Critical Ubuntu security updates (USN-8084-1) patch multiple high-impact curl vulnerabilities. Urgent patches address OAuth2 bearer token leakage during redirects, connection reuse flaws exposing Negotiate credentials, and a potential SMB heap overflow DoS/RCE. Verify your Ubuntu 22.04 LTS, 24.04 LTS, or 25.10 version and apply the updated curl packages (libcurl4, etc.) immediately to prevent credential compromise and service disruption.
Imagine your application’s OAuth2 bearer token—the digital key to your user's data—being silently copied and handed over to a malicious third-party server during a routine redirect.
This isn't a hypothetical scenario; it's the reality of CVE-2026-3783, one of several severe vulnerabilities now patched in curl for all major Ubuntu releases.
On March 11, 2026, Canonical released Ubuntu Security Notice USN-8084-1, addressing five distinct Common Vulnerabilities and Exposures (CVEs) in the ubiquitous curl data transfer tool.
These flaws range from credential exposure and connection mismanagement to a potential denial of service (DoS) and possible remote code execution (RCE) . For systems engineers and security architects, this update is not optional—it's mission-critical.
The Vulnerabilities: A Technical Breakdown
This update resolves vulnerabilities that undermine curl's fundamental trust boundaries in connection handling and credential management. Here’s an analysis of the most critical threats:
OAuth2 Bearer Token Leak on Redirect (CVE-2026-3783)
The Threat: When curl follows an HTTP redirect, it can inadvertently leak OAuth2 bearer tokens to the destination host of the redirect. If an attacker controls or compromises that destination, they can capture these tokens and gain unauthorized access to APIs and user data.
Expert Insight: This bypasses the fundamental security model of OAuth2, where tokens are scoped to specific resources.
Negotiate Credential Cross-Connection Reuse (CVE-2026-1965)
The Threat: Discovered by Zhicheng Chen, this flaw causes curl to incorrectly reuse a connection for Negotiate-authenticated requests. This can result in credentials from one connection being used for a different, unintended request, breaking authentication isolation.
Proxy Connection Credential Mismatch (CVE-2026-3784)
The Threat: Muhamad Arga Reksapati found that curl could reuse an existing HTTP proxy connection for a request requiring different proxy credentials. This leads to requests being processed with incorrect authentication levels, potentially bypassing access controls.
.netrc Credential Cross-Host Leak (CVE-2025-0167)
The Threat: When following redirects, curl could reuse credentials from a
.netrcfile for a different host. This violates the principle of least privilege and can expose credentials intended for one server to another.
SMB Heap Overflow Leading to DoS/RCE (CVE-2026-3805)
Impacted Systems: Ubuntu 25.10 only.
The Threat: Daniel Wade identified a memory handling flaw during sequential SMB requests to the same host. This constitutes a classic heap overflow vulnerability, which an attacker can trigger to crash the service (DoS) or, under specific conditions, achieve arbitrary code execution.
Immediate Patching Strategy: Affected Versions & Remediation
The first question from any engineer is, "Am I vulnerable?" If you are running any of the following Ubuntu releases with an unpatched curl version, your answer is yes.
Affected Ubuntu Releases:
Ubuntu 24.04 LTS (including derivatives)
Ubuntu 22.04 LTS (including derivatives)
The Solution: Update curl and libcurl packages immediately.
A standard system update (sudo apt update && sudo apt upgrade) will apply the necessary fixes. The corrected package versions are:
| Ubuntu Release | Package | Fixed Version |
|---|---|---|
| 25.10 | curl, libcurl4t64 | 8.14.1-2ubuntu1.2 |
| 24.04 LTS | curl, libcurl4t64 | 8.5.0-2ubuntu10.8 |
| 22.04 LTS | curl, libcurl4 | 7.81.0-1ubuntu1.23 |
Verification Command: After updating, verify the installation with
curl --versionand check your package manager logs.
Frequently Asked Questions (FAQ)
Q: How can an attacker exploit the OAuth2 token leak (CVE-2026-3783) remotely?
A: An attacker would need to control a server to which your application is redirected. If your curl request follows a redirect (e.g., HTTP 301/302) to that malicious server, the bearer token is leaked in the subsequent request. This is a client-side vulnerability triggered by server behavior.Q: Do these vulnerabilities affect all curl installations, or just Ubuntu?
A: These are vulnerabilities in the curl codebase itself. While this specific advisory (USN-8084-1) is for Ubuntu, other distributions using the affected curl versions should also release their own patches. Check with your specific Linux distribution's security team.Q: Is there a workaround if I cannot immediately apply the patch?
A: As a temporary measure, you can disable following redirects (--max-redirs 0) in your curl commands or applications. For OAuth2 flows, ensure your application strictly validates redirect targets against a whitelist. However, patching is the only complete remediation.Conclusion: Securing the Data Pipeline
The curl utility is the silent workhorse of countless applications, cloud instances, and CI/CD pipelines. The vulnerabilities fixed in USN-8084-1 represent a direct threat to the confidentiality and integrity of data in transit.
By delaying this patch, you risk leaking high-value credentials like OAuth2 tokens and exposing internal systems to connection-based attacks. Act now to update your Ubuntu systems, verify the package versions, and ensure your data transfer layer remains resilient and trustworthy.
Action:
Run sudo apt update && sudo apt upgrade curl libcurl4 -y on your affected systems now. Review your application logs for any unusual redirect activity that may indicate attempted exploitation.
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