validation (CWE-20) and carries a CVSS 3.1 score of 7.5 (HIGH severity) due to network accessibility, low attack complexity, and complete availability impact.

Business impact

Services that depend on relibc for date-time parsing face sudden unavailability when processing untrusted date input. Web services, APIs, data pipelines, and any networked application parsing user-supplied timestamps become susceptible to crash-triggered outages. Because the flaw requires no authentication and operates at the parsing layer, threat actors can weaponize it against public-facing endpoints. Organizations should assess whether their Rust ecosystem relies on relibc and whether that code path receives external input.

Affected systems

relibc is a lightweight standard C library implementation used in embedded and bare-metal Rust projects. The vulnerability exists in commit ab6a2e and potentially earlier versions. Applications or libraries that bundle or depend on relibc and expose date-parsing functionality to external input are at risk. The vendor product list is empty in the current advisory, so affected software must be identified through build manifests and dependency tracking in your own environment.

Exploitability

This vulnerability is highly exploitable. An attacker can trigger it with a simple network request containing a malformed date string; no sophisticated techniques, user interaction, or elevated privileges are required. The attack surface is broad for any application that accepts and parses date input from untrusted sources (HTTP requests, message queues, file uploads, API parameters). Exploitation is reliable and repeatable.

Remediation

Upgrade relibc to a patched version that properly validates month input in the parse_month function. Verify the fix against relibc's official repository and release notes. Organizations unable to upgrade immediately should implement input validation at the application layer to reject or sanitize malformed month values before passing them to strptime. Consider using safer parsing libraries or frameworks that provide additional validation guardrails.

Patch guidance

Consult the official relibc repository and release notes to identify the version that resolves commit ab6a2e's vulnerability. Apply the patched version to all builds and deployments that depend on relibc. Test date-parsing logic against known malformed inputs in your validation suite before promoting to production. If relibc is vendored into your codebase, audit your supply chain to ensure timely updates are propagated.

Detection guidance

Monitor application logs for parsing errors, crashes, or timeouts associated with date-handling functions, especially when processing untrusted input. Implement input validation at the API or middleware layer to reject month values outside the valid range (1–12) or containing non-numeric characters before they reach strptime. Network-based detection is challenging at the vulnerability level but becomes feasible if you can identify HTTP request patterns that consistently trigger parsing failures.

Why prioritize this

HIGH severity (CVSS 7.5) with zero barriers to exploitation make this a priority for any team operating Rust services that parse dates. The attack is remote, requires no authentication, and causes total service unavailability. Organizations should patch within one to two weeks depending on deployment complexity and risk tolerance.

Risk score, explained

CVSS 3.1 assigns a score of 7.5 (HIGH) because the attack vector is network-based, attack complexity is low, no privileges are required, no user interaction is needed, and the impact on availability is high (complete denial of service). Confidentiality and integrity are not affected, keeping the score below critical. The score reflects a straightforward remote crash condition with minimal operational friction for an attacker.

Frequently asked questions