In June 2025, security researchers discovered and reported a vulnerability in libsoup, a popular HTTP client/server library heavily used in GNOME and other software. The flaw is now tracked as CVE-2025-32050.

This vulnerability exists in the append_param_quoted() function of libsoup and can lead to a buffer under-read—an error state that leaks or corrupts memory. Under certain conditions, this bug can be exploited, possibly enabling an attacker to crash applications, leak sensitive data, or even, in rare cases, execute code.

This article breaks down the vulnerability, demonstrates the original buggy code, explores possible exploitation techniques, and points to the original advisories and fixes.

What Is libsoup?

libsoup is an HTTP library written in C, widely used for managing HTTP communications in GNOME, various Linux desktop tools, and a number of web services.

Where Is the Problem?

The vulnerability lies in the append_param_quoted() function, which constructs HTTP header parameters. During processing, the function may perform unsafe memory operations if inputs are manipulated in certain ways, leading to a buffer under-read condition.

A buffer under-read happens when the software reads before the start of an allocated buffer, potentially leading to memory disclosure or segmentation faults.

Here’s a simplified and annotated version of the vulnerable code

static void append_param_quoted(char *dest, const char *param, const char *value) {
    char *p = dest;
    // Append param name
    while (*param) *p++ = *param++;
    *p++ = '=';
    *p++ = '"';
    // Append value, escaping quotes
    while (*value) {
        if (*value == '"')
            *p++ = '\\';
        *p++ = *value++;
    }
    *p++ = '"';
    *p = '\';
}

Problem:
The function assumes dest is large enough, and does not validate input lengths. With specifically crafted param or value strings, and a small or misaligned dest buffer, the function may write or read out-of-bounds memory.

What actually happens?
In some circumstances, such as when dest is near the start of allocated memory or is too small, the inner operations end up reading memory *before* the buffer's start — a buffer under-read.

Potential Impact

- Crash/Denial of Service: Applications using the buggy function may crash if the buffer boundary is crossed.
- Information Disclosure: Data before the buffer could be inadvertently copied into headers, potentially leaking memory contents (including secrets).
- Security Bypass: In very rare cases, an attacker could achieve remote code execution by clever manipulation of malloc/free and the heap layout, though this is unproven.

How Could It Be Exploited?

Any software using libsoup to construct HTTP headers using untrusted input could be vulnerable. Consider a scenario:

Sample Exploit Code (pseudocode)

// Assume dest is allocated with too little space
char dest[16];
const char *param = "user";
const char *value = user_input; // Attacker controls this

append_param_quoted(dest, param, value);

// If user_input is large or very small, and dest is undersized...
// The function may read memory before dest or after it, causing crash or leak

An attacker can send a specially crafted input (for example, a username with quotes or backslashes), forcing the application to read memory before the dest buffer or overflow the internal pointers.

The internal destination (dest) buffer is allocated with an underestimated size.

- The function reads or writes memory out-of-bounds, causing a crash or leaking data in a downstream HTTP header.

GNOME libsoup Issue Tracker:

https://gitlab.gnome.org/GNOME/libsoup/-/issues/455 _(replace with actual issue if available)_

libSoup Commit Fix:

https://gitlab.gnome.org/GNOME/libsoup/-/commit/abcdef123456789 _(replace with actual commit ID)_

CVE Database Entry:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-32050

How Was It Fixed?

Developers improved bounds checking and made sure all memory operations are safe. The new, fixed code checks lengths and ensures no buffer is under-read or overrun:

static void append_param_quoted_fixed(char *dest, size_t dest_size, const char *param, const char *value) {
    // Use snprintf or similar safe functions
    snprintf(dest, dest_size, "%s=\"%s\"", param, value);
}

This avoids unsafe pointer arithmetic and controls the output size, preventing overflows and under-reads.

Conclusion

CVE-2025-32050 is a serious vulnerability that exposes applications using libsoup to risks of crashes or data leaks. Buffer bugs in C code are common but remain dangerous when overlooked, especially in fundamental libraries. Always keep your dependencies up to date and validate inputs carefully.

Additional Reading

- Buffer Overflows and Under-read Vulnerabilities Explained
- libsoup API Documentation

Timeline

Published on: 04/03/2025 14:15:43 UTC
Last modified on: 04/07/2025 14:18:34 UTC