Critical Security Response: Addressing CVE-2026-25537 and Stack Exhaustion in Fedora 43

Critical Fedora 43 Security Update: Patch CVE-2026-25537 & Stack Exhaustion Flaw Now – An urgent Fedora 43 advisory addresses a severe stack exhaustion vulnerability (CVE-2026-25727) in the time crate and a related JSON Web Token authorization bypass (CVE-2026-25537), impacting tuigreet and key packages.  

A critical vulnerability in a foundational time crate triggered widespread rebuilds across Fedora 43, patching not one but nine major applications, including the tuigreet display manager.

Immediate Action Required

For Fedora 43 users and administrators, an urgent system update is mandatory. A critical stack exhaustion vulnerability, tracked as CVE-2026-25727, has been identified in the widely used Rust time crate. 

This severe denial-of-service (DoS) flaw necessitated a mass rebuild of core packages, including the tuigreet graphical console greeter. Concurrently, a related authorization bypass vulnerability (CVE-2026-25537) in the jsonwebtoken library was patched. 

These coordinated advisories (RUSTSEC-2026-0007 through 0009) represent a significant security event for the Linux ecosystem, demanding prompt remediation to prevent system instability and unauthorized access.

This update, FEDORA-2026-f400579a21, underscores a modern reality of software supply chain security: a single flaw in a common dependency can cascade into a widespread threat. 

The following analysis breaks down the vulnerabilities, their impact on system security and operations, and provides a clear, actionable guide for securing your systems.

1 Vulnerability Analysis and Technical Impact

1.1 The Core Vulnerabilities: CVE-2026-25727 & CVE-2026-25537

The recent Fedora 43 security advisory addresses two distinct but critical threats arising from compromised software dependencies.

The first and primary threat is CVE-2026-25727, a stack exhaustion vulnerability within the Rust time library. In technical terms, this flaw allows a maliciously crafted input to trigger uncontrolled or deeply nested recursion within the library's parsing functions. This consumes all available stack memory, leading to an immediate application crash and denial of service. 

For a display manager like tuigreet, exploitation could prevent user login entirely, effectively locking access to the graphical environment. The flaw's CVSS score is rated high, reflecting its potential to destabilize any application that processes date/time data from untrusted sources.

Simultaneously, the update patches CVE-2026-25537, a type confusion vulnerability in the jsonwebtoken library. 

This flaw could allow an attacker to bypass signature verification—a cornerstone of the library's security—by manipulating token structures. A successful exploit could lead to an authorization bypass, where an attacker crafts a token that is incorrectly validated as genuine, granting them privileges they should not have. 

The advisory also includes patches for RUSTSEC-2026-0007 (bytes crate) and RUSTSEC-2026-0008 (git2/libgit2 crates), demonstrating a comprehensive dependency tree remediation.

1.2 Cascading Risk in the Software Supply Chain

This incident is a textbook example of software supply chain risk. A single vulnerable component, the time crate, created a transitive vulnerability in every application that depended on it.

• Widespread Impact: The advisory lists nine directly affected packages—from tuigreet and rustup to keylime-agent-rust and uv. This is not an exhaustive list of all impacted software, just those explicitly rebuilt for this advisory.

• Systemic Threat: Applications do not need to use the time crate's vulnerable function directly. Merely linking to it as a transitive dependency (a dependency of a dependency) is enough to expose the application to risk. This hidden propagation makes vulnerability management in modern, dependency-heavy ecosystems like Rust and Linux distributions exceptionally challenging.

• Real-World Consequence: Consider a server running keylime-agent-rust for security compliance. An attacker exploiting this stack overflow could crash the agent, disabling security monitoring and potentially creating a window for further intrusion. The system stability implications are severe and immediate.

The table below summarizes the key components and their associated security advisories:

1.3 The Fedora Security Response: Mass Rebuild

The Fedora Project's response mechanism is a critical part of the Linux security model. Upon the disclosure of these RustSec advisories, the Fedora Security Team initiated a mass rebuild of affected packages.

• Proactive Patching: Rather than waiting for individual maintainers, the team coordinated a bulk update to the latest, patched versions of the vulnerable crates (time 0.3.47, git2 0.20.4, etc.).

• Comprehensive Coverage: As noted in the advisory, "All applications that statically link libgit2 via the git2 Rust bindings were also rebuilt" to pull in other fixes. This ensures that the fix is deployed consistently and efficiently across the entire distribution.

• Single Advisory: The result is the unified advisory FEDORA-2026-f400579a21, which provides a single, actionable update command for users to resolve multiple interrelated threats.

This process highlights the strength of a curated distribution: centralized security response that protects users from fragmented and delayed patching.

2 Remediation and Proactive System Hardening

2.1 Immediate Patch Deployment

The mitigation path is clear and standardized. All Fedora 43 systems must apply the update immediately.

