[PR] avformat/dashdec: reject a negative fragment index (PR #24093)

PR #24093 opened by michaelni
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24093
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24093.patch

A live manifest whose startNumber decreases across a refresh drives
cur_seq_no negative in move_segments(); get_current_fragment() only checked
the upper bound before indexing fragments[]. Add a lower-bound check and
clamp the negative delta at its source.

Fixes: out of array read



>From dc2a53b5b4ee3ea89029e0c602ff78df021dbda9 Mon Sep 17 00:00:00 2001
From: Joshua Rogers <[email protected]>
Date: Tue, 4 Aug 2026 12:11:55 +0000
Subject: [PATCH] avformat/dashdec: reject a negative fragment index

A live manifest whose startNumber decreases across a refresh drives
cur_seq_no negative in move_segments(); get_current_fragment() only checked
the upper bound before indexing fragments[]. Add a lower-bound check and
clamp the negative delta at its source.

Fixes: out of array read
---
 libavformat/dashdec.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/libavformat/dashdec.c b/libavformat/dashdec.c
index d4a05ace7e..740ac6bc1c 100644
--- a/libavformat/dashdec.c
+++ b/libavformat/dashdec.c
@@ -1537,8 +1537,11 @@ static void move_segments(struct representation *rep_src, struct representation
         free_fragment_list(rep_dest);
         if (rep_src->start_number > (rep_dest->start_number + rep_dest->n_fragments))
             rep_dest->cur_seq_no = 0;
-        else
+        else {
             rep_dest->cur_seq_no += rep_src->start_number - rep_dest->start_number;
+            if (rep_dest->cur_seq_no < 0)
+                rep_dest->cur_seq_no = 0;
+        }
         rep_dest->fragments    = rep_src->fragments;
         rep_dest->n_fragments  = rep_src->n_fragments;
         rep_dest->parent  = rep_src->parent;
@@ -1658,7 +1661,7 @@ static struct fragment *get_current_fragment(struct representation *pls)
     int reload_count = 0;
 
     while (( !ff_check_interrupt(c->interrupt_callback)&& pls->n_fragments > 0)) {
-        if (pls->cur_seq_no < pls->n_fragments) {
+        if (pls->cur_seq_no >= 0 && pls->cur_seq_no < pls->n_fragments) {
             seg_ptr = pls->fragments[pls->cur_seq_no];
             seg = av_mallocz(sizeof(struct fragment));
             if (!seg) {
-- 
2.52.0

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