File ffmpeg-4-CVE-2026-75142.patch of Package A_tw-ffmpeg-4

From 9d786e4b5e9b8482651928574de33772aeee7be1 Mon Sep 17 00:00:00 2001
From: Joshua Rogers <MegaManSec@users.noreply.github.com>
Date: Tue, 4 Aug 2026 12:11:55 +0000
Subject: [PATCH] avformat/mpegenc: reject stream counts that overflow the
 system header

put_system_header() writes 12 + 3*N bytes after the pack header into a
fixed 128-byte stack buffer, but is handed a PutBitContext sized past the
real buffer, so its own bounds check never fires; ~35+ streams overflow the
stack. Reject at mux init when the system header would not fit.

Fixes: out of array access
Fixes: many.mkv
---
 libavformat/mpegenc.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/libavformat/mpegenc.c b/libavformat/mpegenc.c
index 128dfe2885..cea91d80da 100644
--- a/libavformat/mpegenc.c
+++ b/libavformat/mpegenc.c
@@ -473,6 +473,16 @@ static av_cold int mpeg_mux_init(AVFormatContext *ctx)
         if (!stream->fifo)
             return AVERROR(ENOMEM);
     }
+
+    /* The system header is emitted, right after the pack header (which is at
+     * most 14 bytes), into the fixed 128-byte buffer used by flush_packet().
+     * Reject configurations whose system header would not fit. */
+    if (get_system_header_size(ctx) > 128 - 14) {
+        av_log(ctx, AV_LOG_ERROR,
+               "Too many streams to fit the MPEG program stream system header\n");
+        return AVERROR(EINVAL);
+    }
+
     bitrate       = 0;
     audio_bitrate = 0;
     video_bitrate = 0;
-- 
2.49.0