diff -ruNp a/src/lib/OpenEXR/ImfIDManifest.cpp b/src/lib/OpenEXR/ImfIDManifest.cpp
--- a/src/lib/OpenEXR/ImfIDManifest.cpp	2026-06-25 11:37:35.541229769 +0200
+++ b/src/lib/OpenEXR/ImfIDManifest.cpp	2026-06-25 11:39:22.576872688 +0200
@@ -359,6 +359,13 @@ void IDManifest::init(const char* data,
       //
       // previous string had more than 255 characters? 
       //
+      const size_t minPrefixLen =
+          stringList[i - 1].size () > 255 ? size_t (2) : size_t (1);
+      if (stringList[i].size () < minPrefixLen)
+      {
+          throw IEX_NAMESPACE::InputExc (
+              "IDManifest string too small for common prefix length");
+      }
       if(stringList[i-1].size()>255)
       {
           common = size_t( ((unsigned char) (stringList[i][0])) <<8 ) + size_t( (unsigned char) (stringList[i][1]));