In a recent blog post, curl creator Daniel Stenberg addressed a security report from Mythos that claimed five vulnerabilities in the popular data transfer tool. After a thorough manual review, Stenberg determined that three of the reported issues were false positives, one was a minor bug rather than a security flaw, leaving only a single genuine vulnerability. This Q&A breaks down the findings, the review process, and what it means for curl users.
1. What exactly did Mythos claim regarding curl?
Mythos, a security research group, publicly asserted that they had identified five distinct vulnerabilities in curl, a widely used command-line tool and library for transferring data with URLs. Their report garnered attention, especially given curl's critical role in countless applications and systems. The claims suggested potential security risks that could affect many users. However, the initial announcement lacked detailed technical evidence, prompting Stenberg to conduct his own analysis.
2. How did Daniel Stenberg respond to Mythos' findings?
Stenberg, the original author and lead maintainer of curl, did not take the report at face value. He performed a manual review of each claimed vulnerability, cross-referencing the reported issues against curl's source code and known behavior. His response was measured and transparent: he acknowledged the importance of security reports but also highlighted the need for accuracy. He published his findings on his personal blog at daniel.haxx.se, emphasizing that not every reported code flaw constitutes a security vulnerability.
3. What were the results of the manual review?
Out of the five vulnerabilities Mythos claimed, Stenberg's manual review concluded that three were outright false positives—meaning the reported behaviors either did not exist or were not exploitable. A fourth was described as "just a bug," i.e., a coding error that did not have security implications. Only the fifth issue was confirmed as a genuine vulnerability, though its severity was relatively low. This breakdown underscores the importance of rigorous verification before labeling issues as security flaws.
4. Why does the distinction between a bug and a vulnerability matter?
The term "vulnerability" implies a security risk that can be exploited to compromise a system. A "bug" may simply cause incorrect behavior without exposing data or allowing unauthorized access. Stenberg stressed that conflating the two can lead to unnecessary panic, wasted resources, and misdirected development efforts. In open-source projects, maintaining a clear classification helps prioritize fixes: security vulnerabilities require immediate patches, while non-security bugs can be scheduled normally. This nuanced view protects both users and maintainers from overreaction.
5. What does this mean for the overall security of curl?
The review reaffirms that curl remains a robust and well-maintained tool. Stenberg's careful vetting process is part of curl's long-standing security posture—most reported issues are quickly triaged. The one confirmed vulnerability from the Mythos report was minor and has been patched. Users are advised to update to the latest curl version to ensure they are protected. The episode also highlights the value of community feedback and responsible disclosure, as even flawed reports can lead to improvements when handled properly.
6. How does this event relate to earlier publicity from Anthropic?
In April 2026, AI company Anthropic generated significant media attention by concluding a similar analysis of curl's security. While the exact details of Anthropic's findings are not covered here, Stenberg's response to Mythos reflects a broader pattern: external security assessments often overstate risks. Stenberg has consistently advocated for evidence-based reporting and has called on researchers to thoroughly test their claims before publication. The Mythos incident reinforces his message that hype should not replace careful examination.
7. What is the proper process for reporting vulnerabilities in curl?
curl follows a responsible disclosure policy. Researchers are encouraged to report potential vulnerabilities privately to the curl security team via the project's website or security contacts. This allows maintainers to verify the issue, create a fix, and release a coordinated update before public disclosure. Stenberg recommends that reporters provide clear, reproducible proof-of-concept examples. The team strives to respond promptly and credits reporters in the release notes. For more details, visit curl's security page.