1. Apply the Update: Execute the following command with root privileges. This command specifically targets the advisory to ensure the correct packages are installed.

   bash

   sudo dnf upgrade --advisory=FEDORA-2026-f400579a21

2. Standard System Update: Alternatively, a full system update will also incorporate this fix:

   bash

   sudo dnf update

3. Verify Installation: After the update, you can verify the updated version of tuigreet is installed:

   bash

   rpm -q tuigreet

   The output should show version 0.9.1-7.fc43 or later.

Reboot Considerations: While a full reboot is the most thorough action, restarting the affected services is often sufficient. For tuigreet, this typically means restarting the greetd service:

bash

sudo systemctl restart greetd

For other affected applications like keylime-agent-rust or rustup, consult their respective documentation for proper restart procedures.

2.2 Long-Term Security Posture Enhancement

Beyond patching, this event should inform your long-term DevSecOps and system hardening strategies.

• Automate Vulnerability Scanning: Implement tools like cargo-audit for Rust projects or dnf plugin for system updates to automatically detect known vulnerabilities in dependencies. For enterprise environments, consider a dedicated Software Composition Analysis (SCA) tool.

• Enhance Monitoring: Increase monitoring for abnormal crashes or resource exhaustion (particularly stack-related errors) in applications known to use Rust crates. Logs from greetd or other patched services should be watched closely in the days following the update.

• Adopt a Zero-Trust Model for Auth: The jsonwebtoken flaw underscores that authentication mechanisms are prime targets. Where possible, implement multi-factor authentication (MFA) and adhere to the principle of least privilege, ensuring that a single compromised token does not grant extensive access.

• Stay Informed: Subscribe to security mailing lists like package-announce@lists.fedoraproject.org and monitor official channels such as the Fedora Project Security Advisories page. For Rust ecosystems, regularly check the RustSec Advisory Database.

3 Implications for Developers and Enterprise

This incident carries significant lessons for software developers and enterprise security architects.

For developers, especially in the Rust ecosystem, it reinforces the critical need for dependency hygiene. Regularly auditing your Cargo.toml with cargo audit, minimizing unnecessary dependencies, and pinning dependency versions with a Cargo.lock file in applications are essential practices. 

The Rust language's memory safety guarantees are powerful, but they do not eliminate logical bugs like stack exhaustion or type confusion in safe code, making proactive security just as vital.

For enterprises, this is a case study in third-party risk management. The software bill of materials (SBOM) concept moves from theoretical to essential. 

Can you quickly identify all systems that might be affected by a vulnerability in a mid-level library like the time crate? Establishing an SBOM practice and integrating vulnerability feeds into your asset management system is crucial for reducing mean time to remediation (MTTR) in such events.

Consider this: If a simple date parsing function in a widely trusted library can bring down critical services, how well do you understand the deep dependency graph of your own production software stack?

4 Frequently Asked Questions (FAQ)

Q: What is the actual risk if I don't update immediately?

A The primary risk is a denial-of-service attack that could crash tuigreet (preventing logins) or other critical services like keylime-agent. The secondary risk is a potential authorization bypass in applications using the vulnerable jsonwebtoken library for authentication.

Q: I'm not using tuigreet. Is my system still vulnerable?

A: Yes, potentially. The vulnerability resides in the time crate, not just tuigreet. The advisory lists nine affected packages (atuin, keylime-agent-rust, maturin, rustup, tbtools, uv), and others not listed may also be impacted if they use the vulnerable crate version. Applying the full system update is the safest course of action.

Q: How does a "stack exhaustion" attack work in practice?

A: An attacker sends a maliciously crafted date/time string to an application. When the vulnerable time library parses it, flawed logic causes excessive recursion, consuming all memory allocated for the "call stack." This leads to a segmentation fault (SIGSEGV) and crashes the application.

Q: Can this vulnerability lead to remote code execution (RCE)?

A: Based on the advisory, CVE-2026-25727 is classified as a denial-of-service flaw. There is no public indication that it allows for arbitrary code execution. However, crashing a security agent or authentication service can be a critical first step in a broader attack chain.

Q: Where can I find more technical details about these CVEs?

A: You can follow the references in the official advisory:

• For CVE-2026-25727 (time): Bug #2438164

• For CVE-2026-25537 (jsonwebtoken): Bug #2437470

The Fedora 43 tuigreet security update is more than a routine patch; it's a response to a systemic supply chain vulnerability. By understanding the technical depth of CVE-2026-25727 and CVE-2026-25537, administrators can appreciate the urgency of the update. 

By implementing the provided remediation steps and adopting the suggested hardening practices, you do not just fix a single flaw—you bolster your entire system's resilience against the complex, interconnected threats that define modern cybersecurity. 

Take action now: run the update command, restart critical services, and review your long-term vulnerability management strategy.