CVE-2025-9086: Haxx Curl Buffer Overflow Vulnerability

CVE-2025-9086 Overview

CVE-2025-9086 is a heap-based out-of-bounds read vulnerability in cURL, the widely-used command line tool and library for transferring data with URLs. The flaw exists in the cookie path comparison logic and can be triggered when cURL handles secure cookies during HTTP/HTTPS redirection scenarios.

The vulnerability occurs when a secure cookie is set for an HTTPS site, and cURL is subsequently redirected to the same hostname over HTTP. When a malicious clear-text site attempts to set the same cookie name with a minimal path value (path="/"), a bug in the path comparison logic causes cURL to read beyond the boundaries of a heap buffer.

Critical Impact

This vulnerability can cause application crashes (denial of service) or potentially allow an insecure HTTP site to override secure cookie contents, bypassing expected cookie security protections.

Affected Products

• Haxx cURL (multiple versions)
• Debian Linux 11.0

Discovery Timeline

• 2025-09-10 - Vulnerability disclosed via Openwall OSS-Security Mailing List
• 2025-09-12 - CVE CVE-2025-9086 published to NVD
• 2026-01-20 - Last updated in NVD database

Technical Details for CVE-2025-9086

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read), a memory corruption flaw that occurs when the application reads data past the end of an allocated buffer. The issue resides in cURL's cookie handling mechanism, specifically within the path comparison logic used to validate cookies during HTTP redirections.

The expected behavior when a cookie has been set as "secure" on an HTTPS site is that any subsequent attempt to override that cookie from an insecure HTTP connection should be rejected. However, the flawed path comparison logic causes cURL to read outside the boundaries of a single-byte heap allocation that holds the path, leading to undefined behavior.

The outcome depends on the memory contents immediately following the single-byte allocation. In the crash scenario, the out-of-bounds read triggers a segmentation fault or similar memory access violation. In the more subtle exploitation scenario, the incorrect memory read causes the comparison to produce a wrong result, allowing the insecure site to override the secure cookie's contents.

Root Cause

The root cause is a boundary condition error in the cookie path comparison function within cURL. When processing a cookie with a minimal path value (single slash), the comparison logic fails to properly validate the buffer boundaries before reading memory. The single-byte allocation holding the path string is insufficient for the comparison operation being performed, resulting in the read extending past the allocated heap memory.

Attack Vector

The attack is network-based and can be triggered through the following sequence:

1. A legitimate HTTPS site sets a secure cookie for a specific hostname
2. The victim's cURL client is redirected (or otherwise directed) to communicate with an HTTP version of the same hostname
3. The malicious HTTP site responds with a Set-Cookie header containing the same cookie name but with a minimal path (path="/")
4. cURL's cookie handling code processes this cookie, triggering the out-of-bounds read during path comparison

The vulnerability requires no authentication and can be exploited without user interaction beyond the initial request. The attack complexity is low, as the exploit requires only the ability to control or influence HTTP responses to a cURL client.

Detection Methods for CVE-2025-9086

Indicators of Compromise

• Unexpected cURL process crashes, particularly during HTTP/HTTPS redirect operations
• Memory corruption errors in application logs referencing cURL library functions
• Abnormal cookie handling behavior where secure cookies are unexpectedly modified
• Segmentation faults in systems heavily utilizing cURL for web communications

Detection Strategies

• Monitor for cURL crashes with memory access violations in cookie-related code paths
• Implement network monitoring to detect suspicious HTTP-to-HTTPS redirect patterns targeting the same hostname
• Review application logs for cookie integrity warnings or unexpected cookie value changes
• Deploy memory sanitization tools (ASan, Valgrind) in development and staging environments to detect out-of-bounds reads

Monitoring Recommendations

• Enable crash reporting for applications utilizing cURL to identify potential exploitation attempts
• Monitor network traffic for patterns involving HTTPS requests immediately followed by HTTP requests to identical hostnames
• Track Set-Cookie headers with minimal path values (path="/") in HTTP responses that follow secure sessions
• Implement application-level logging for cookie modification events

How to Mitigate CVE-2025-9086

Immediate Actions Required

• Update cURL to the latest patched version immediately
• Review all systems and applications that depend on libcurl for potential exposure
• Audit network configurations to minimize exposure to HTTP downgrade scenarios
• Consider implementing strict HTTPS-only policies where possible

Patch Information

Haxx has released security updates to address this vulnerability. System administrators should consult the cURL CVE-2025-9086 Documentation for specific version information and patch details. Debian users should refer to the Debian LTS Announcement for distribution-specific updates.

Additional technical details are available via the HackerOne Report #3294999.

Workarounds

• Enforce HTTPS-only connections by configuring --proto =https to prevent HTTP downgrade attacks
• Use the --no-sessionid and strict redirect policies to control redirect behavior
• Implement application-level cookie validation to detect unexpected modifications
• Consider network-level controls to block HTTP traffic to sensitive hostnames that should only use HTTPS
• Deploy web application firewalls (WAF) rules to filter suspicious redirect patterns

# Configuration example: Force HTTPS-only connections in curl
# Add to ~/.curlrc or specify on command line
--proto =https

# Alternative: Use HSTS preload lists and certificate pinning
# to prevent downgrade attacks at the application level