[bug] Heap-overflow (read) via `FLAC__metadata_object_vorbiscomment_set_vendor_string`

Hi, there is a potential bug in utf8len_ reachable by FLAC__metadata_object_vorbiscomment_set_vendor_string.

This bug was reproduced on https://github.com/xiph/flac/commit/9547dbc2ddfca06a70ea937dbb605bbe78ea5f90.

Description

The crash occurs in utf8len_ (format.c:343) which checks for 4-byte UTF-8 sequences by reading utf8[0..3] without ensuring that at least 4 bytes remain in the buffer. When FLAC__metadata_object_vorbiscomment_set_vendor_string() validates the provided vendor string via FLAC__format_vorbiscomment_entry_value_is_legal(), a one-byte buffer beginning with 0xF0 (a 4-byte UTF-8 lead byte) triggers the path that dereferences utf8[1], utf8[2], and utf8[3], causing a heap OOB read.

Note that the size of the vendor string is provided to FLAC__format_vorbiscomment_entry_value_is_legal, but it invokes utf8len_ without providing this information.

POC

The following testcase demonstrates the bug:

testcase.cpp

#include <cstdlib>
extern "C" {
#include "/fuzz/install/include/FLAC/metadata.h"
}
int main() {
    // Create a VORBIS_COMMENT object
    FLAC__StreamMetadata *obj = FLAC__metadata_object_new(FLAC__METADATA_TYPE_VORBIS_COMMENT);
    if (!obj) return 0;
    // 1-byte buffer starting with 0xF0 => 4-byte UTF-8 lead without continuation bytes
    FLAC__byte *buf = (FLAC__byte*)malloc(1);
    buf[0] = 0xF0;
    FLAC__StreamMetadata_VorbisComment_Entry entry;
    entry.entry = buf;
    entry.length = 1; // valid per API (non-zero length with non-NULL pointer)
    // Triggers OOB read in utf8len_/validation
    FLAC__metadata_object_vorbiscomment_set_vendor_string(obj, entry, /*copy=*/0);
    return 0;
}

stdout

=================================================================
==1==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x502000000011 at pc 0x557d3196af45 bp 0x7fffb5d3caa0 sp 0x7fffb5d3ca98
READ of size 1 at 0x502000000011 thread T0
    #0 0x557d3196af44 in utf8len_ format.c
    #1 0x557d3196aaec in FLAC__format_vorbiscomment_entry_value_is_legal (/fuzz/test+0x156aec) (BuildId: 73032547d555f3196deb8ae4a3007a887ac3a7cf)
    #2 0x557d31925d26 in FLAC__metadata_object_vorbiscomment_set_vendor_string (/fuzz/test+0x111d26) (BuildId: 73032547d555f3196deb8ae4a3007a887ac3a7cf)
    #3 0x557d319206ed in main /fuzz/testcase.cpp:16:5
    #4 0x7fb433393d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #5 0x7fb433393e3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #6 0x557d31845374 in _start (/fuzz/test+0x31374) (BuildId: 73032547d555f3196deb8ae4a3007a887ac3a7cf)

0x502000000011 is located 0 bytes after 1-byte region [0x502000000010,0x502000000011)
allocated by thread T0 here:
    #0 0x557d318e171e in malloc (/fuzz/test+0xcd71e) (BuildId: 73032547d555f3196deb8ae4a3007a887ac3a7cf)
    #1 0x557d31920588 in main /fuzz/testcase.cpp:10:36
    #2 0x7fb433393d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16

SUMMARY: AddressSanitizer: heap-buffer-overflow format.c in utf8len_
Shadow bytes around the buggy address:
  0x501ffffffd80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x501ffffffe00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x501ffffffe80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x501fffffff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x501fffffff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x502000000000: fa fa[01]fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x502000000080: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x502000000100: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x502000000180: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x502000000200: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x502000000280: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==1==ABORTING

stderr

# Ubuntu 22.04 with some packages pre-installed
FROM hgarrereyn/stitch_repro_base@sha256:3ae94cdb7bf2660f4941dc523fe48cd2555049f6fb7d17577f5efd32a40fdd2c

