AVIF Parsing Memory Leaks and Buffer Overflows

This article examines common edge cases encountered when parsing AVIF (AV1 Image File Format) files that frequently result in memory leaks, heap corruptions, and buffer overflows. By analyzing the structural complexities of the underlying ISO Base Media File Format (ISOBMFF) container and the embedded AV1 bitstream, this overview highlights the primary failure modes in memory allocation, pointer arithmetic, and cleanup logic.

1. Integer Overflows in Grid Image Dimensions

AVIF supports tiling large images using the grid derived item reference. The parser reads properties specifying the number of rows, columns, and tile dimensions.

• The Failure Mode: When calculating the destination canvas buffer size (e.g., rows * columns * tile_width * tile_height * bytes_per_pixel), naive integer multiplication without saturation checks can overflow a 32-bit integer.
• Resulting Impact: An undersized memory buffer is allocated on the heap. When the individual tiles are decoded and copied into the composite canvas, memory is written out of bounds, resulting in a heap-based buffer overflow.

2. Truncated and Inconsistent Box Sizes in ISOBMFF

The ISOBMFF container consists of nested data structures called "boxes" (or atoms), each defining its length and four-character code (FourCC).

• The Failure Mode: Attackers can craft box headers where the declared box length extends beyond the physical boundary of the parent box or the total file payload. Conversely, specifying a box size smaller than the box header (typically 8 or 16 bytes for 64-bit extended sizes) can lead to unsigned integer underflow during boundary checks.
• Resulting Impact: Inadequate validation allows parsers to read beyond allocated input streams (heap or stack out-of-bounds read) or execute negative-offset pointer arithmetic.

3. Circular References and Deeply Nested Boxes

Certain metadata boxes, such as iprp (item properties) and iref (item references), can point to other items or boxes.

• The Failure Mode: Parsers that resolve references recursively can be trapped by circular references (Box A references Box B, which references Box A) or malicious deeply nested structures.
• Resulting Impact: This can cause stack exhaustion (stack overflow) or excessive memory allocation loops where reference graph resolution allocates structures indefinitely, terminating in denial-of-service via memory exhaustion.

4. Asymmetric Item Location (iloc) Specifications

The iloc box maps image items to specific byte ranges and offsets within the file or within the idat (item data) box.

• The Failure Mode: Parsers must process offset, length, and base_offset fields across variable bit-widths (e.g., 4-bit, 8-bit, or dynamic size indicators). Adding base_offset + extent_offset + extent_length without strict bounds validation often yields arithmetic wraparound.
• Resulting Impact: If the computed slice points past the actual buffer, the decoder reads past its allocated slice. If an item slice claims to overlap invalid memory ranges, decoding routines write frame data to arbitrary heap offsets.

5. Incomplete Cleanup on Decode Interruption

AVIF parsing is a multi-stage process: container parsing, property extraction, decoder initialization (such as libaom or dav1d), and raster output generation.

• The Failure Mode: Malformed data injected late in the sequence (e.g., a corrupted Sequence Header in the AV1 bitstream) often triggers an early return or exception.
• Resulting Impact: If the parser allocates container tracking structures or decoder contexts without robust RAII (Resource Acquisition Is Initialization) or centralized error-handling cleanup labels, intermediate allocations are orphaned. Over repeated parsing operations in server environments, this causes severe memory leaks.

Mitigation Strategies

Preventing these vulnerabilities requires standard defensive parsing patterns:

• Employ safe math libraries with checked arithmetic for all dimension, offset, and size calculations.
• Use fuzzing integrated with AddressSanitizer (ASan) and UndefinedBehaviorSanitizer (UBSan) specifically targeted at malformed box hierarchies.
• Enforce hard limits on image dimensions, tile counts, and nesting depths before committing to memory allocations.