Curl Exposure of Sensitive Information Vulnerability (CVE-2025-0167)

1. Introduction

In February 2025, the curl project team disclosed a new vulnerability identified as CVE-2025-0167. Curl is a foundational networking tool used across billions of devices and applications worldwide. This new security issue affects how curl handles .netrc files during HTTP redirects, potentially leaking sensitive credentials to unintended hosts.

Given curl's widespread usage — both directly by users and embedded within applications — understanding, mitigating, and carefully updating curl is critical for maintaining a secure environment.

This article explains everything you need to know about CVE-2025-0167, the risks, how to patch it, and most importantly, why you should be cautious when updating curl, especially on Windows systems.

2. What is CVE-2025-0167?

CVE-2025-0167 is a vulnerability where curl could accidentally leak credentials during HTTP redirects if a .netrc file is used for authentication and the .netrc file has a default entry without a login and password.

Summary from the curl advisory:

When curl uses a .netrc file to retrieve login credentials, and an HTTP redirect occurs, curl might mistakenly send the first site's password to the redirected (second) site — leaking sensitive credentials.

This behavior happens only under a very specific configuration:

• .netrc is enabled
• A default section exists in .netrc without login/password entries
• HTTP redirects are followed (-L option)

3. How Critical is CVE-2025-0167?

The official severity rating assigned to CVE-2025-0167 is Low.

However:

• Impact is serious if exploited because passwords could be leaked to malicious or unintended hosts.
• Exploitability is limited because specific .netrc file structure and redirect behavior are required.
• Likelihood is rare, but consequences could be high if sensitive environments are exposed.

It is therefore recommended to patch immediately, even though the rating is low.

4. Technical Analysis of the Vulnerability

Here’s how the vulnerability works technically:

machine nn.tld
  login mary
  password maryspassword
default
    

If a curl transfer is initiated to nn.tld, and the server responds with a redirect (e.g., Location: http://zz.tld/path), curl might wrongly apply the same Authorization (with mary:marryspassword) to the new host zz.tld.

Normally, authorization headers should not be reused across different hosts without explicit permission. This is where the security breach happens.

The root cause: Curl treated a default entry without credentials as a match, incorrectly propagating authentication details.

5. Affected Versions and Fix

Affected versions:

• curl 7.76.0 to 8.11.1 (inclusive)

Fixed in:

• curl 8.12.0 and later

Git commit that fixed the issue:
0e120c5b925e8ca75d5319e

The fix ensures that a .netrc default entry without login/password is not considered a match, thus preventing credential leakage.

6. How to Patch or Upgrade curl

You have several options:

A. Recommended: Upgrade to curl 8.12.0+

If you use Linux/macOS and your package manager already provides curl 8.12.0 or later:

sudo apt update && sudo apt install curl      # Debian/Ubuntu
sudo dnf upgrade curl                         # RHEL/Fedora
brew update && brew upgrade curl              # macOS (Homebrew)
    

On Windows, if you installed curl manually (e.g., via Chocolatey):

choco upgrade curl
    

B. Manual Patch (Source Code)

If you cannot upgrade, you can patch manually by applying the commit:

git clone https://github.com/curl/curl.git
cd curl
git checkout curl-8_11_1  # or your current branch
git cherry-pick 0e120c5b925e8ca75d5319e
./buildconf
./configure
make
sudo make install
    

C. Temporary Mitigation Without Upgrade

If immediate patching isn't possible:

• Avoid using --netrc with -L (--location) redirects.
• Manually pass credentials using -u username:password.

Example:

curl -u user:pass https://example.com
    

7. Manual Code Changes to Work Around

For developers, the manual fix implemented in the source code (lib/netrc.c) is:

if(!retcode) {
    if(!password && our_login) {
        password = strdup("");
        if(!password)
            retcode = 1;
    }
    else if(!login && !password)
        /* a default with no credentials */
        retcode = NETRC_FILE_MISSING;
}
    

Meaning: If both login and password are missing in the default section, treat it as a "missing" file — so no default credentials are assumed.

8. Should We Directly Upgrade curl on Windows?

Important: If curl is bundled with Windows (typically in C:\Windows\System32\curl.exe):

• You should NOT manually upgrade it by replacing the file.
• You should wait for Microsoft to release a patch through Windows Update.

Why?

• Windows ties system binaries (like curl.exe) deeply into the OS.
• Manual replacement could break PowerShell, scripts, security components, or even OS upgrades.
• Microsoft officially maintains and patches system curl via Cumulative Updates (CU).

9. Risks of Manually Upgrading Windows System curl

Manually overwriting C:\Windows\System32\curl.exe could cause:

10. Best Practices and Final Recommendations

Additionally:

• Always check your current curl version:

curl --version
    

• On Windows, check if it's System32 curl:

where curl
    

11. Conclusion

CVE-2025-0167 may have a Low severity rating officially, but the potential real-world impact can be significant depending on your environment. Because curl is deeply embedded into so many systems, services, and devices, even a relatively "niche" bug like this demands a careful and deliberate response.

Summary key points:

• Patch or upgrade curl where possible.
• Avoid modifying Windows-bundled curl manually.
• Understand that .netrc + redirects = risky combination.
• Always follow vendor guidance for system-managed software components.

Security is not just about reacting to critical CVEs. It's about continuous vigilance and systematic risk management — and CVE-2025-0167 is a perfect example.

📚 References

• CVE-2025-0167 NVD Entry
• curl Security Advisory for CVE-2025-0167
• Patch Commit on GitHub