RUN git clone https://github.com/xiph/flac.git /fuzz/src && \
    cd /fuzz/src && \
    git checkout 9547dbc2ddfca06a70ea937dbb605bbe78ea5f90 && \
    git submodule update --init --remote --recursive

ENV LD_LIBRARY_PATH=/fuzz/install/lib
ENV ASAN_OPTIONS=hard_rss_limit_mb=1024:detect_leaks=0

RUN echo '#!/bin/bash\nexec clang-17 -fsanitize=address -O0 "$@"' > /usr/local/bin/clang_wrapper && \
    chmod +x /usr/local/bin/clang_wrapper && \
    echo '#!/bin/bash\nexec clang++-17 -fsanitize=address -O0 "$@"' > /usr/local/bin/clang_wrapper++ && \
    chmod +x /usr/local/bin/clang_wrapper++

# Install build dependencies
RUN apt-get update && apt-get install -y --no-install-recommends \
    ca-certificates \
    cmake \
    ninja-build \
    make \
    pkg-config \
    build-essential \
 && rm -rf /var/lib/apt/lists/*

# Configure, build, install static libs to /fuzz/install
ENV CC=clang_wrapper CXX=clang_wrapper++
WORKDIR /fuzz/build
RUN cmake -S /fuzz/src -B . -G Ninja \
    -DCMAKE_INSTALL_PREFIX=/fuzz/install \
    -DCMAKE_BUILD_TYPE=Release \
    -DBUILD_SHARED_LIBS=OFF \
    -DBUILD_PROGRAMS=OFF \
    -DBUILD_EXAMPLES=OFF \
    -DBUILD_TESTING=OFF \
    -DBUILD_DOCS=OFF \
    -DWITH_OGG=OFF \
    -DINSTALL_MANPAGES=OFF \
    -DBUILD_CXXLIBS=ON
RUN cmake --build . -j$(nproc)
RUN cmake --install .

# Provide shim for FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION allocation-check globals
RUN printf "int alloc_check_threshold=2147483647;\nint alloc_check_counter=0;\nint alloc_check_keep_failing=0;\n" > /fuzz/build/flac_fuzz_shim.c \
 && clang_wrapper -c /fuzz/build/flac_fuzz_shim.c -o /fuzz/build/flac_fuzz_shim.o \
 && ar rcs /fuzz/install/lib/libflac_fuzz_shim.a /fuzz/build/flac_fuzz_shim.o

clang++-17 -fsanitize=address -g -O0 -o /fuzz/test /fuzz/testcase.cpp -I/fuzz/install/include -L/fuzz/install/lib -lFLAC++ -lFLAC -lflac_fuzz_shim -lm && /fuzz/test

docker build . -t repro --platform=linux/amd64

docker run \
    -it --rm \
    --platform linux/amd64 \
    --mount type=bind,source="testcase.cpp",target=/fuzz/testcase.cpp \
    repro \
    bash -c "clang++-17 -fsanitize=address -g -O0 -o /fuzz/test /fuzz/testcase.cpp -I/fuzz/install/include -L/fuzz/install/lib -lFLAC++ -lFLAC -lflac_fuzz_shim -lm && /fuzz/test"

Additional Info

This testcase was discovered by STITCH, an autonomous fuzzing system. All reports are reviewed manually (by a human) before submission.

Start at utf8len_ in format.c:343 and trace how FLAC__format_vorbiscomment_entry_value_is_legal is called by FLAC__metadata_object_vorbiscomment_set_vendor_string. Build the provided Dockerfile and run testcase.cpp with AddressSanitizer to reproduce the one-byte 0xF0 input. Done means the testcase no longer reports a heap-buffer-overflow during validation.