Bug 2543296 (CVE-2026-102560)

Summary: CVE-2026-102560 libsoup: libsoup: Heap buffer overflow during outgoing permessage-deflate buffer growth
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Priority: high    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
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OS: Linux   
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A flaw was found in libsoup. When the permessage-deflate WebSocket extension compresses a very large outgoing message, truncated size calculations used for GByteArray growth could wrap, causing zlib to write past the allocated buffer and resulting in a heap buffer overflow.
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Description OSIDB Bzimport 2026-09-29 16:50:24 UTC 
A heap buffer overflow was found in libsoup's WebSocket permessage-deflate outgoing path.

soup_websocket_extension_deflate_process_outgoing_message() stored deflateBound()'s uLong result in a guint and grew a GByteArray by that amount. For payloads approaching 4 GiB, the bound exceeds G_MAXUINT; truncation allowed buffer->len + max_length to wrap on later growth, and zlib wrote through next_out past the allocation.

Fixed by refusing payloads whose compressed bound cannot fit in a GByteArray and by checking each incremental growth (commit dbd0417e, libsoup 3.7.3).

References:
https://gitlab.gnome.org/GNOME/libsoup/-/work_items/554 (Bug 8)
https://gitlab.gnome.org/GNOME/libsoup/-/commit/dbd0417e